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Effect of in situ exopolysaccharide production on physicochemical, rheological, sensory, and microstructural properties of the yogurt drink ayran: An optimization study based on fermentation kinetics

Open ArchivePublished:December 26, 2014DOI:https://doi.org/10.3168/jds.2014-8936

      Abstract

      Exopolysaccharide (EPS)-producing starter cultures are preferred for the manufacture of fermented milk products to improve rheological and technological properties. However, no clear correlation exists between EPS production and the rheological and technological properties of fermented milk products such as the yogurt drink ayran. In this study, 4 different strain conditions (EPS and EPS+ Streptococcus thermophilus strains) were tested as a function of incubation temperature (32, 37, or 42°C) and time (2, 3, or 4 h) to determine the effect of culture type and in situ EPS production on physicochemical, rheological, sensory, and microstructural properties of ayran. Furthermore, we assessed the effect of fermentation conditions on amounts of EPS production by different EPS-producing strains during ayran production. A multifactorial design of response surface methodology was used to model linear, interaction, and quadratic effects of these variables on steady shear rheological properties of ayran samples and in situ EPS production levels. The physicochemical and microbiological characteristics of ayran samples altered depending on incubation conditions and strain selection. Steady shear tests showed that ayran samples inoculated with EPS+ strains exhibited pseudoplastic flow behavior. Production of ayran with EPS strain (control sample) resulted in the lowest apparent viscosity values (η50), whereas those produced with the combination of 2 EPS+ strains yielded ayran with notably increased η50 values. We concluded that incubation time was the variable with the greatest effect on η50, consistency coefficient (K), and flow behavior index (n) values. In situ EPS production was also affected by these conditions during ayran fermentation in which strain-specific metabolism conditions were found to be the most important factor for EPS production. In addition, these findings correlated the amount of in situ EPS produced with the rheological properties of ayran. Scanning electron microscopy images of the samples showed differences in structural features, revealing a prominent network strand structure in the ayran samples inoculated with the admixture of 2 EPS-producing strains incubated at 37°C for 3 h. These results provide useful information for large-scale production of ayran by the dairy industry.

      Key words

      Introduction

      Demand is increasing worldwide for fermented dairy products because of their potential health-promoting properties, and these products have been traditionally produced in every country in the world. Ayran is a Turkish yogurt drink traditionally produced by adding water and salt to yogurt (homemade) or by addition of yogurt starter cultures to standardized milk for fermentation on an industrial scale with an annual production of approximately 1 million tonnes (
      • Yildiz F.
      ;
      • Altay F.
      • Karbancioglu-Guler F.
      • Daskaya-Dikmen C.
      • Heperkan D.
      A review on traditional Turkish fermented non-alcoholic beverages: Microbiota, fermentation process and quality characteristics.
      ). The main technological problem in ayran is its tendency to exhibit rheological instability during storage similar to that of yogurt (it is a diluted yogurt product;
      • Koksoy A.
      • Kilic M.
      Use of hydrocolloids in textural stabilization of a yoghurt drink, ayran.
      ;
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). The rheological properties of ayran are affected by heat treatment of milk, DM, salt content of the final product, addition of transglutaminase, fermentation temperature, and final pH, as well as starter culture type, such as ropy and nonropy starters (
      • Köksoy A.
      • Kılıç M.
      Effects of water and salt level on rheological properties of Ayran, a Turkish yoghurt drink.
      , 2004;
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ;
      • Tamucay-Özünlü B.
      • Kocak C.
      The effect of different heat treatments of milk on quality of Ayran.
      ;
      • Sanli T.
      • Sezgin E.
      • Senel E.
      • Benli M.
      Effects of using transglutaminase on properties of ayran in traditional production of Ayran.
      ). Similarly, use of stabilizers is reported to enhance the rheological properties of ayran although such use is not preferred because of their potential negative effect on the overall acceptability of fermented milk beverages (
      • Koksoy A.
      • Kilic M.
      Use of hydrocolloids in textural stabilization of a yoghurt drink, ayran.
      ).
      Several lactic acid bacteria (LAB) have been shown to produce exopolysaccharides (EPS) that may be attached to the bacterial cell wall or directly secreted to the environment. Lactic acid bacteria can structurally produce homopolysaccharides, which contain only one type of sugar molecule, and heteropolysaccharides, which are composed of different sugar monomers (
      • Dertli E.
      • Colquhoun I.J.
      • Gunning A.P.
      • Bongaerts R.J.
      • Le Gall G.
      • Bonev B.B.
      • Mayer M.J.
      • Narbad A.
      Structure and biosynthesis of two exopolysaccharides produced by Lactobacillus johnsonii FI9785.
      ). Exopolysaccharides have unique characteristics because of the differences in the sugar subunits and glycosidic linkages present in their repeating units, which explains the great diversity among bacterial EPS and novel EPS structures (
      • De Vuyst L.
      • Degeest B.
      Heteropolysaccharides from lactic acid bacteria.
      ;
      • Dertli E.
      • Colquhoun I.J.
      • Gunning A.P.
      • Bongaerts R.J.
      • Le Gall G.
      • Bonev B.B.
      • Mayer M.J.
      • Narbad A.
      Structure and biosynthesis of two exopolysaccharides produced by Lactobacillus johnsonii FI9785.
      ).
      Exopolysaccharides have crucial roles in physicochemical and rheological properties of fermented dairy products as natural bio-thickening agents and in situ-produced stabilizers (
      • Duboc P.
      • Mollet B.
      Applications of exopolysaccharides in the dairy industry.
      ). Moreover, higher viscosity values are reported for fermented milk products, including yogurt, when EPS-producing cultures are used in fermentation compared with non-EPS-producing cultures (
      • Marshall V.M.
      • Rawson H.L.
      Effects of exopolysaccharide-producing strains of thermophilic lactic acid bacteria on the texture of stirred yoghurt.
      ;
      • Folkenberg D.M.
      • Dejmek P.
      • Skriver A.
      • Skov Guldager H.
      • Ipsen R.
      Sensory and rheological screening of exopolysaccharide producing strains of bacterial yoghurt cultures.
      ). These EPS were reported to enhance the sensory characteristics, including mouthfeel, shininess, ropiness, and creaminess, in which the characteristics of EPS play an important role (
      • Folkenberg D.M.
      • Dejmek P.
      • Skriver A.
      • Ipsen R.
      Relation between sensory texture properties and exopolysaccharide distribution in set and in stirred yoghurts produced with different starter cultures.
      ;
      • Purwandari U.
      • Shah N.P.
      • Vasiljevic T.
      Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt.
      ). In addition to their potential role on the improvement of the rheological properties of yogurt and fermented dairy products, their GRAS (generally regarded as safe) status also increased the interest in these biopolymers (
      • De Vuyst L.
      • Degeest B.
      Heteropolysaccharides from lactic acid bacteria.
      ;
      • Hassan A.N.
      • Ipsen R.
      • Janzen T.
      • Qvist K.B.
      Microstructure and rheology of yogurt made with cultures differing only in their ability to produce exopolysaccharides.
      ).
      Several factors, including monosaccharide composition, linkage type, side chains, net charge, molecular weight of EPS, and their interaction with milk constituents, especially proteins and ions, affect the rheological functions of these biopolymers in fermented milk products (
      • Kleerebezem M.
      • van Kranenburg R.
      • Tuinier R.
      • Boels I.C.
      • Zoon P.
      • Looijesteijn E.
      • Hugenholtz J.
      • de Vos W.M.
      Exopolysaccharides produced by Lactococcus lactis: From genetic engineering to improved rheological properties.
      ;
      • Duboc P.
      • Mollet B.
      Applications of exopolysaccharides in the dairy industry.
      ;
      • Ruas-Madiedo P.
      • Hugenholtz J.
      • Zoon P.
      An overview of the functionality of exopolysaccharides produced by lactic acid bacteria.
      ;
      • Broadbent J.R.
      • McMahon D.J.
      • Welker D.L.
      • Oberg C.J.
      • Moineau S.
      Biochemistry, genetics, and applications of exopolysaccharide production in Streptococcus thermophilus: A review.
      ;
      • Hassan A.N.
      • Ipsen R.
      • Janzen T.
      • Qvist K.B.
      Microstructure and rheology of yogurt made with cultures differing only in their ability to produce exopolysaccharides.
      ). Similarly, the degree and length of branches in EPS molecules can significantly affect the compactness of the EPS, which determines their rheological role (
      • Duboc P.
      • Mollet B.
      Applications of exopolysaccharides in the dairy industry.
      ). In situ EPS production level was also among the factors having the most remarkable effect on rheological properties. However, previous reports showed that EPS concentration was not always positively correlated with yogurt viscosity, and that production of EPS might not always result in improved viscosity or gel firmness (
      • Rawson H.L.
      • Marshall V.M.
      Effect of ‘ropy’ strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus on rheology of stirred yogurt.
      ;
      • Ruas-Madiedo P.
      • Hugenholtz J.
      • Zoon P.
      An overview of the functionality of exopolysaccharides produced by lactic acid bacteria.
      ). It has also been reported that no difference was detected in the viscosity values of ayran samples produced with ropy and nonropy cultures (
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). These contrary results might be due to the alterations in protein–EPS interactions during yogurt and ayran fermentation process that depend on final EPS structure and production levels as well as the chemical composition of the final product, which can be affected by the starter culture type, incubation temperature, and time. However, these studies have been performed with a limited number of bacterial cultures and under different conditions, which makes it hard to compare these results directly with those noted previously (
      • Folkenberg D.M.
      • Dejmek P.
      • Skriver A.
      • Skov Guldager H.
      • Ipsen R.
      Sensory and rheological screening of exopolysaccharide producing strains of bacterial yoghurt cultures.
      ). Thus, to fully understand the role of EPS in the rheology of dairy products such as ayran, a comprehensive study is required to test the relationship between EPS production and culture incubation conditions and the chemical properties of these products along with their microstructural properties.
      Several strategies exist to test the relationship between EPS production and culture incubation conditions and between amount of EPS produced and rheological properties. One of the most effective ways to do this is to determine the factors having the greatest effect on the fermentation process and to find the optimum or limit values of these factors. Given the production steps of fermented milk products and yogurt, incubation temperature and time appear to be among the most effective parameters in EPS production. To find the optimum or limit values of these factors, response surface methodology (RSM) is known to be a useful statistical tool, allowing improvement and optimization of processes by finding the experimental relationship between the input and output variables considered in the experimental design (
      • Hejazi T.H.
      • Bashiri M.
      • Diaz-Garcia J.
      • Noghondarian K.
      Optimization of probabilistic multiple response surfaces.
      ). Accurate assessment of the effect of factors relies, to a large extent, on determination of the simultaneous effects of multiple factors, as opposed to conclusions based on a single factor. However, in the literature, a considerable number of studies based on RSM have been conducted using an approach with a single-response problem and only limited attention has been given to multi-response problems. However, to achieve the best EPS-producing conditions in our case, a multiple-response optimization study should be conducted. To the best of our knowledge, a limited number of studies have been conducted on optimization of fermentation conditions for EPS production and effect of EPS amounts on rheological properties of fermented milk products based on multiple response optimization (
      • Kimmel S.A.
      • Roberts R.F.
      • Ziegler G.R.
      Optimization of exopolysaccharide production by Lactobacillus delbrueckii ssp. bulgaricus RR grown in a semidefined medium.
      ;
      • Erkaya T.
      • Başlar M.
      • Şengül M.
      • Ertugay M.F.
      Effect of thermosonication on physicochemical, microbiological and sensorial characteristics of ayran during storage.
      ). Furthermore, no study has investigated the effect of in situ EPS production on rheological properties of ayran, a yogurt drink. Therefore, the objectives of this study were (1) to use RSM to determine the effect of fermentation conditions (incubation temperature and time) on in situ EPS production levels in ayran; (2) to investigate the effect of in situ EPS production on physicochemical, rheological, sensory, and microstructural properties of ayran; and (3) to determine if a correlation exists between the amount of EPS produced and rheological properties of ayran.

      Materials and Methods

       Materials

      Standardized cow milk (pH 6.6, DM and fat of 8 and 2%, respectively) was used for the preparation of ayran samples. The following strain conditions were used: (1) EPS strain (non-EPS-producing Streptococcus thermophilus strain) as control; (2) strain 1, an EPS-producing S. thermophilus strain (YO-MIX 499 LYO 100 DCU, Danisco Canada Inc., Scarborough, Canada); (3) strain 2, an EPS-producing S. thermophilus strain (YOG CY-340 DSL, Delvo, Delvo-Yog, DSM, Heerlen, the Netherlands); and (4) a mixture of strains 1 and 2. All strain conditions also included Lactobacillus delbrueckii ssp. bulgaricus in the starter culture.

       Ayran Production

      For ayran production, after the pasteurization (85°C for 15 min) process, milk was cooled to the relevant incubation temperatures (32, 37, and 42°C), as shown in Table 1, and each strain (EPS strain, 2 EPS+ strains, and their mixture) was added at a 1% concentration. The samples were incubated at the relevant incubation times (2, 3, and 4 h) as presented in Table 1. After incubation, the ayran samples were prepared with the addition of water (1:1), and then homogenized by using a mixer (Ultra Turrax, T18, IKA, Stauffen, Germany), and salt solution (1%) was added as described previously (
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). After homogenization, the ayran samples incubated at different incubation temperatures and times were stored at 4°C overnight until analysis.
      Table 1The design matrix indicating the levels of coded and actual values for each run
      RunCoded levels of factorsActual levels of factors
      Incubation temperature (°C)Incubation time (h)Incubation temperature (°C)Incubation time (h)
      R1−1−1322.0
      R2−10323.0
      R3−11324.0
      R40−1372.0
      R500373.0
      R600373.0
      R701374.0
      R81−1422.0
      R910423.0
      R1011424.0

       Physicochemical and Microbiological Analysis

      For the physicochemical analysis of ayran samples, pH, percent DM, fat and protein contents, and titratable acidity values were determined. The pH values were determined at 25°C using a pH meter (WTW 315i Set Model, Weilheim, Germany). To determine percent DM content, ayran samples were dried at 105°C for 4 h in a drying oven (Daihan, WOF sterilizator, Gangwon-do, South Korea). Protein (micro-Kjeldahl method, 6.38×) and fat contents (Gerber method) of ayran samples were determined using previously described methods (
      • Yöney Z.
      ). Titratable acidity was determined after titrating with 0.1 N NaOH and expressed as a percentage of lactic acid, as described previously (
      • Metin M.
      ). For the microbiological analysis of ayran samples, serial dilutions were prepared, plated onto M17 (Oxoid, Basingstoke, UK) and de Man, Rogosa, and Sharpe (MRS; Oxoid) plates, and incubated at 37°C for 48 and 72 h for enumeration of S. thermophilus and Lb. delbrueckii ssp. bulgaricus counts, respectively.

       Isolation and Quantification of EPS in Ayran Samples

      For isolation of EPS from ayran samples, an equal volume of chilled ethanol was added to 20 mL of ayran sample followed by an overnight incubation at 4°C. After centrifugation at 6,000 × g for 30 min at 4°C, the formed pellets containing EPS were resuspended in 5 mL of distilled water. For removal of proteins, TCA was added at a final concentration of 20% and the suspension was further incubated for 2 h at 4°C under gentle agitation. Precipitated proteins were removed by centrifugation at 13,000 × g for 20 min at 4°C, and supernatants were collected. Two volumes of cold ethanol were added to the supernatants for EPS precipitation, and the centrifugation process was followed as described above. The resuspended EPS were dialyzed (12,000–14,000-Da visking dialysis membrane, Medicell International, London, UK) against 5 L of distilled water for 2 d by changing the water 3 times per day. After isolation of EPS from ayran samples, EPS levels in each sample were determined by using the phenol-sulfuric acid method (
      • DuBois M.
      • Gilles K.A.
      • Hamilton J.K.
      • Rebers P.A.
      • Smith F.
      Colorimetric method for determination of sugars and related substances.
      ) and expressed as milligrams of EPS per liter of sample.

       Steady Shear Rheological Analysis

      A strain/stress controlled rheometer (MRC 302, Anton Paar, Graz, Austria) equipped with a Peltier temperature controller was used to determine steady shear rheological characteristics of ayran samples. The measurements were carried out using a parallel plate configuration (plate diameter 35 mm, angle 4°, gap size 0.5 mm) in the shear rate range of 0.1 to 100 s−1 at a constant measurement temperature (5°C). A 1.0-mL sample was placed between plates, and measurement was started immediately. In total, 25 data points were recorded at 10-s intervals during the shearing. Each measurement was replicated 3 times in 2 different samples. Apparent viscosity was determined as a function of shear rate, and shear stress versus shear rate was plotted by increasing shear rate. The obtained data were fitted to the Ostwald de Waele model using Toolmaster (Graz, Austria), and consistency coefficient and flow behavior index values were calculated according to the following model used to describe shear-induced behavior of the ayran samples:
      σ=Kγ˙n,
      [1]


      where σ is the shear stress (Pa), K is the consistency coefficient (Pa·sn), γ˙ is the shear rate (s−1), and n is the flow behavior index (dimensionless).

       Sensory Analysis

      The sensory analysis of ayran samples were determined based on protocol described previously (
      • Yilmaz M.T.
      • Karaman S.
      • Cankurt H.
      • Kayacier A.
      • Sagdic O.
      Steady and dynamic oscillatory shear rheological properties of ketchup-processed cheese mixtures: Effect of temperature and concentration.
      ). In brief, 100-mL ayran samples were presented and served in coded glass containers to a panel consisting 5 women and 5 men. Panelists were trained before evaluation to familiarize them with the sensory analysis, samples, and methodology. All coded ayran samples were evaluated for color, odor, appearance, taste, viscosity, and general acceptance properties in a scale from 1 to 9 describing low to high acceptability, respectively.

       Scanning Electron Microscopy

      Microscopic structure of the ayran samples was analyzed by scanning electron microscopy. Samples were examined under high vacuum in a field emission scanning electron microscope (JSM-5510, JEOL Ltd., Tokyo, Japan) with a working distance of 8 mm. Secondary electron images were acquired at an accelerating voltage of 5 kV. For processing of the images, SEM Control User Interface (version 5.21; JEOL Ltd.) was used.

       Data Analysis, Modeling, and Optimization

      Response surface methodology was performed to determine changes in the amount of EPS production and rheological parameters of ayran as a function of incubation temperature and time. For this purpose, a 3-level, 2-variable central composite design was used. The 2 factors, levels, and experimental design in terms of coded and uncoded (actual values) are given in Table 1. The model used was
      Yϵ=β0+i=1Nβixi+i=1Nβiixi2+i=1i<jj=i+1βijxixj,
      [2]


      where Y is the corresponding predicted response value, ε is the error term, β0 is the intercept term, βi is the linear term, βii is the quadratic term, βij is the interaction term, and Xi and Xj are the coded levels of the independent variables. The regression coefficients of linear, quadratic and interaction terms were determined by using Design Expert package software (version 7.0; Stat-Ease Inc., Minneapolis, MN) for each output parameter.
      The best fitting models were determined using multiple linear regressions with backward elimination regression, in which insignificant factors and interactions were removed from the models and only the variables significant at P < 0.01, P < 0.05, and P < 0.1 levels were selected for the model construction using backward elimination regression. For this procedure, let the model with all possible covariates be
      Y=β0+β1X1++βr1Xr1+ϵ.
      [3]


      Then, the following r − 1 tests were carried out: H0j : βj = 0, j = 1, 2, …, r – 1. The lowest partial F-test value Fl corresponding to H0l : βl = 0 or t-test value tl is compared with the preselected significance values F0 and t0.
      In this study, both single and multiple response optimization procedures were performed. Single response optimization is useful for finding the optimum values of factors for only one response. However, for industrial applications, optimization should be synchronously performed for all the responses involved. Moreover, competition occurs between these responses in many cases; thus, improving one response may lead another response to deteriorate, further complicating the situation. To overcome this problem, multiple responses are solved through use of a desirability function that combines all the responses into one measurement. The operating conditions, where x provides the “most desirable” response values, can be found by this method. In this respect, different desirability functions di(Yi) can be used depending on whether a particular response Yi is to be maximized or minimized (
      • Derringer G.
      • Suich R.
      Simultaneous optimization of several response variables.
      ). Let Li, Ui, and Ti be the lower, upper, and target values, respectively, that are desired for response Yi with Li, Ui, and Ti.
      If a response is to be maximized, then its individual desirability function is with the exponent s that determines how significant it is to hit the target value. For s = 1, the desirability function increases linearly toward Ti, which indicates a large adequate value for the response; for s < 1, the function is convex, and for s > 1, the function is concave:
      diyˆi=0yˆix<LiyˆixLiTiLi2LiyˆixTi1yˆix>Ti.
      [4]


      If a response is to be minimized, then its individual desirability function is with Ti, which indicates a small adequate value for the response:
      diyˆi=1yˆix<TiyˆixUiTiUi2TiyˆixUi0yˆix>Ui.
      [5]


      Having computed for each response variable, desirability values were combined into a single desirability index, D. For this purpose, each response was transformed in a dimensionless function, called partial desirability function, di, which reflects the desirable ranges for each response. The desirable ranges varies from 0 to 1 (least to most desirable, respectively. Definition of the partial desirability functions allows the global desirability function D to calculate the weighted geometric mean of n individual desirability functions (all transformed responses) [Eq. (6)]. The simultaneous objective function is a geometric mean of all transformed responses (
      • Myers R.H.
      • Montgomery D.C.
      Response Surface Methodology.
      ):
      D=d1p1×d2p2×d3p3×...×dnpi1pi=i=1ndipi1pi,
      [6]


      where pi is the weighting of the ith desirability function, and normalized so that i=1npi=1. By weighting the partial desirability functions, it is possible to enable the optimization process to take the relative importance of each response into consideration. By examining the form of the desirability function, it is possible to find the region where the function was close to 1 and determine the compromise optimum conditions.
      The computational work including designation of experimental points, randomization, ANOVA, fitting of the second-order polynomial models, graphical representations, and optimization was performed using the statistical package Design Expert (version 7.0; Stat-Ease Inc.). The SPSS Statistics package (17.0; SPSS Statistics/IBM, Armonk, NY) was used to conduct an ANOVA to show the differences between experimental runs and between treatments (P < 0.05).

      Results and Discussion

      The effects of strain selection as a function of EPS production as well as incubation temperature and time on chemical properties of the ayran samples are presented in Table 2. The final pH of ayran after incubation period is critical in terms of the development of a desired final product due to its role in chemical, physicochemical, and rheological properties of ayran (
      • Tamucay-Özünlü B.
      • Kocak C.
      Effect of ending the incubation at different pHs on quality of Ayran.
      ). Our study revealed that the pH values of ayran samples were significantly affected (P < 0.05) both by starter culture type with different EPS production characteristics and by fermentation temperature and time, and several trends were observed. As expected, ayran samples achieved lower pH values when the incubation temperature and time were increased from 32°C to 42°C and from 2 to 4 h, respectively. Similar observations have been reported during yogurt production (
      • Purwandari U.
      • Shah N.P.
      • Vasiljevic T.
      Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt.
      ). The pH of ayran samples ranged between 4.78 and 3.94 for all strain conditions depending on the incubation temperature and time (Table 2). Similar to our results, the pH of ayran samples is reported to range between 3.96 and 4.52 on the first day of incubation (
      • Tamucay-Özünlü B.
      • Kocak C.
      The effect of different heat treatments of milk on quality of Ayran.
      ;
      • Sanli T.
      • Sezgin E.
      • Senel E.
      • Benli M.
      Effects of using transglutaminase on properties of ayran in traditional production of Ayran.
      ). In general, the effect of incubation time on alterations of pH values of ayran samples was greater than that of incubation temperature. Previously, 2 S. thermophilus strains were compared for their fermentation time required to reach pH 4.5 in yogurt samples depending on incubation temperature and this time was found to be different due to the thermophilic or mesophilic character of the strains (
      • Purwandari U.
      • Shah N.P.
      • Vasiljevic T.
      Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt.
      ). We cannot clearly correlate the pH differences with the temperature requirements of the strains tested, which could be because of the short incubation period used in our study. Importantly, strain type, with different EPS production characteristics, affected the pH values of ayran samples, and the lowest pH values were detected in the EPS control group, a finding in agreement with previous observations where nonropy cultures more rapidly decreased the final pH of ayran samples than did ropy cultures. Final pH of ayran is reported to be important for ayran rheology (
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). In fact, this is expected because EPS production is an energy-intensive process (
      • Broadbent J.R.
      • McMahon D.J.
      • Welker D.L.
      • Oberg C.J.
      • Moineau S.
      Biochemistry, genetics, and applications of exopolysaccharide production in Streptococcus thermophilus: A review.
      ) and the catabolic mechanism of the non-EPS-producing strain might be faster than that of EPS-producing strains, which may result in more lactic acid formation and a greater reduction in final pH. The higher titratable acidity values (P < 0.05) of the control ayran group compared with the other groups may also support this observation (Table 2). We also observed some significant differences (P < 0.05) in pH of ayran samples produced with EPS-producing strains that could be related to strain-specific conditions. It has been reported that a slimy EPS-producing strain S. thermophilus showed a faster decrease in pH of yogurt samples compared with a capsular EPS-producing strain (
      • Purwandari U.
      • Shah N.P.
      • Vasiljevic T.
      Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt.
      ).
      Table 2Physicochemical and microbiological properties
      Control=non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2=EPS-producing S. thermophilus strain; mixture=admixture of strains 1 and 2.
      and exopolysaccharide (EPS) yield in ayran samples inoculated with different strains
      Control=non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2=EPS-producing S. thermophilus strain; mixture=admixture of strains 1 and 2.
      RunControlStrain 1Strain 2MixtureControlStrain 1Strain 2Mixture
      pHDM (%)
      R14.38
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.58
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.75
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.78
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.77
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.83
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.87
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.76
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R24.27
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.46
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.47
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.53
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.61
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.83
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.77
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.61
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R34.16
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.34
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.38
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.41
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.64
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.78
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.76
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.69
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R44.53
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.54
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.45
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.44
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.55
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.92
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.77
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.63
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R54.18
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.38
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.41
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.24
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.48
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.81
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.94
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.59
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R64.24
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.37
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.40
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.26
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.47
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.98
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.83
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.65
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R74.05
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.28
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.34
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.08
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.50
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.98
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.97
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.55
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R84.17
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.52
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.42
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.65
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.97
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.93
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.72
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R94.02
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.39
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.26
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.54
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.42
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.81
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.62
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.47
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R103.94
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.28
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.21
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.43
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.50
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.69
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.73
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.68
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      Fat (%)Protein (%)
      R12.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.89
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.82
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.87
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R21.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.86
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.76
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.96
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.07
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R32.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.91
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R41.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.84
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.65
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.92
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.01
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R51.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.93
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.98
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.05
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R61.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.75
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.92
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.93
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R71.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.86
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.94
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.87
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.07
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R81.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.85
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.91
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.75
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.91
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R91.75
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.73
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.89
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.92
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R101.70
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.80
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.97
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.76
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.89
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.98
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      Titratable acidity (%)MRS
      Growth on M17 and de Man, Rogosa, and Sharpe (MRS) medium.
      (log cfu/mL)
      R10.58
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.49
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.41
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.41
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.238
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.086
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.538
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.427
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R20.71
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.51
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.52
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.55
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.359
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.255
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.821
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.412
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R30.84
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.56
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.54
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.57
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.467
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.500
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.023
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.382
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R40.61
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.58
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.53
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.53
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.289
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.488
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.700
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.419
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R50.82
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.64
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.56
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.75
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.482
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.408
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.927
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.577
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R60.83
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.64
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.57
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.74
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.440
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.387
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.225
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.649
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R70.87
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.68
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.59
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.87
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.573
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.394
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.996
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.701
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R80.82
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.61
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.57
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.45
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.801
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.396
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.332
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.338
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R90.88
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.69
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.73
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.61
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.557
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.456
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.273
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.584
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R100.90
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.73
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.76
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.74
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.593
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.506
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.303
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.622
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      M17
      Growth on M17 and de Man, Rogosa, and Sharpe (MRS) medium.
      (log cfu/mL)
      EPS yield (mg/L)
      R18.009
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.826
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.122
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.519
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND
      ND=not detected.
      14.53
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.08
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.70
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R28.052
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.764
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.088
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.551
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND14.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      3.75
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.12
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R38.221
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.944
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.177
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.600
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND7.74
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.88
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      9.14
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R48.085
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.482
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.193
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.588
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND5.61
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      3.65
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      10.81
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R58.264
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.334
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.203
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.500
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND8.02
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      10.84
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      22.15
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R68.250
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.345
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.819
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.540
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND7.88
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      10.15
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      23.08
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R78.258
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.167
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.193
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.637
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND10.10
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      13.60
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      29.31
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R88.058
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.586
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.235
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.690
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND4.72
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.21
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      10.59
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R98.236
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.542
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.982
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.710
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND3.84
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.98
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      14.34
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R108.283
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.378
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.159
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.745
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      ND4.95
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      7.00
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      16.36
      Different uppercase superscript letters show differences between the strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      A-D Different uppercase superscript letters show differences between the strains within the same run (P < 0.05).
      a–j Different lowercase superscript letters show differences between runs within the same strain (P < 0.05).
      1 Control = non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2 = EPS-producing S. thermophilus strain; mixture = admixture of strains 1 and 2.
      2 Growth on M17 and de Man, Rogosa, and Sharpe (MRS) medium.
      3 ND = not detected.
      The DM content of ayran samples was also affected by inoculation and incubation conditions and varied between 6.98 and 6.42%. Generally, a short incubation period resulted in higher DM content and incubation period on DM content of ayran was more likely to be important than the type of strain used in ayran production at each incubation temperature. For instance in the first, second, and third runs (in which ayran was produced at 32°C), we found no significant difference in DM content of ayran samples, but different incubation periods resulted in significant differences for different strains (P < 0.05; control and strain mixture groups). Similar trends were observed for other conditions tested. The DM values of ayran samples in our study were similar to previous observations (
      • Gülmez M.
      • Güven A.
      • Sezer C.
      • Duman B.
      Evaluation of microbiological and chemical quality of ayran samples marketed in Kars and Ankara cities in Turkey.
      ). Another important chemical characteristic of dairy products such as ayran is their fat content. Similar to DM content, we did not observe large fluctuations in the fat contents of ayran samples. No significant differences were detected for different EPS-producing strains except in runs 4, 5, and 10 (Table 2). Similarly, we did not detect a linear relationship between incubation temperature and time and the fat content of ayran samples, although several trends observed. The highest and lowest fat contents were found to be 2.00 and 1.70%, respectively, for all ayran samples tested, which was in accordance with previous observations (
      • Gülmez M.
      • Güven A.
      • Sezer C.
      • Duman B.
      Evaluation of microbiological and chemical quality of ayran samples marketed in Kars and Ankara cities in Turkey.
      ). Additionally, the protein contents of ayran samples were evaluated and ranged between 1.73 and 2.07% (Table 2); protein content differed depending on the conditions tested. These alterations among different strain groups were found to be important (P < 0.05), showing strain-specific effects on the fermentation process. These results were also in agreement with previous observations, in which the protein content of ayran samples was reported to be between 1.44 and 3.48% (
      • Altay F.
      • Karbancioglu-Guler F.
      • Daskaya-Dikmen C.
      • Heperkan D.
      A review on traditional Turkish fermented non-alcoholic beverages: Microbiota, fermentation process and quality characteristics.
      ).
      The titratable acidity values of ayran samples, expressed as the amount of lactic acid, were determined in this study for all conditions tested (Table 2). We noted significant alterations (P < 0.05) in acidity values of ayran samples among different groups depending on EPS production as well as different incubation temperatures and times. The acidity ranged between 0.49 and 0.73% for the ayran samples produced with strain 1 and between 0.41 and 0.76% for the ayran samples produced with strain 2. Notably, the highest acidity values in ayran samples were detected in the control group and ranged between 0.58 and 0.90%. This could be related to the faster growth and acidification rates in EPS strains, as discussed above (
      • De Vuyst L.
      • Degeest B.
      Heteropolysaccharides from lactic acid bacteria.
      ;
      • Duboc P.
      • Mollet B.
      Applications of exopolysaccharides in the dairy industry.
      ;
      • Broadbent J.R.
      • McMahon D.J.
      • Welker D.L.
      • Oberg C.J.
      • Moineau S.
      Biochemistry, genetics, and applications of exopolysaccharide production in Streptococcus thermophilus: A review.
      ;
      • Purwandari U.
      • Shah N.P.
      • Vasiljevic T.
      Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt.
      ). The combination of 2 EPS-producing strains widened the range of acidity of ayran samples, which were found to be between 0.41 and 0.87%. Our results were also in agreement with previous observations, in which the acidity of ayran samples was found to be between 0.2 and 1.0% (
      • Gülmez M.
      • Güven A.
      • Sezer C.
      • Duman B.
      Evaluation of microbiological and chemical quality of ayran samples marketed in Kars and Ankara cities in Turkey.
      ;
      • Tamucay-Özünlü B.
      • Kocak C.
      The effect of different heat treatments of milk on quality of Ayran.
      ;
      • Sanli T.
      • Sezgin E.
      • Senel E.
      • Benli M.
      Effects of using transglutaminase on properties of ayran in traditional production of Ayran.
      ). Additionally, incubation conditions affected the acidity values of ayran samples; the increase in incubation temperature and time from 32 to 42°C and from 2 to 4 h increased acidity values in all treatment groups (Table 2). These observations were similar to alterations in the pH values of ayran sample.
      We also evaluated the effect of EPS production and fermentation temperature and time on the initial counts of S. thermophilus and Lb. delbrueckii ssp. bulgaricus in all ayran samples, and these counts were clearly both dependent on inoculation and incubation conditions (P < 0.05, Table 2). We did not observe a clear correlation between the increase in incubation temperature and S. thermophilus counts but, in general, S. thermophilus counts increased with increasing incubation time. It was previously reported that the numbers of viable cells in yogurt are higher at 30 and 42°C than at 37°C and that counts of viable cells of ropy strain are permanently higher than that of capsular strain, regardless of incubation conditions (
      • Purwandari U.
      • Shah N.P.
      • Vasiljevic T.
      Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt.
      ). Our results also revealed that the numbers of S. thermophilus in ayran samples were dependent on strain characteristics (P < 0.05); however, we detected no clear dependency of S. thermophilus count on EPS production. In general, S. thermophilus counts of ayran samples produced with EPS strain and strain 2 and ayran samples produced with strain 1 and strain mixture, respectively, did not show significant differences (P > 0.05) and notably the amount of EPS produced under these conditions were significantly different (P < 0.05, Table 2). Similar results were previously reported, in which EPS production of S. thermophilus strains was dependent on the strain-specific conditions (
      • Vaningelgem F.
      • Zamfir M.
      • Mozzi F.
      • Adriany T.
      • Vancanneyt M.
      • Swings J.
      • De Vuyst L.
      Biodiversity of exopolysaccharides produced by Streptococcus thermophilus strains is reflected in their production and their molecular and functional characteristics.
      ) and, for particular strains, growth-associated EPS production was also reported (
      • De Vuyst L.
      • Vanderveken F.
      • Van de Ven S.
      • Degeest B.
      Production by and isolation of exopolysaccharides from Streptococcus thermophilus grown in a milk medium and evidence for their growth-associated biosynthesis.
      ). Notably, higher growth rates in LAB strains might not always reflect higher EPS production because both cell wall biosynthesis and EPS production require the availability of isoprenoid phosphate lipid carrier and sugar nucleotides (
      • Whitfield C.
      Bacterial extracellular polysaccharides.
      ). Our findings also revealed a trend between counts of S. thermophilus in ayran samples and chemical characteristics such as pH and titratable acidity, especially for the control group. In contrast to S. thermophilus numbers, increasing the incubation temperature significantly increased counts of Lb. delbrueckii ssp. bulgaricus in ayran samples (P < 0.05) and the same trend was observed with increasing the incubation temperature (Table 2). The highest and lowest counts of Lb. delbrueckii ssp. bulgaricus were detected in ayran samples produced with strain 1 in the first run and with the control strain in the tenth run, respectively. Similarly, in general, the lowest counts of Lb. delbrueckii ssp. bulgaricus were detected for ayran samples produced with strain 1, in which S. thermophilus counts were also observed to be lower than in the other strain conditions (Table 2). The ratio of S. thermophilus to Lb. delbrueckii ssp. bulgaricus is important in terms of technological properties of ayran because of the limited proteolytic activities of S. thermophilus; the ratio in our study was lower than previous observations in ayran (
      • Tamucay-Özünlü B.
      • Kocak C.
      The effect of different heat treatments of milk on quality of Ayran.
      ) but similar to that of reported for yogurt fermentation (
      • Akın S.
      • Sıddık G.
      Effect of viscous cultures on the rheological and sensory properties, flavor substances and starter bacteria counts of set yogurt.
      ).
      The analysis of in situ EPS production during ayran fermentation revealed that EPS production was significantly (P < 0.05) affected by incubation temperature and time and by strain-specific conditions (Table 2). For instance, the highest and lowest EPS production were observed in runs 1 and 9, in runs 7 and 8, and in runs 7 and 1 for ayran samples produced with strain 1, strain 2, and strain mixture, respectively (Table 2). At an incubation temperature of 32°C, we observed no increase in EPS production with increasing incubation time when strain 1 was used but EPS production significantly (P < 0.05) increased for strain 2 and strain mixture with the increasing incubation period at this temperature; however, the highest EPS production was recorded for strain 1 after 2 h of incubation. Optimum EPS production temperature when the strain mixture was used was 37°C; in general, maximum EPS yields were recorded for the strain mixture. This may explain the fact that the highest apparent viscosity values were recorded for ayran produced using the strain mixture (Table 3). However, the relationship between EPS yields and rheological parameters of ayran samples were not always clear and it has been reported previously that no clear relationship exists between EPS production and viscosity, especially when comparing different strains (
      • Ruas-Madiedo P.
      • Tuinier R.
      • Kanning M.
      • Zoon P.
      Role of exopolysaccharides produced by Lactococcus lactis ssp. cremoris on the viscosity of fermented milks.
      ;
      • De Vuyst L.
      • Zamfir M.
      • Mozzi F.
      • Adriany T.
      • Marshall V.
      • Degeest B.
      • Vaningelgem F.
      Exopolysaccharide-producing Streptococcus thermophilus strains as functional starter cultures in the production of fermented milks.
      ). At this temperature (37°C), in situ EPS production during ayran production showed a consistent significant increase (P < 0.05) for all strain conditions tested. The highest EPS production in ayran was recorded at 37°C for the strain mixture, with a yield of 29.31 mg/L. We should also note that neither chemical characteristics such as pH nor microbiological results demonstrated the highest number of S. thermophilus at this temperature. This may suggest that EPS production is not only related to intrinsic strain metabolism conditions but it can also be affected by extrinsic factors such as incubation conditions, which can alter the final EPS yields, monomer composition, and molecular mass of the final EPS (
      • Grobben G.J.
      • van Casteren W.H.M.
      • Schols H.A.
      • Oosterveld A.
      • Sala G.
      • Smith M.R.
      • Sikkema J.
      • de Bont J.A.M.
      Analysis of the exopolysaccharides produced by Lactobacillus delbrueckii ssp. bulgaricus NCFB 2772 grown in continuous culture on glucose and fructose.
      ;
      • De Vuyst L.
      • Vanderveken F.
      • Van de Ven S.
      • Degeest B.
      Production by and isolation of exopolysaccharides from Streptococcus thermophilus grown in a milk medium and evidence for their growth-associated biosynthesis.
      ;
      • De Vuyst L.
      • Degeest B.
      Heteropolysaccharides from lactic acid bacteria.
      ;
      • Dertli E.
      • Colquhoun I.J.
      • Gunning A.P.
      • Bongaerts R.J.
      • Le Gall G.
      • Bonev B.B.
      • Mayer M.J.
      • Narbad A.
      Structure and biosynthesis of two exopolysaccharides produced by Lactobacillus johnsonii FI9785.
      ). At 42°C, EPS production did not increase with the increasing incubation time for strain 1, but we did observe a significant increase (P < 0.05) in EPS production for the other 2 strain conditions with increasing incubation time (Table 2). Interestingly, the lowest EPS yield for all conditions tested was recorded for strain 2 at 42°C after a 2-h incubation period, which was consistent with the low apparent viscosity value in this run (Table 3). Similar to the general EPS yields, EPS levels of the strain mixture were significantly higher (P < 0.05) than those of the other strain conditions at this incubation temperature (Table 2). Several variations on EPS production were recorded for different strain conditions, which can be related to the biosynthesis mechanism of EPS in S. thermophilus, as this species produces heteropolymeric-type EPS, which may show genetic instability depending on environmental conditions (
      • De Vuyst L.
      • Degeest B.
      Heteropolysaccharides from lactic acid bacteria.
      ;
      • Duboc P.
      • Mollet B.
      Applications of exopolysaccharides in the dairy industry.
      ;
      • Broadbent J.R.
      • McMahon D.J.
      • Welker D.L.
      • Oberg C.J.
      • Moineau S.
      Biochemistry, genetics, and applications of exopolysaccharide production in Streptococcus thermophilus: A review.
      ).
      Table 3Ostwald de Waele variables, η0 values, and related determination coefficients (R2) of ayran samples inoculated with different strains
      Control=non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2=EPS-producing S. thermophilus strain; mixture=admixture of strains 1 and 2.
      Variable
      K=consistency coefficient; η50=apparent viscosity; n=flow behavior index
      and run
      ControlStrain 1Strain 2Mixture
      K (Pa·sn)
       R10.550
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.025
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.159
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.648
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R21.007
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.109
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.885
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.236
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R31.091
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.579
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.034
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      2.485
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R40.752
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.062
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.323
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.950
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R53.501
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.414
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.049
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.618
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R63.569
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.369
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.176
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.260
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R74.855
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.433
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.775
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      5.365
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R81.644
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.255
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.500
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.924
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R92.799
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.220
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      3.563
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      3.767
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R103.013
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      4.346
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      8.384
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      6.752
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      η50 (Pa·s)
       R10.040
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.006
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.028
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.105
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R20.067
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.116
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.162
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.220
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R30.069
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.212
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.182
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.251
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R40.051
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.043
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.112
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.118
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R50.107
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.121
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.178
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.251
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R60.102
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.124
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.172
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.248
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R70.115
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.197
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.186
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.272
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R80.094
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.126
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.087
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.161
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R90.127
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.204
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.188
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.236
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R100.136
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.264
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.216
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.293
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      n
       R10.347
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.662
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.581
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.562
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R20.325
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      1.007
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.578
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.555
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R30.307
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.742
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.558
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.423
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R40.326
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.924
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.730
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.456
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R50.123
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.641
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.551
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.276
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R60.140
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.553
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.478
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.298
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R70.079
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.501
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.412
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.263
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R80.281
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.816
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.572
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.369
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R90.210
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.548
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.261
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.300
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
       R100.221
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.301
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.081
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      0.191
      Different uppercase superscript letters show differences between strains within the same run (P<0.05).
      ,
      Different lowercase superscript letters show differences between runs within the same strain (P<0.05).
      R2
       R10.99910.98990.97740.9227
       R20.99380.97500.98810.9754
       R30.98930.98840.98770.9934
       R40.99850.96980.99750.9693
       R50.92060.98930.99430.9957
       R60.96250.98100.98590.9961
       R70.81070.97910.98600.9949
       R80.97920.99280.99750.9569
       R90.97290.98740.97800.9751
       R100.97360.99470.85490.9654
      A–D Different uppercase superscript letters show differences between strains within the same run (P < 0.05).
      a–i Different lowercase superscript letters show differences between runs within the same strain (P < 0.05).
      1 Control = non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2 = EPS-producing S. thermophilus strain; mixture = admixture of strains 1 and 2.
      2 K = consistency coefficient; η50 = apparent viscosity; n = flow behavior index
      Overall, in situ EPS production by different strains during ayran fermentation was determined by incubation and strain-specific conditions, and the effect of these conditions on final EPS yields was modeled by using RSM. Table 4 shows the F-values of model parameters generated for final EPS yields for ayran samples produced with strains 1 and 2 and the strain mixture. As can be seen in Table 4, the R2 values for all strain conditions tested were close to 1, suggesting that the models demonstrated sufficient predictability for EPS yields depending on the parameters tested. Results revealed that linear effects of both incubation temperature and time significantly (P < 0.01; P < 0.1) affected EPS production. Previous studies also reported the crucial role of incubation temperature and time on EPS yields of LAB strains (
      • Petry S.
      • Furlan S.
      • Crepeau M.J.
      • Cerning J.
      • Desmazeaud M.
      Factors affecting exocellular polysaccharide production by Lactobacillus delbrueckii ssp. bulgaricus grown in a chemically defined medium.
      ;
      • Broadbent J.R.
      • McMahon D.J.
      • Welker D.L.
      • Oberg C.J.
      • Moineau S.
      Biochemistry, genetics, and applications of exopolysaccharide production in Streptococcus thermophilus: A review.
      ;
      • Tallon R.
      • Bressollier P.
      • Urdaci M.C.
      Isolation and characterization of two exopolysaccharides produced by Lactobacillus plantarum EP56.
      ). In Figure 1, these effects are illustrated as 3-dimensional response surfaces, in which the direction of the effects of incubation temperature and time on EPS production for all ayran strains and the second-order regression model equations can be seen. These graphs show that final EPS yields in all 3 strain conditions increased with the increase in incubation time. Similarly, incubation temperature had a positive effect on EPS yields for strain 2 and the mixture but had a negative effect for strain 1, in which the effect of incubation temperature was the most determining factor for EPS production. Furthermore, the quadratic effect of incubation temperature was found to be the most determinant factor for strain 2 and the strain mixture in EPS production, suggesting an optimum incubation temperature for EPS production during fermentation process (Figure 1). Previous reports revealed the complex relationship between the EPS production of LAB and incubation temperature, in which it was suggested that EPS production is maximal when the incubation temperature is lower than the optimum growth temperature. This finding is related to the lesser formation of cell wall at lower temperatures, which means the availability of more sugar nucleotides for EPS production (
      • Sutherland I.W.
      Bacterial exopolysaccharides.
      ;
      • van den Berg D.
      • Robijn G.W.
      • Janssen A.C.
      • Giuseppin M.
      • Vreeker R.
      • Kamerling J.P.
      • Vliegenthart J.
      • Ledeboer A.M.
      • Verrips C.T.
      Production of a novel extracellular polysaccharide by Lactobacillus sake 0−1 and characterization of the polysaccharide.
      ); in another study, however, the optimum growth temperature was reported to be the optimum temperature for EPS production (
      • Kojic M.
      • Vujcic M.
      • Banina A.
      • Cocconcelli P.
      • Cerning J.
      • Topisirovic L.
      Analysis of exopolysaccharide production by Lactobacillus casei CG11, isolated from cheese.
      . Our results suggest that selection of incubation temperature depending on strain-specific conditions with regard to EPS production is crucial to improve the final quality of ayran.
      Table 4Significance of the regression models (F values) and effects of the processing variables
      K=consistency coefficient; η50=apparent viscosity; n=flow behavior index.
      on rheological properties and exopolysaccharide (EPS) yield in ayran samples inoculated with different strains
      Control=non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2=EPS-producing S. thermophilus strain; mixture=admixture of strains 1 and 2.
      FactorControlStrain 1Strain 2MixtureControlStrain 1Strain 2Mixture
      K (Pa·sn)n
      ModelModifiedQuadraticQuadraticQuadraticModified2FI
      Two-factor interaction.
      QuadraticQuadratic
       x1 (temperature)52.45
      P<0.01.
      357.34
      P<0.01.
      68.87
      P<0.01.
      21.33
      P<0.01.
      21.87
      P<0.01.
      5.68†69.97
      P<0.01.
      36.05
      P<0.01.
       x2 (time)114.59
      P<0.01.
      650.80
      P<0.01.
      131.23
      P<0.01.
      38.98
      P<0.01.
      56.71
      P<0.01.
      7.50
      P<0.05;
      74.92
      P<0.01.
      23.94
      P<0.01.
       x1x21.004.300.0255.40†35.67
      P<0.01.
      2.28
       x1x162.40
      P<0.01.
      0.234.223.3650.09
      P<0.01.
      18.74
      P<0.05;
      4.49
       x2x27.97
      P<0.05;
      2.5613.36
      P<0.05;
      2.527.72
      P<0.05;
      2.500.011
       x12x244.98
      P<0.01.
      24.10
      P<0.01.
       x1x22
      Lack of fit41.6367.2410.3032.434.791.450.436.93
      R20.98480.99610.98210.94400.97390.75600.98040.9436
      Adjusted R20.96580.99110.95980.87390.94130.63390.95590.8732
      Predicted R20.84540.96770.82430.57830.66330.11260.86340.6249
      Adequate precision22.53946.81720.82311.44916.7067.21121.04611.487
      η50 (Pa·s)EPS yield (mg/L)
      ModelQuadraticQuadraticQuadraticQuadraticCubicCubic
      The ratio of maximum response to minimum was >10; the data were analyzed after base 10 log-transformation was performed. †P<0.10;
      Cubic
       x1 (temperature)63.03
      P<0.01.
      16.49
      P<0.05;
      4.91†9.17
      P<0.05;
      254.43
      P<0.01.
      5.71
      P<0.01.
      15.44†
       x2 (time)34.63
      P<0.01.
      38.55
      P<0.01.
      44.15
      P<0.01.
      114.81
      P<0.01.
      49.72
      P<0.05;
      113.45101.37
      P<0.01.
       x1x20.5013.40
      P<0.05;
      0.332.3160.66
      P<0.05;
      0.00010.80
       x1x10.130.0170.680.0342.79135.57
      P<0.01.
      154.92
      P<0.01.
       x2x26.93†3.686.95†19.17
      P<0.05;
      3.828.61†2.19
      x12x299.13
      P<0.01.
      7.6038.14
      P<0.05;
       x1x2224.54
      P<0.05;
      7.380.27
      Lack of fit9.4468.7035.32141.6740.762.656.81
      R20.96360.94750.93550.97340.99680.99490.9938
      Adjusted R20.91800.88180.85480.94010.98550.97690.9720
      Predicted R20.62980.45190.32650.74680.55740.47100.2356
      Adequate precision14.57211.5389.33914.48627.41323.42921.156
      1 K = consistency coefficient; η50 = apparent viscosity; n = flow behavior index.
      2 Control = non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2 = EPS-producing S. thermophilus strain; mixture = admixture of strains 1 and 2.
      3 Two-factor interaction.
      4 The ratio of maximum response to minimum was >10; the data were analyzed after base 10 log-transformation was performed.P < 0.10;
      * P < 0.05;
      ** P < 0.01.
      Figure thumbnail gr1
      Figure 1Effect of incubation temperature (T) and time (t) on amounts of exopolysaccharide (EPS) production by different EPS-producing strains in ayran samples. Strains 1 and 2 = EPS-producing Streptococcus thermophilus strains; mixture = admixture of strains 1 and 2.
      Figure 2 shows the shear rate versus shear stress data for ayran samples produced under different inoculation and incubation conditions. As can be seen, the apparent viscosity of ayran samples produced with all strain conditions decreased depending on the shear rate, indicating that all ayran samples showed shear thinning behavior. Importantly, both inoculation and incubation conditions affected the rheological conditions of ayran samples but the latter were more important for the determination of ayran rheology (Figure 2). The obtained shear rate versus shear stress data for all ayran samples were fitted to an Ostwald de Waele model and model parameters were determined for all conditions tested. The model parameters and the apparent viscosity values measured at 50 s−1 (η50) for all ayran samples are presented in Table 4. As can be seen in Table 4, the coefficient of determination values (R2) for the Ostwald de Waele model of ayran samples produced with the control strain, strains 1 and 2, and strain mixture ranged between 0.8107 and 0.9991, 0.9698 and 0.9947, 0.8549 and 0.9975, and 0.9227 and 0.9961, respectively. The closeness of the R2 values to 1 show that this model successfully determined the flow behavior of ayran samples. The apparent viscosity of ayran samples significantly increased (P < 0.05) with increasing incubation temperature and time for all strain conditions. For instance, the highest η50 values were detected at 42°C after 4 h of incubation for all strains. The increase in the η50 values of ayran with increasing fermentation temperature were also reported for both ropy and nonropy strains and it was suggested that the increase in the hydrophobic interactions of proteins at higher incubation temperatures could explain the increased η50 values in ayran (
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). Similarly, the effect of incubation time on η50 values of ayran might be related to increasing cross-linkages in the protein network but this process can be determined by several factors other than incubation time, including chemical characteristics such as pH and acidification rates and EPS-protein gel interactions depending on the incubation period (
      • Beal C.
      • Skokanova J.
      • Latrille E.
      • Martin N.
      • Corrieu G.
      Combined effects of culture conditions and storage time on acidification and viscosity of stirred yogurt.
      ;
      • de Kruif C.G.
      • Tuinier R.
      Polysaccharide protein interactions.
      ;
      • Hassan A.N.
      • Ipsen R.
      • Janzen T.
      • Qvist K.B.
      Microstructure and rheology of yogurt made with cultures differing only in their ability to produce exopolysaccharides.
      ;
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). The type of strain used for inoculation significantly (P < 0.05) affected the η50 of ayran samples depending on the interaction of EPS production and culture-incubation temperature (Table 4). Generally, the η50 of ayran samples produced with strain 2 was higher than those samples produced with strain 1, whereas the lowest η50 values were detected for ayran samples produced with the control strain, indicating the important role of EPS in ayran rheology. The mixture of 2 EPS-producing strains resulted in the highest η50 values, indicating the relationship between EPS production levels and ayran viscosity (Table 4). In fact, these results were consistent with the in situ EPS production levels by these strains (Table 2), showing the crucial role of EPS production in rheological properties of ayran. Similarly, production of 2 different EPS can also explain the higher η50 values in ayran samples produced with strain mixture, as previously reported (
      • Faber E.J.
      • Zoon P.
      • Kamerling J.P.
      • Vliegenthart J.F.G.
      The exopolysaccharides produced by Streptococcus thermophilus RS and STS have the same repeating unit but differ in viscosity of their milk cultures.
      ). Our results were in contrast to previous observations in which ayran produced with nonropy cultures demonstrated higher η50 values than those produced with ropy cultures, and EPS production, rather than differences in acidification properties of nonropy and ropy cultures, was stated to be the reason for the lower η50 values in ayran produced with ropy cultures (
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). The potential negative role of EPS in ayran or yogurt samples is thought to be associated with the potential prevention of protein–protein interactions by EPS during the fermentation process (
      • Hassan A.N.
      • Frank J.F.
      • Schmidt K.A.
      • Shalabi S.I.
      Rheological properties of yogurt made with encapsulated nonropy lactic cultures.
      ;
      • Marshall V.M.
      • Rawson H.L.
      Effects of exopolysaccharide-producing strains of thermophilic lactic acid bacteria on the texture of stirred yoghurt.
      ;
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). In contrast, EPS production increases the aggregation in the protein network and interaction between EPS and casein matrix in yogurt and therefore increases yogurt viscosity (
      • Rawson H.L.
      • Marshall V.M.
      Effect of ‘ropy’ strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus on rheology of stirred yogurt.
      ;
      • Hassan A.N.
      • Ipsen R.
      • Janzen T.
      • Qvist K.B.
      Microstructure and rheology of yogurt made with cultures differing only in their ability to produce exopolysaccharides.
      ). On the other hand, it has been reported that 2 S. thermophilus strains produced significantly different amounts of EPS but the η50 values of fermented milk products with these strains were found to be similar; this similarity was reported to be related to the difference in final EPS structures (
      • De Vuyst L.
      • Zamfir M.
      • Mozzi F.
      • Adriany T.
      • Marshall V.
      • Degeest B.
      • Vaningelgem F.
      Exopolysaccharide-producing Streptococcus thermophilus strains as functional starter cultures in the production of fermented milks.
      ). It should be noted here that LAB can produce a great variety of EPS differing in yield and structural and compositional characteristics, including net charge and molecular mass, which affect the rheological characteristics of EPS. The contrasting results reported are probably related to differences in these properties (
      • Rawson H.L.
      • Marshall V.M.
      Effect of ‘ropy’ strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus on rheology of stirred yogurt.
      ;
      • Kleerebezem M.
      • van Kranenburg R.
      • Tuinier R.
      • Boels I.C.
      • Zoon P.
      • Looijesteijn E.
      • Hugenholtz J.
      • de Vos W.M.
      Exopolysaccharides produced by Lactococcus lactis: From genetic engineering to improved rheological properties.
      ;
      • Duboc P.
      • Mollet B.
      Applications of exopolysaccharides in the dairy industry.
      ;
      • Ruas-Madiedo P.
      • Hugenholtz J.
      • Zoon P.
      An overview of the functionality of exopolysaccharides produced by lactic acid bacteria.
      ). Furthermore, carbohydrate-based stabilizers reportedly increase the apparent viscosity values of ayran, which supports our observations, and EPS produced in situ by the strains used in this study could be used as a stabilizer to enhance the rheological properties of ayran (
      • Koksoy A.
      • Kilic M.
      Use of hydrocolloids in textural stabilization of a yoghurt drink, ayran.
      ).
      Figure thumbnail gr2
      Figure 2Shear stress versus shear rate data for ayran samples. Control = non-exopolysaccharide (EPS)-producing Streptococcus thermophilus strain; strains 1 and 2 = EPS-producing S. thermophilus strain; mixture = admixture of strains 1 and 2.
      The model parameters consistency coefficient (K) and flow behavior index (n) were found to be significantly (P < 0.05) altered depending on the strain-specific conditions and incubation temperature and time for ayran samples (Table 4). The K values of ayran samples increased and n values decreased with increasing incubation temperature and time, and the effect of incubation time was greater. For instance, the highest K value (8.384 Pa·s) and the lowest n value (0.081) were determined for the sample incubated at 42°C for 4 h and produced with strain 2 (Table 4). Several studies have reported that higher incubation temperatures lead to increased K values because of the formation of stronger gels compared with lower incubation temperatures in yogurt samples (
      • Skriver A.
      • Roemer H.
      • Qvist K.B.
      Rheological characterization of stirred yoghurt: viscometry.
      ;
      • Haque A.
      • Richardson R.K.
      • Morris E.R.
      Effect of fermentation temperature on the rheology of set and stirred yogurt.
      ;
      • Shaker R.R.
      • Abu-Jdayil B.
      • Jumah R.Y.
      • Ibrahim S.A.
      Rheological properties of set yogurt as influenced by incubation temperature and homogenization.
      ). Furthermore, a trend was reported for ayran that was in agreement with our observations (
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ) and, at higher temperatures, not only the alterations in protein network but also chemical and microbiological changes that might trigger alterations in protein network can explain the observed higher K values. The effect of incubation time on K values can be related to alterations in the protein network of ayran, as previously reported (
      • Ozdemir U.
      • Kilic M.
      Influence of fermentation conditions on rheological properties and serum separation of Ayran.
      ). It should be noted that water and ionic contents of ayran samples were found to be important for K and n values (
      • Köksoy A.
      • Kılıç M.
      Effects of water and salt level on rheological properties of Ayran, a Turkish yoghurt drink.
      ), but the effect of DM of ayran on these values was not clear in our study. The values of K and n were significantly affected by EPS production and generally the smallest changes in these values were detected for the control strain group. Furthermore, these values changed differently in other strain groups which in total suggest the enhancing role of EPS production in ayran rheology depending on EPS structure and production levels produced in situ during ayran fermentation (Table 4). Previous reports suggest that yogurt viscosity was strain dependent, in which the structure of EPS and its interactions with milk proteins and cultures are thought to be important in this process, which can also be the case in ayran (
      • Rawson H.L.
      • Marshall V.M.
      Effect of ‘ropy’ strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus on rheology of stirred yogurt.
      ;
      • Faber E.J.
      • van den Haak M.J.
      • Kamerling J.P.
      • Vliegenthart J.F.
      Structure of the exopolysaccharide produced by Streptococcus thermophilus S3.
      ;
      • Folkenberg D.M.
      • Dejmek P.
      • Skriver A.
      • Skov Guldager H.
      • Ipsen R.
      Sensory and rheological screening of exopolysaccharide producing strains of bacterial yoghurt cultures.
      ;
      • Purwandari U.
      • Shah N.P.
      • Vasiljevic T.
      Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt.
      ).
      Steady shear rheology parameters of ayran samples in all groups were significantly affected by incubation temperature and time (Table 4). Therefore, these parameters were modeled to determine the effect of incubation temperature and time. Table 4 shows the F-values of model parameters generated for K, n, and η50 values for ayran samples. To check the competence of the models, R2, adjusted R2, predicted R2, and adequate precision values were determined (Table 4). In these models, R2 values for η50 were calculated to be 0.9558, 0.9083, 0.8899, and 0.9731 for samples produced with strain 1, strain 2, control strain, and the mixture, respectively. Similarly, R2 values for K and n parameters ranged between 0.8012 and 0.9960 and between 0.7573 and 0.9910, respectively, depending on the strain tested. All parameters used in the models were necessary to construct the correct model, as can be seen by the closeness of the R2 and adjusted R2 values to 1. Similarly, the adequate precision values for each parameter, which reflects the signal to noise ratio, were in the desirable range (
      • Toker O.S.
      • Dogan M.
      • Ersöz N.B.
      • Yilmaz M.T.
      Optimization of the content of 5−hydroxymethylfurfural (HMF) formed in some molasses types: HPLC−DAD analysis to determine effect of different storage time and temperature levels.
      ).
      The effects of incubation temperature and time on K, n, and η50 of ayran samples are reported in Table 4. Results revealed that linear effects of both incubation temperature and time significantly (P < 0.01; P < 0.05) affected the model parameters for all strain conditions in ayran samples. Similarly, as can be seen in Table 4, several trends were observed for the effect of interaction of factors (temperature × time) on model parameters for samples. In Figure 3, these effects are illustrated as 3-dimensional response surfaces in which the direction of the effects of incubation temperature and time on these model parameters for all ayran strains can be seen. Furthermore, the second-order regression model equations predicting effects of incubation temperature and time are presented in Figure 3. These graphs show that the K values of ayran samples produced with EPS-producing strains increased notably with incubation temperature and time, and several trends were observed for different strain conditions for these factors. For instance, the quadratic effect of temperature was the most determining factor for K in the control group, showing a negative effect that indicates the presence of an optimum temperature for this value in the control sample. In contrast, the linear effect of incubation time was the most determining factor for K in samples produced with strains 1 and 2 and the mixture, respectively, and positively affected this value. Consistent with the observations in K values, n values of all samples decreased notably with incubation temperature and time (Figure 3). The effect of incubation time was the most important factor to explain the n values of ayran samples produced with control strain and strain 1, whereas incubation temperature had a more important effect on n value of samples produced with the mixture of strains. Both incubation temperature and time similarly affected the n value of ayran produced with strain 2. As can be seen in Figure 3, η50 values of all ayran samples increased with incubation temperature and time. Incubation temperature was the most effective factor in apparent viscosity of ayran samples produced with control strain and strain 1, respectively, whereas incubation time was the most effective variable for the apparent viscosity of samples produced with strain 2 and the mixture of strains, respectively (Figure 3). This value was negatively affected when these 2 factors interacted in ayran manufactured with strain 1 and with the quadratic effect of incubation period in ayran produced with strain 2 and the mixture of strains (Figure 3).
      Figure thumbnail gr3
      Figure 3Effect of temperature (T) and time (t) on K (consistency coefficient), η50 (apparent viscosity), and n (flow behavior index) values of ayran samples. Control = non-exopolysaccharide (EPS)-producing Streptococcus thermophilus strain; strains 1 and 2 = EPS-producing S. thermophilus strain; mixture = admixture of strains 1 and 2.
      To calculate the optimum incubation temperature and time on rheological parameters of ayran and EPS production levels and to determine the EPS yields resulting in maximum and minimum K, n, and η50 values, single-response (SRO) and multiple-response (MRO) optimization analyses were applied and the results are presented in Table 5. As can be seen from the SRO analysis, EPS yield was minimal at 32°C for 2.1 h incubation and final EPS yield was found to be maximal when the fermentation was conducted at 37.4°C for 3.9 h (Table 5). Additionally, the incubation parameters that resulted in minimum and maximum K, n, and η50 values were 32.1°C for 0.2 h, 41.1°C for 4 h, and 32°C for 2 h (minimum values) and 42°C for 4 h, 32°C for 2 h, and 42°C for 3.9 h (maximum values), respectively. Several trends were observed for optimal factor levels for SRO for EPS yield and rheological parameters and are given in Table 5. The optimal conditions for the rheological parameters that resulted in minimum and maximum EPS yields for different strain conditions were determined by using MRO analysis (Table 5). For instance, the minimum K value was recorded at 32.1°C for 0.2 h for the strain mixture; at the same temperature but with an increase of incubation time to 2.1 h, EPS yield was minimal for the strain mixture, and, at this condition, the value of K was nearly doubled, showing the crucial role of incubation temperature and EPS production on this parameter (Table 5). A similar trend was observed for the maximum EPS yield incubation conditions affecting the K value, showing the crucial role of both EPS yield and incubation conditions on physicochemical properties of ayran. In contrast to the strain mixture, the minimal EPS yields for strains 1 and 2 were obtained at 41.8°C for 2.7 h and 41.9°C for 2 h incubation, respectively, after SRO analysis but similar trends were observed for the rheological parameters except the n value for strain 1, showing the crucial role of incubation temperature and time on rheological parameters (Table 5). Addition of MRO analysis confirmed this role because at maximum EPS yield at 32°C for strain 1, the K value was 0.1 Pa·sn and at minimum EPS yield at 41.8°C, this value was 0.7 Pa·sn, which can be related to the stronger protein network at 41.8°C that resulted in higher K values. Table 6 gives Pearson correlation coefficients between rheological parameters and EPS yield for different strain conditions. At all strain conditions, a significant positive correlation was found between K and η50, as expected (Table 6). Interestingly, no significant positive correlation was observed between EPS yield and K value for strains 1 and 2, which can be can be explained by different gelling conditions. For the strain mixture, however, this correlation was positive. Similarly, a positive correlation between η50 and EPS yield was noted for all strain conditions except strain 1 (Table 6). Exopolysaccharide is the focus of much recent interest in the food industry and optimization studies should be applied to determine the complex interactions between EPS yield and fermentation kinetics. For such a product such as ayran, in which the use of gelling agents is not permitted, these studies are important.
      Table 5Single and multiple response optimization
      SRO=single response optimization analysis for calculation of optimum incubation temperature (T) and time (t) values, yielding minimum and maximum values for each response. MRO=multiple response optimization analysis for simultaneous calculation for values of rheological parameters based on T and t values, yielding (A) minimum and (B) maximum EPS amount values calculated by SRO. Desirability for each MRO ranged between 0.91 and 0.99.
      for calculation of maximum (Max) and minimum (Min) values for responses of ayran samples produced with exopolysaccharide (EPS)-producing strains 1 and 2 and their admixture
      Response
      EPS yield=amount of exopolysaccharide produced; K=consistency coefficient; η50=apparent viscosity; n=flow behavior index.
      Strain 1Strain 2Mixture
      Optimum factor levels for SROSROMROOptimum factor levels for SROSROMROOptimum factor levels for SROSROMRO
      T (°C)t (h)MinMaxABT (°C)t (h)MinMaxABT (°C)t (h)MinMaxAB
      EPS yield (mg/L)41.82.73.841.92.00.232.02.15.7
      32.22.314.536.04.013.737.43.929.4
      K (Pa·sn)36.12.30.10.735.52.50.10.732.10.20.40.7
      42.04.04.00.142.04.07.62.242.04.06.65.5
      n41.83.90.30.641.93.90.10.641.14.00.20.6
      42.02.00.80.836.62.00.70.532.02.00.60.2
      η50 (Pa·s)32.72.00.040.232.52.00.00.132.02.00.10.1
      42.04.00.30.040.23.70.20.242.03.90.30.3
      1 SRO = single response optimization analysis for calculation of optimum incubation temperature (T) and time (t) values, yielding minimum and maximum values for each response. MRO = multiple response optimization analysis for simultaneous calculation for values of rheological parameters based on T and t values, yielding (A) minimum and (B) maximum EPS amount values calculated by SRO. Desirability for each MRO ranged between 0.91 and 0.99.
      2 EPS yield = amount of exopolysaccharide produced; K = consistency coefficient; η50 = apparent viscosity; n = flow behavior index.
      Table 6Pearson correlation coefficients (r) between rheological parameters and amounts of exopolysaccharide (EPS) produced by different strains
      K=consistency coefficient, η50=apparent viscosity at 50 s−1, EPS=amount of EPS produced by different strains. K, η50, and EPS amount values were used to perform the Pearson correlation analysis.
      Parameter
      K=consistency coefficient, η50=apparent viscosity at 50 s−1, EPS=amount of EPS produced by different strains. K, η50, and EPS amount values were used to perform the Pearson correlation analysis.
      Ayran samples inoculated with different strains
      Control=non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2=EPS-producing S. thermophilus strain; mixture=admixture of strains 1 and 2.
      ControlStrain 1Strain 2Mixture
      Kη50EPSKη50EPSKη50EPSKη50EPS
      K1.0001.0001.0001.000
      η500.817
      P<0.05,
      1.0000.749
      P<0.05,
      1.0000.598
      P<0.05,
      1.0000.855
      P<0.01).
      1.000
      EPS
      Correlation analysis was not performed because control Ayran samples produced no EPS.
      1.000–0.384–0.4811.0000.2010.733
      P<0.05,
      1.0000.780
      P<0.01).
      0.628
      P<0.05,
      1.000
      1 K = consistency coefficient, η50 = apparent viscosity at 50 s−1, EPS = amount of EPS produced by different strains. K, η50, and EPS amount values were used to perform the Pearson correlation analysis.
      * P < 0.05,
      ** P < 0.01).
      2 Control = non-EPS-producing Streptococcus thermophilus strain; strains 1 and 2 = EPS-producing S. thermophilus strain; mixture = admixture of strains 1 and 2.
      3 Correlation analysis was not performed because control Ayran samples produced no EPS.
      Figures 4 and 5 show representative scanning electron micrographs of the ayran samples produced with different strains, which allowed analysis of the interactions of EPS with ayran components, especially proteins, and observation of the bacterial cells within the ayran microstructure. As can be seen in Figure 4A, B, and C, the ayran produced with the non-EPS-producing control strain had a compact appearance under different incubation conditions, consistent with that reported for other milk products (
      • Hassan A.N.
      • Frank J.F.
      • Elsoda M.
      Observation of bacterial exopolysaccharide in dairy products using cryo-scanning electron microscopy.
      ); this compact structure is related to the aggregation of proteins caused by the heat treatment process during buttermilk production (
      • Ayala-Hernandez I.
      • Goff H.D.
      • Corredig M.
      Interactions between milk proteins and exopolysaccharides produced by Lactococcus lactis observed by scanning electron microscopy.