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Low-fat akawi cheese made from bovine-camel milk blends: Rheological properties and microstructural characteristics

Open AccessPublished:April 01, 2022DOI:https://doi.org/10.3168/jds.2021-21367

      ABSTRACT

      Camel milk (CM) can be used as an ingredient to produce various dairy products but it forms weak rennet-induced and acid-induced gels compared with bovine milk (BM). Therefore, in this study, we aimed to investigate the effect of blending bovine milk with camel milk on the physicochemical, rheological (amplitude sweep and frequency sweep), and microstructural properties of low-fat akawi (LFA) cheese. The cheeses were made of BM only or BM blended with 15% (CM15%) or 30% (CM30%) camel milk and stored at 4°C for 28 d. The viscoelastic properties as a function of temperature were assessed. The LFA cheeses made from blended milks had higher moisture, total Ca, and soluble Ca contents, and had higher pH 4.6–water-soluble nitrogen compared with those made from BM. Analysis by scanning electron microscopy demonstrated that the microstructures formed in BM cheese were rough with granular surfaces, whereas those in blended milk cheeses had smooth surfaces. Hardness was lower for LFA cheeses made from blended milk than for those made from BM only. The LFA cheeses demonstrated viscoelastic behavior in a linear viscoelastic range from 0.1 to 1.0% strain. The storage modulus (G′) was lower in LFA cheese made from BM over a range of frequencies. Adding CM reduced the resistance of LFA cheeses to flow as temperature increased. Blended cheeses exhibited lower complex viscosity values than BM cheeses during temperature increases. Thus, the addition of camel milk improved the rheological properties of LFA cheese.

      Key words

      INTRODUCTION

      Camel milk (CM) contains proximate constituents comparable to those of bovine milk (BM), ranging from 2.7 to 4.8 g/100 g, 3.1 to 4.2 g/100 g, and 4. 4.8 g/100 g for protein, fat, and carbohydrate contents, respectively (
      • Hailu Y.
      • Hansen E.B.
      • Seifu E.
      • Eshetu M.
      • Ipsen R.
      • Kappeler S.
      Functional and technological properties of camel milk proteins: A review.
      ;
      • Mohamed H.
      • Nagy P.
      • Agbaba J.
      • Kamal-Eldin A.
      Use of near and mid infra-red spectroscopy for analysis of protein, fat, lactose and total solids in raw cow and camel milk.
      ). Unlike other nonbovine milks, which are often produced seasonally, CM is available throughout the year (
      • Al Haj O.A.
      • Al Kanhal H.A.
      Compositional, technological and nutritional aspects of dromedary camel milk.
      ). African, middle Asian (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan, and Mongolia), and Gulf Cooperation Council (GCC) countries are the main home for camels and camel milk production (
      • Swelum A.A.
      • El-Saadony M.T.
      • Abdo M.
      • Ombarak R.A.
      • Hussein E.O.S.
      • Suliman G.
      • Alhimaidi A.R.
      • Ammari A.A.
      • Ba-Awadh H.
      • Taha A.E.
      • El-Tarabily K.A.
      • Abd El-Hack M.E.
      Nutritional, antimicrobial and medicinal properties of camel's milk: A review.
      ). According to a recent assessment report by the Food and Agriculture Organization (FAO) of the United Nations, global annual camel milk production is around 3.1 million tonnes (
      • FAO (Food and Agriculture Organization of the United Nations)
      Value of agricultural production: Gross production value (current thousand us$): milk, whole fresh camel.
      ). Various health benefits are attributed to CM, including antihypertensive, antidiabetic, anticancer, and antiautism activities (
      • Solanki D.
      • Hati S.
      Fermented camel milk: A review on its bio-functional properties.
      ;
      • Swelum A.A.
      • El-Saadony M.T.
      • Abdo M.
      • Ombarak R.A.
      • Hussein E.O.S.
      • Suliman G.
      • Alhimaidi A.R.
      • Ammari A.A.
      • Ba-Awadh H.
      • Taha A.E.
      • El-Tarabily K.A.
      • Abd El-Hack M.E.
      Nutritional, antimicrobial and medicinal properties of camel's milk: A review.
      ). However, processing of CM into products such as set-style and stirred yogurt and cheese products is a major challenge (
      • Konuspayeva G.
      • Faye B.
      Recent advances in camel milk processing.
      ). Because of the large casein micelles, low κ-casein content, and high β-casein content in CM, acid- and rennet-induced CM gels are fragile (
      • Hailu Y.
      • Hansen E.B.
      • Seifu E.
      • Eshetu M.
      • Ipsen R.
      • Kappeler S.
      Functional and technological properties of camel milk proteins: A review.
      ). There are a few reports of making cheese from CM (
      • Mbye M.
      • Sobti B.
      • Al Nuami M.K.
      • Al Shamsi Y.
      • Al Khateri L.
      • Al Saedi R.
      • Saeed M.
      • Ramachandran T.
      • Hamed F.
      • Kamal-Eldin A.
      Physicochemical properties, sensory quality, and coagulation behavior of camel versus bovine milk soft unripened cheeses.
      ), but the resulting cheeses had a labneh-like structure (
      • Al-Zoreky N.S.
      • Almathen F.S.
      Using recombinant camel chymosin to make white soft cheese from camel milk.
      ). This particular issue makes CM unattractive to the large sector of dairy yogurt and cheese manufacturers, despite consumer interest based on the associated health benefits of CM.
      The weak gel formation of CM needs to be investigated so that CM can be included in gelled products and to allow the functional characteristics of current camel milk dairy products, such as cheese, to be altered. This could enable CM to be used as an ingredient in dairy products (
      • Elbarbary H.A.
      • Saad M.A.
      Improvement of the quality of buffalo's milk soft cheese by camel's whey protein concentrate.
      ), which would create significant economic value for CM farms and industry. Researchers have reported mixing CM with various hydrocolloids or with BM to produce set yogurt with satisfactory properties (
      • Kamal-Eldin A.
      • Alhammadi A.
      • Gharsallaoui A.
      • Hamed F.
      • Ghnimi S.
      Physicochemical, rheological, and micro-structural properties of yogurts produced from mixtures of camel and bovine milks.
      ;
      • Sobti B.
      • Mbye M.
      • Alketbi H.
      • Alnaqbi A.
      • Alshamisi A.
      • Almeheiri M.
      • Seraidy H.
      • Ramachandran T.
      • Hamed F.
      • Kamal-Eldin A.
      Rheological characteristics and consumer acceptance of camel milk yogurts as affected by bovine proteins and hydrocolloids.
      ). Blending CM with BM could also be considered for cheese making, particularly in markets where camel milk is popular. In these markets, akawi cheese is also a popular product. Akawi cheese (full- or low-fat) is a white brined cheese that is largely consumed in North Africa, Middle Eastern, and GCC countries and is well known for its use in making a sweet called kunafah (
      • Al-Dhaheri A.S.
      • Al-Hemeiri R.
      • Kizhakkayil J.
      • Al-Nabulsi A.
      • Abushelaibi A.
      • Shah N.P.
      • Ayyash M.
      Health-promoting benefits of low-fat akawi cheese made by exopolysaccharide-producing probiotic Lactobacillus plantarum isolated from camel milk.
      ). Kunafah should be produced using an unripened rennet-coagulated cheese; for example, low-fat akawi (LFA) cheese, with good stretching properties especially at high cooking temperatures. Low-fat akawi cheese has a greater market than other white cheeses in the above-mentioned countries because of its low fat content and greater elasticity compared with its full-fat counterpart (
      • Al-Dhaheri A.S.
      • Al-Hemeiri R.
      • Kizhakkayil J.
      • Al-Nabulsi A.
      • Abushelaibi A.
      • Shah N.P.
      • Ayyash M.
      Health-promoting benefits of low-fat akawi cheese made by exopolysaccharide-producing probiotic Lactobacillus plantarum isolated from camel milk.
      ). In general, low-fat cheeses are often preferable because of the adverse health effects of fat. Therefore, it is worth investigating the effects of incorporating CM into BM on the functional properties of LFA cheese. The objective of this study was to investigate the rheological properties (amplitude sweep, viscoelastic, and thermal behavior), texture profile, and microstructural characteristics of LFA cheeses made of blended bovine-camel milk at different ratios and stored under vacuum packaging at 4°C for 28 d.

      MATERIALS AND METHODS

      Low-Fat Akawi Cheese Making

      Low-fat pasteurized CM (2.7% protein, 1.0% fat, and 4.2% carbohydrates) and BM (3.2% protein, 1.0% fat, and 4.6% carbohydrates) were obtained from a local manufacturer. Initially, BM and CM blends (with CM from 0 to 50%) were prepared and evaluated by renneting using camel-chymosin FAR-M (Chr. Hansen Holding A/S) to determine the maximum CM ratio for the experimental design. A proportion of 30% CM in the blend provided acceptable curd firmness, close to that of BM. Therefore, LFA cheeses were made from BM alone and from blends of BM with 15% CM (CM15%) and with 30% CM (CM30%). The cheesemaking procedure followed was described by
      • Ayyash M.
      • Abdalla A.
      • Alameri M.
      • Baig M.A.
      • Kizhakkayil J.
      • Chen G.
      • Huppertz T.
      • Kamal-Eldin A.
      Biological activities of the bioaccessible compounds after in vitro digestion of low-fat akawi cheese made from blends of bovine and camel milk.
      . Briefly, low-fat milk (12 L) was pasteurized at 73°C for 15 s and tempered at 40°C for 30 min in a 13-L temperature-controlled cheese vat. A CH-1 starter culture, consisting of Lactobacillus bulgaricus and Streptococcus thermophilus (Chr. Hansen Holding A/S), was added (0.3% wt/wt) followed by incubation at 40°C for 60 min. Subsequently, chymosin (Chr. Hansen Holding A/S) was added [60 international milk clotting units (IMCU)/L] and curd was allowed to be formed for 45 min at 40°C. The curd was subsequently cut into ∼1-cm3 cubes, followed by stirring for 20 min at 40°C. After subsequent whey drainage, the curd was transferred to cheesecloth and molded into ∼250-g blocks followed by pressing (35 g/cm3) for 90 min at room temperature. Cheese blocks were brined in 7% NaCl solution at 4°C overnight. The brined blocks were dried, vacuum-packaged, and stored at 4°C for 28 d. The samples were taken for analysis on d 1 and 28. The LFA cheese was made twice (2 experimental units) and sampled in triplicate (3 blocks).

      Chemical Composition

      The moisture content was determined by the oven-drying method at 105°C, ash content by muffle furnace method, fat content by Gerber method, and protein content by the Kjeldahl method, according to
      • AOAC International
      Official Methods of Analysis.
      . For pH measurement, grated cheese (25 g) was homogenized with 25 mL of distilled water, and the pH was measured using a digital pH meter Stater3100 (Ohaus Corp.).

      Proteolysis Assessment

      Water-soluble extracts (WSE) from the cheese samples were prepared according to
      • Kuchroo C.
      • Fox P.
      Fractionation of the water-soluble-nitrogen from cheddar cheese: Chemical methods.
      by homogenizing grated cheese with deionized, distilled water at ratio of 1:2. The slurries were centrifuged at 6,000 × g for 15 min at 4°C. The nitrogen content of the WSE; that is, the water-soluble nitrogen (WSN), was assessed using the Kjeldahl method (
      • AOAC International
      Official Methods of Analysis.
      ) and expressed as a percentage of total nitrogen (TN). The peptide profile of the WSE was assessed by HPLC according to the method previously described by
      • Ayyash M.M.
      • Shah N.P.
      Proteolysis of low-moisture mozzarella cheese as affected by substitution of NaCl with KCl.

      Total and Soluble Calcium

      The total and soluble Ca contents were assessed according to
      • Metzger L.E.
      • Barbano D.M.
      • Kindstedt P.S.
      Effect of milk preacidification on low fat mozzarella cheese: Iii. Post-melt chewiness and whiteness.
      . The total Ca content was assessed after cheese samples were ashed and mixed with acid solution. The soluble Ca content was assessed in cheese extract prepared by homogenizing 5 g of cheese with 50 mL of deionized, distilled H2O at 60°C for 30 s. The Ca contents in whole cheese and the soluble phase were analyzed using inductively coupled plasma-optical emission spectrometry.

      Texture Profile Analysis

      Cylindrical samples (25 mm diameter × 20 mm height) were cut, in duplicate, from the center of LFA cheese blocks. Texture profile analysis on these samples was performed according to
      • Ayyash M.
      • Abu-Jdayil B.
      • Hamed F.
      • Shaker R.
      Rheological, textural, microstructural and sensory impact of exopolysaccharide-producing Lactobacillus plantarum isolated from camel milk on low-fat akawi cheese.
      . Hardness (the force necessary to attain a given deformation), adhesiveness (the work necessary to pull the compressing plunger away from the sample), cohesiveness (a measurement of how well the structure of the cheese withstands compression), springiness (the return rate of deformed cheese to its original shape), gumminess (the energy required to disintegrate cheese to a state ready for swallowing), and chewiness (the energy needed to chew the cheese) of all cheese specimens were tested on the same day of sampling using a Texture Analyzer CT3 (Brookfield Ametek).

      Microstructure by Scanning Electron Microscopy

      The microstructure of cheese samples was studied by scanning electron microscopy according to
      • Ayyash M.
      • Abu-Jdayil B.
      • Hamed F.
      • Shaker R.
      Rheological, textural, microstructural and sensory impact of exopolysaccharide-producing Lactobacillus plantarum isolated from camel milk on low-fat akawi cheese.
      . Small pieces of cheese were fixed on an aluminum holder and coated with a thin layer of gold using a Cressington 108 Auto Sputter Coater (Ted Pella Inc.). The analysis of the gold-coated cheese samples was conducted using a JEOL JSM–6010LA scanning electron microscope (JEOL) operating at an accelerating voltage of 20 kV. The scanning electron micrographs were collected in secondary electron imaging mode and images were recorded at various magnifications.

      Rheological Properties

      Rheological analyses of cheese samples were carried out according to
      • Ayyash M.
      • Abu-Jdayil B.
      • Hamed F.
      • Shaker R.
      Rheological, textural, microstructural and sensory impact of exopolysaccharide-producing Lactobacillus plantarum isolated from camel milk on low-fat akawi cheese.
      . Briefly, samples were cut from at least 3 mm deep into the cheese blocks. These samples were immediately placed in small airtight plastic containers and equilibrated at room temperature (25 ± 1°C) for at least 20 min. Small oscillatory amplitude measurements were performed with a Discovery Hybrid Rheometer HR-2 (TA Instruments). The measuring geometry consisted of 2 parallel plates with a diameter of 40 mm operating at a 2.6-mm gap size (sample thickness). Excessive cheese was trimmed carefully, and the sample was allowed to rest for 60 s on the rheometer to allow the stresses induced during sample handling to relax. All rheological properties were measured in duplicate.
      The linear viscoelastic range was determined by performing a strain sweep at a frequency of 1.0 Hz, with strain values ranging from 0.1 to 10%. A strain in the linear region (0.1–1%) was then selected for a frequency sweep test, where strain was set at 0.5% and frequency was varied from 0.1 to 20 Hz at 25°C. The dynamic parameters storage modulus (elastic component; G′), loss modulus (viscous component; G″), and loss tangent (tan δ) were documented. The rheological properties of the LFA cheeses as a function of temperature were determined according to
      • Guinee T.P.
      • Feeney E.P.
      • Auty M.A.E.
      • Fox P.F.
      Effect of pH and calcium concentration on some textural and functional properties of mozzarella cheese.
      . The cheese samples were heated from 20 to 85°C at a heating rate of 3°C/min with a strain of 0.5% and a frequency of 1.0 Hz. The dynamic parameters G′, G″, and tan δ, and the complex viscosity (η*) were recorded. Activation energy (Ea) was calculated by determining the slope of the plot of ln η* versus 1/T using the following equation: η* = A exp(−Ea/RT), where A is a pre-exponential factor, R is a universal gas constant, and T is absolute temperature (in Kelvin).

      Statistical Analysis

      One-way ANOVA was carried out to investigate the effect of cheese type on cheese parameters at the same storage time. Within the same cheese type, one-way ANOVA was carried out to investigate the effect of storage period on cheese parameters. Means comparisons at the same storage time or within cheese type were performed using Tukey's test. All statistical analyses were carried out using Minitab 20.0 software (Minitab Inc.).

      RESULTS AND DISCUSSION

      Chemical Composition of the Experimental Cheeses

      Compositional properties of LFA cheese made from BM or CM-BM blends are shown in Table 1. Cheese made from blended milk (CM15% and CM30%) had significantly (P < 0.05) lower fat, fat in dry matter (FDM), and protein content compared with BM cheese. Dry matter content was numerically but not significantly lower and moisture content higher for CM15% and CM30% compared with BM cheese. Ash, total Ca, and soluble Ca contents were significantly (P < 0.05) higher in CM15% and CM30% than in BM cheese. We found no difference in NaCl content (P > 0.05) among samples. The higher moisture contents in cheeses made from blended milk may be due to the greater water-holding capacity of CM proteins compared with BM (
      • Mbye M.
      • Sobti B.
      • Al Nuami M.K.
      • Al Shamsi Y.
      • Al Khateri L.
      • Al Saedi R.
      • Saeed M.
      • Ramachandran T.
      • Hamed F.
      • Kamal-Eldin A.
      Physicochemical properties, sensory quality, and coagulation behavior of camel versus bovine milk soft unripened cheeses.
      ).
      • Shahein M.
      • Hassanein A.
      • Zayan A.
      Evaluation of soft cheese manufactured from camel and buffalo milk.
      also reported higher moisture content in cheese made from blended camel and buffalo milk compared with the control cheese (made only from buffalo milk).
      Table 1Composition (g/100 g of cheese unless otherwise noted) and properties of low-fat akawi cheese made from bovine milk (BM) or bovine milk blended with 15% (CM15%) or 30% (CM30%) camel milk at specific storage times (d)
      Values are mean ± SD of n = 6.
      ParameterStorage time (d)Low-fat akawi cheese
      BMCM15%CM30%
      Moisture159.7 ± 1.00
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      61.9 ± 2.50
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      62.3 ± 2.73
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      Protein124.8 ± 1.20
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      23.3 ± 0.69
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      23.7 ± 1.00
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      Fat112.2 ± 0.62
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      9.8 ± 1.60
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      9.9 ± 1.07
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      DM140.3 ± 1.00
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      38.1 ± 2.50
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      37.6 ± 2.73
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      Fat in DM130.3 ± 1.77
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      25.6 ± 2.60
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      26.4 ± 1.06
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      Ash content13.3 ± 0.41
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      3.9 ± 0.24
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      3.9 ± 1.17
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      Salt content13.9 ± 0.20
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      3.73 ± 0.31
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      3.7 ± 0.27
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      Total Ca (mg/100 g)1520.0 ± 9.44
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      577.1 ± 7.06
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      575.1 ± 9.78
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      Soluble Ca (mg/100 g)178.2 ± 1.23
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      88.1 ± 2.11
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      90.2 ± 1.43
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      2896.2 ± 0.99
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      113 ± 1.87
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      124 ± 2.34
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      pH16.26 ± 0.48
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      6.19 ± 0.67
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      6.27 ± 0.38
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      286.01 ± 1.10
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      5.90 ± 1.27
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      5.91 ± 1.09
      Means in same row with different lowercase letters differ significantly (P < 0.05).
      a,b Means in same row with different lowercase letters differ significantly (P < 0.05).
      1 Values are mean ± SD of n = 6.
      The significantly (P < 0.05) higher total Ca content in cheese made from blended milk compared with BM only (Table 1) may be attributed to the higher Ca content in CM compared with BM (
      • Hailu Y.
      • Hansen E.B.
      • Seifu E.
      • Eshetu M.
      • Ipsen R.
      • Kappeler S.
      Functional and technological properties of camel milk proteins: A review.
      ). Moreover, the nature of CM casein micelles might increase Ca retention in curd after whey drainage. The pH at whey drainage has a crucial effect on Ca distribution between whey and curd (Table 1). In contrast, the soluble Ca contents in cheese made from blended milk were higher (P < 0.05) than those in cheese from BM at d 1 and 28 (Table 1). Soluble Ca increased (P < 0.05) during storage, possibly due to the decline in pH (Table 1;
      • Lucey J.A.
      • Fox P.F.
      Importance of calcium and phosphate in cheese manufacture—A review.
      ). In addition to soluble Ca, differences were also apparent in colloidal Ca, which can be calculated as the difference between total and soluble Ca. When considering data from Table 1, colloidal Ca was notably higher in LFA cheese made from blended milk than that from BM. During storage, colloidal Ca levels decreased somewhat but the differences between cheese samples remained.
      On d 1, the pH values of the cheeses did not differ significantly, but after 28 d of storage, the pH values of the CM15% and CM30% cheeses were significantly lower (P < 0.05) than those of BM cheese (Table 1). This reduction in pH during storage could be due to bacterial activity (∼8.0 log cfu/g) in cheese during storage (
      • Ayyash M.
      • Abdalla A.
      • Alameri M.
      • Baig M.A.
      • Kizhakkayil J.
      • Chen G.
      • Huppertz T.
      • Kamal-Eldin A.
      Biological activities of the bioaccessible compounds after in vitro digestion of low-fat akawi cheese made from blends of bovine and camel milk.
      ). The greater reduction in pH observed in cheeses made from blended milk is contrary to the higher buffering capacity of CM and its slower acidification during yogurt fermentation (
      • Attia H.
      • Kherouatou N.
      • Dhouib A.
      Dromedary milk lactic acid fermentation: Microbiological and rheological characteristics.
      ). This observation requires further investigation. The cheese yield was around 11% (∼1,100 g) with no significant difference between the cheese trials.

      Proteolysis Assessment by WSN and Peptide Profile

      The WSN content and peptide profile of the WSE of BM, CM15%, and CM30% LFA cheeses at d 1 and 28 of storage are presented in Figure 1. On d 1 and 28, the cheeses made from blended milk had higher (P < 0.05) WSN compared with cheese made from BM. Although changes in moisture content were insignificant, the slightly higher moisture contents in cheeses made from blended milk may contribute to higher WSN in these cheeses (Table 1). In contrast, the higher WSN% observed in mature cheeses made from blended milk could be related to the primary protein hydrolysis initiated by the action of chymosin residues (
      • Ardö Y.
      • McSweeney P.L.H.
      • Magboul A.A.A.
      • Upadhyay V.K.
      • Fox P.F.
      Biochemistry of cheese ripening: Proteolysis.
      ). This implies that CM proteins may be more susceptible to proteolytic breakdown than BM proteins, possibly because of the differences in casein composition and conformation.
      Figure thumbnail gr1
      Figure 1Water-soluble nitrogen (WSN) expressed as a percent of total nitrogen (TN) (a) and peptide profile of water-soluble extract at d 1 (b) and d 28 (c) of storage of low-fat akawi cheeses made from bovine milk (BM) or bovine milk blended with 15% (CM15%) or 30% (CM30%) camel milk. Error bars represent SD.
      Differences were also observed in the peptide profiles of LFA cheese made from blended milk and BM (Figure 1). The peptide profile of fresh cheese (d 1) showed prominent hydrophobic peptides eluting late (73–75 min) in CM15% and CM30% cheeses that were absent in BM cheese (Figure 1b). As cheese ages, caseins and polypeptides are hydrolyzed into smaller peptides (
      • McSweeney P.L.H.
      Biochemistry of cheese ripening: Introduction and overview.
      ). After 28 d of storage, changes in the peptide profile were noted at low retention times, indicating changes in hydrophilic peptide contents (Figure 1c). The most notable peaks occurred at retention times of 25 to 30 min and 35 to 40 min, which could represent degradation products of caseins (
      • Baptista D.P.
      • Gigante M.L.
      Bioactive peptides in ripened cheeses: Release during technological processes and resistance to the gastrointestinal tract.
      ). These products were less pronounced in CM15% and CM30% cheeses (Figure 1c). We postulate that proteolytic enzymes acted differently on CM than on BM caseins. The proteolytic activities of these enzymes on CM caseins require further investigation.

      Cheese Microstructure

      Scanning electron micrographs displaying the microstructure of the cheeses at d 1 and 28 are shown in Figure 2. Variations in cheese microstructure between BM cheese and cheese made from CM-BM blends were clearly visible at d 1 and 28 (Figure 2). At d 1, the microstructure of BM cheese was rough with a granular surface, whereas that of the CM15% and CM30% cheeses exhibited a smooth surface (Figure 2). At d 28, BM cheese had a thick network of aggregated caseins and narrower pores, most likely due to the higher protein fraction, whereas pores were absent in the structures of CM15% and CM30% cheeses (Figure 2). These findings suggest that addition of CM altered the microstructural properties of LFA cheese made from blended milk, in agreement with
      • Mbye M.
      • Sobti B.
      • Al Nuami M.K.
      • Al Shamsi Y.
      • Al Khateri L.
      • Al Saedi R.
      • Saeed M.
      • Ramachandran T.
      • Hamed F.
      • Kamal-Eldin A.
      Physicochemical properties, sensory quality, and coagulation behavior of camel versus bovine milk soft unripened cheeses.
      . Therefore, these observations need further investigation to evaluate and determine the relevant factors.
      Figure thumbnail gr2
      Figure 2Scanning electron microscopy images of low-fat akawi cheeses made from bovine milk (BM) or bovine milk blended with 15% (CM15%) or 30% (CM30%) camel milk at d 1 and d 28 of storage. Scale bar = 10 μm.

      Texture Profile Analysis

      Texture profile analysis results for LFA cheeses made from BM and from BM-CM blends at d 1 and 28 are shown in Table 2. Analysis of variance indicated significant (P < 0.05) differences in texture characteristics between the experimental cheeses at both storage times (Table 2). At d 1, fresh cheeses made from blended milk had significantly (P < 0.05) lower hardness values compared with BM cheese, which may be explained by the weak protein network of camel milk (Table 1), WSN% (Figure 1), and level of proteolysis in LFA (Figure 1) in cheeses made from blended milk, despite the fact that colloidal Ca was higher in the LFA cheese from blended milk (Table 1). Negative correlations between hardness and moisture content, and hardness and proteolysis rate, have been well documented (
      • Lucey J.A.
      • Johnson M.
      • Horne D.
      Invited review: Perspectives on the basis of the rheology and texture properties of cheese.
      ;
      • Ardö Y.
      • McSweeney P.L.H.
      • Magboul A.A.A.
      • Upadhyay V.K.
      • Fox P.F.
      Biochemistry of cheese ripening: Proteolysis.
      ). After 28 d of storage, the hardness of all LFA cheeses had decreased significantly (P < 0.05), although the CM15% and CM30% cheeses had lower (P < 0.05) hardness, cohesiveness, and chewiness than the BM cheese. This change may be attributed to proteolysis (Figure 1) and solubilization of Ca (Table 1) during storage.
      Table 2Texture profile analysis of low-fat akawi cheese made from bovine milk (BM) or bovine milk blended with 15% (CM15%) or 30% (CM30%) camel milk at d 1 and 28
      Values are mean (n = 6) ± SD.
      ParameterStorage time (d)BMCM15%CM30%
      Hardness (kg)13.31 ± 0.89
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.95 ± 0.70
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.92 ± 0.12
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      283.07 ± 0.67
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.70 ± 0.20
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.62 ± 0.14
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Cohesiveness10.75 ± 0.04
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      0.75 ± 0.03
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      0.74 ± 0.05
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      280.83 ± 0.06
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      0.82 ± 0.01
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      0.80 ± 0.02
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Springiness (mm)18.69 ± 0.22
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      8.45 ± 0.26
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      9.02 ± 0.81
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      286.40 ± 0.40
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      6.23 ± 0.09
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      6.14 ± 0.26
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Gumminess (kg)12.46 ± 0.60
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.60 ± 0.19
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.16 ± 0.20
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      282.05 ± 0.39
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.25 ± 0.61
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      2.00 ± 0.74
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Chewiness (mJ)1253.16 ± 13.36
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      234.79 ± 18.74
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      263.40 ± 20.11
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      28193.46 ± 5.28
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      135.51 ± 3.63
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      90.19 ± 6.82
      Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      a,b Means in a column within a textural parameter with different lowercase letters differ significantly (P < 0.05).
      A–C Means in a row with different uppercase letters differ significantly (P < 0.05).
      1 Values are mean (n = 6) ± SD.
      Milks with smaller micelles (e.g., BM) have been found to form a more compact and therefore firmer gel network than milks with larger micelles (e.g., CM;
      • Li Q.
      • Zhao Z.
      Acid and rennet-induced coagulation behavior of casein micelles with modified structure.
      ). Hence, the larger camel micelles presumably disrupt the continuity of the bovine-dominant para-casein network, creating weak points in the matrix and thereby reducing firmness. Also, the fewer protein rearrangements of the gel network could be caused by differences in casein micelle size (average 260–300 nm in CM compared with 100–140 nm in BM;
      • Hailu Y.
      • Hansen E.B.
      • Seifu E.
      • Eshetu M.
      • Ipsen R.
      • Kappeler S.
      Functional and technological properties of camel milk proteins: A review.
      ), leading to cheeses with high moisture contents and reduced firmness (
      • Lucey J.A.
      • Johnson M.
      • Horne D.
      Invited review: Perspectives on the basis of the rheology and texture properties of cheese.
      ). The results of the present study show that addition of CM significantly affected the texture properties of LFA cheese made from blended milk. However, whether and how CM and BM micelles actually interact and fuse to form a coherent matrix remains unclear and requires additional investigation of pertinent factors.

      Rheological Properties

      Linear Viscoelastic Region (Amplitude Sweep)

      Figure 3 (a–f) displays the strain sweep test results for the experimental cheeses on d 1 and 28. As can be seen, LFA cheeses retained their ability to dissipate applied energy up to ∼1.0% strain. Because the LFA cheeses exhibited viscoelastic behavior in a linear viscoelastic range from 0.1 to 1.0% strain, a strain of 0.5% was selected for subsequent rheological tests. The differences in rheological properties among cheeses were obvious at d 1 and 28. For all cheeses, G′ > G″, in agreement with
      • Ayyash M.
      • Abu-Jdayil B.
      • Hamed F.
      • Shaker R.
      Rheological, textural, microstructural and sensory impact of exopolysaccharide-producing Lactobacillus plantarum isolated from camel milk on low-fat akawi cheese.
      and
      • Raphaelides S.
      • Antoniou K.
      • Vasilliadou S.
      • Georgaki C.
      • Gravanis A.
      Ripening effects on the rheological behaviour of halloumi cheese.
      , who reported elastic structures for halloumi and akawi cheeses, respectively.
      Figure thumbnail gr3
      Figure 3Linear viscoelastic region (amplitude sweep) of low-fat akawi cheeses: bovine milk only at d 1 (a) and d 28 (b); blend of bovine milk and 15% camel milk (CM15%) at d 1 (c) and d 28 (d); and blend of bovine milk and 30% camel milk (CM30%) at d 1 (e) and d 28 (f). Storage modulus (G′) is shown by blue triangles and loss modulus (G″) by green squares.

      Viscoelastic Properties (Frequency Sweep)

      The frequency sweep test is a quantitative technique for examining and evaluating the viscoelastic features of cheeses at different frequencies (
      • Tunick M.H.
      Activation energy measurements in rheological analysis of cheese.
      ). Figure 4 shows that all the experimental cheeses had G′ > G″ within the tested frequency range (0.1–20 Hz) at d 1 and 28 (Figure 4a–d). At d 1, the G′ and G″ values of the CM15% and CM30% cheeses were lower than those of the BM cheese, which could be attributed to the potential disruptive effect of camel micelles on the bovine matrix, resulting in a cheese with a weak texture, poor protein network, and slightly higher moisture content. At d 28, the differences among the experimental cheeses were more pronounced, with CM15% having lower G′ and G″ values than BM, followed by CM30% (Figure 4b, 4d, 4f). This pattern may be due to higher proteolysis rates during storage in LFA cheeses made from blended milk (Figure 1a). The increase in proteolysis could decrease the network strength between the proteins, affecting the rheological properties (
      • Fox P.F.
      • Guinee T.P.
      • Cogan T.M.
      • McSweeney P.L.
      Cheese: Structure, rheology and texture.
      ). Moreover, reduction in colloidal Ca (increase in soluble Ca; Table 1) during storage also reduces the cross-linkages between the caseins and affects the rheological properties of the cheese (
      • Lucey J.A.
      • Johnson M.
      • Horne D.
      Invited review: Perspectives on the basis of the rheology and texture properties of cheese.
      ).
      Figure thumbnail gr4
      Figure 4Viscoelastic properties (frequency sweep) of low-fat akawi cheeses: storage modulus (G′) at d 1 (a) and d 28 (b); loss modulus (G″) at d 1 (c) and d 28 (d); and loss tangent (tan δ) at d 1 (e) and d 28 (f). Blue = cheeses made from bovine milk; green = cheeses made from a blend of bovine milk and 15% camel milk (CM15%); and red = cheeses made from a blend of bovine milk and 30% camel milk (CM30%).
      Cheese softening, as represented by tan δ values, occurred at a greater frequency on d 1 than on d 28 (Figure 4e and f), implying that LFA cheeses became less resistant to structure transition after 28 d of storage. The increase in proteolysis rate (Figure 1a) and soluble Ca during storage could explain the lower resistance to structural transition in LFA cheeses. A similar trend was noted at d 28; however, the differences in tan δ values among experimental cheeses were distinct, with CM15% and BM cheeses having higher tan δ values relative to CM30% cheese (Figure 4f). The strength and number of bonds between casein particles, as well as the particle composition and distribution pattern of the strands that make up the particles, determine G′, G″, and tan δ (
      • Tunick M.H.
      • Van Hekken D.L.
      Rheology and texture of commercial queso fresco cheeses made from raw and pasteurized milk.
      ). This could be due to the larger camel micelles presumably disrupting the continuity of the para-CN network, creating weak points in the matrix, and thereby reducing firmness. In addition, fewer protein rearrangements in the gel network caused by differences in casein micelle sizes and composition could result in less firm cheese (
      • Lucey J.A.
      • Johnson M.
      • Horne D.
      Invited review: Perspectives on the basis of the rheology and texture properties of cheese.
      ).

      Temperature Sweep

      Changes in G′, G″, and tan δ during the temperature sweep test (25–85°C) at d 1 and 28 are shown in Figure 5 (a–f). At d 1, the experimental cheeses showed similar reductions in G′ as the temperature increased from 25 to 65°C; however, as the temperature increased from 65 to 85°C, G′ values decreased in BM and CM15% cheeses but increased in CM30% (Figure 5a). This increase in CM30% may suggest structural reformation at temperatures >70°C. This observation requires further investigation to understand the mechanism of effect of CM on the protein network in cheese. Presumably because of proteolysis (Figure 1) and solubilization of Ca after 28 d of storage (Table 1), the G′ and G″ values for CM15% and CM30% cheeses were lower than those for BM cheese (Figure 5c and 5d). Of note, the CM30% cheese showed a different pattern at d 28 than at d 1 (Figure 5a). Our results agree with
      • Kern C.
      • Weiss J.
      • Hinrichs J.
      Additive layer manufacturing of semi-hard model cheese: Effect of calcium levels on thermo-rheological properties and shear behavior.
      , who reported that a cheese model with a low Ca content had lower G′ and G″ (as a function of temperature, 20–80°C) than cheese with a high Ca content. Although many factors that change during storage (proteolysis, Ca equilibrium, casein arrangements, moisture content) may affect the viscoelastic properties, this finding requires further investigation to understand the interactions and network arrangements between cheese caseins from different sources.
      Figure thumbnail gr5
      Figure 5Thermal viscoelastic properties (temperature sweep) of low-fat akawi cheeses: storage modulus (G′) at d 1 (a) and d 28 (b); loss modulus (G″) at d 1 (c) and d 28 (d); and loss tangent (tan δ) at d 1 (e) and d 28 (f). Blue = cheeses made from bovine milk; green = cheeses made from a blend of bovine milk and 15% camel milk (CM15%); and red = cheeses made from a blend of bovine milk and 30% camel milk (CM30%).
      The loss tangent (tan δ), as a function of temperature, indicates the gel-sol transition in cheese. All the LFA cheeses were primarily elastic-like (tan δ maximum <0.8) to varying degrees (Figure 5e and 5f). The tan δ values started increasing when temperature exceeded 40°C and reached a maximum at 60 to 70°C, followed by a noticeable decline (Figure 5e and 5f). This result concurs with results of
      • Guinee T.P.
      • Feeney E.P.
      • Auty M.A.E.
      • Fox P.F.
      Effect of pH and calcium concentration on some textural and functional properties of mozzarella cheese.
      and
      • Kern C.
      • Weiss J.
      • Hinrichs J.
      Additive layer manufacturing of semi-hard model cheese: Effect of calcium levels on thermo-rheological properties and shear behavior.
      for mozzarella and model cheeses, respectively. The initial increase in tan δ from 25°C to 45°C may be attributed to the melting of fat (
      • Guinee T.P.
      • Feeney E.P.
      • Auty M.A.E.
      • Fox P.F.
      Effect of pH and calcium concentration on some textural and functional properties of mozzarella cheese.
      ). At d 1, BM cheese reached a maximum tan δ value at ∼65°C, which is higher than the temperature at maximum tan δ for cheeses made from blended milk (Figure 5e). This indicates that adding CM reduced the resistance of LFA cheeses to flow with increasing temperature. The large casein micelle size, low κ-casein content, and high β-casein content in CM (
      • Hailu Y.
      • Hansen E.B.
      • Seifu E.
      • Eshetu M.
      • Ipsen R.
      • Kappeler S.
      Functional and technological properties of camel milk proteins: A review.
      ) might weaken the protein network in CM15% and CM30% cheeses. The difference in transition temperatures became more pronounced between the BM cheese (lower) and other experimental cheeses after 28 d of storage (Figure 5f), which may be attributed to the higher proteolysis rate (Figure 1A) and higher soluble Ca (Table 1) in LFA cheeses made from blended milk during storage. These factors could weaken the protein network in cheese and reduce the gel-sol transition temperature. However, the CM15% and CM30% cheeses had higher tan δ values than the BM cheese, which implies that LFA cheese made from blended milk had greater meltability and flowability than LFA cheese made from BM only. This may be caused by the weakened protein network in LFA cheeses made from blended milk. Weak gel formation of camel milk has been reported. Moreover, we assume that the absence of β-lactoglobulin and low κ-casein in camel milk (
      • Al Haj O.A.
      • Al Kanhal H.A.
      Compositional, technological and nutritional aspects of dromedary camel milk.
      ) could play a role in weakening the protein network in cheese made from blended milk.
      The complex viscosity (η*) is the resistance to flow under oscillatory shear conditions, calculated as the complex modulus G* (total energy required to deform a sample) divided by frequency (
      • Tunick M.H.
      Activation energy measurements in rheological analysis of cheese.
      ). At d 1 and 28, all experimental cheeses showed a decrease in η* with heating (Figure 6a and 6b). At d 1, all experimental cheeses showed similar η* values, whereas at d 28, the differences among experimental cheeses were more obvious, with the BM cheese exhibiting higher η* values than CM15% and CM30% cheeses (Figure 6b). This finding concurs with our results for other rheological parameters (i.e., G′, G″, and tan δ; Figure 4, Figure 5). The determination of activation energy (Ea) of flow is an important indicator of the degree to which a cheese sample melts under heating (
      • Tunick M.H.
      Activation energy measurements in rheological analysis of cheese.
      ). Cheese with an Ea value <60 kJ/mol does not flow, whereas Ea values from 100 to 150 kJ/mol indicate that a cheese will flow, and Ea values >150 kJ/mol indicate rapid collapse of the protein matrix as the energy barrier is breached by heating (
      • Tunick M.H.
      Activation energy measurements in rheological analysis of cheese.
      ). Table 3 shows the average Ea values for the experimental cheeses as determined from temperature sweeps at d 1 and 28. Generally, all experimental cheeses showed a low tendency to melt or flow, most likely because of the pH values of all cheeses (d 1: 6.2–6.5; d 28: 5.9–6.2), all of which exceeded the maximum limit (6.0) for heated cheese to melt or flow (
      • Tunick M.H.
      Activation energy measurements in rheological analysis of cheese.
      ). However, Ea values differed significantly (P < 0.05) among cheeses at d 1 and 28. At d 1, CM15% and CM30% cheeses had higher initial Ea values of 31.6 and 29.9 kJ/mol, respectively, compared with that of BM cheese (27.1 kJ/mol). Similarly, at d 28, CM15% and CM30% cheeses had greater final Ea values of 33.7 and 36.1 kJ/mol, respectively, relative to BM (29.8 kJ/mol). This could be due to the lower pH values in mature CM15% and CM30% cheeses (5.90 and 5.91, respectively) compared with BM cheese (6.01; Table 1), resulting in higher solubilization of colloidal Ca phosphate (Table 1), greater dissociation of the caseins, and improved softening or melting properties. Additionally, CM15% and CM30% cheeses had higher moisture content than BM cheese, which is negatively correlated with rheological parameters; namely, Ea, G′, and G″ (
      • Tunick M.H.
      • Van Hekken D.L.
      Rheology and texture of commercial queso fresco cheeses made from raw and pasteurized milk.
      ). The findings of the current study indicated that addition of CM significantly affected melting properties of LFA cheese made from blended milk.
      Figure thumbnail gr6
      Figure 6Complex viscosity (η*) of low-fat akawi cheeses at d 1 (a) and d 28 (b). Blue = cheeses made from bovine milk; green = cheeses made from a blend of bovine milk and 15% camel milk (CM15%); and red = cheeses made from a blend of bovine milk and 30% camel milk (CM30%).
      Table 3Activation energy of flow (Ea) of low-fat akawi cheese made from bovine milk (BM) or bovine milk blended with 15% (CM15%) or 30% (CM30%) camel milk at d 1 and 28
      Values are mean ± SD of n = 4.
      ParameterStorage time (d)Low-fat akawi cheese
      BMCM15%CM30%
      Ea (kJ/mol)127.1 ± 0.15
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Means in a column with different lowercase letters differ significantly (P < 0.05).
      31.6 ± 0.70
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Means in a column with different lowercase letters differ significantly (P < 0.05).
      29.9 ± 0.85
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Means in a column with different lowercase letters differ significantly (P < 0.05).
      2829.8 ± 1.10
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Means in a column with different lowercase letters differ significantly (P < 0.05).
      33.7 ± 0.25
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Means in a column with different lowercase letters differ significantly (P < 0.05).
      36.1 ± 0.75
      Means in a row with different uppercase letters differ significantly (P < 0.05).
      Means in a column with different lowercase letters differ significantly (P < 0.05).
      A,B Means in a row with different uppercase letters differ significantly (P < 0.05).
      a,b Means in a column with different lowercase letters differ significantly (P < 0.05).
      1 Values are mean ± SD of n = 4.

      CONCLUSIONS

      We conclude that blending CM with BM affected the rheological properties of the resulting LFA cheeses. It was clear that the addition of CM affected the proteolysis rate and soluble Ca content, which in turn affected the functionality of the LFA cheese. The addition of CM affected the hardness and cohesiveness of LFA cheeses. The elastic and viscous properties of the LFA cheese as a function of heating was influenced by the presence of CM. Adding CM improved the meltability and flowability of the LFA cheese, which increases opportunities for its use as an ingredient in other food formulas. Based on the current use of LFA cheese as a main ingredient in the sweet industry, adding CM could improve the functionality of LFA cheese. The microstructure of LFA cheese became smoother when CM was added. The LFA cheeses made from blended milk had a greater activation energy than those made with BM only. Further studies are warranted to investigate casein interactions in CM and BM and to understand the mechanism of Ca distribution in cheese made from blended camel and bovine milk.

      ACKNOWLEDGMENTS

      The authors are grateful to United Arab Emirates University for funding this project. Author contributions were as follows: M. Ayyash: conceptualization, formal analysis, writing-original draft, supervision; A. Abdalla: writing-original draft; B. Abu Al-Jayal: conceptualization, investigation, formal analysis; S. Al Madhani: investigation; T. Huppertz and A. Kamal-Eldin: conceptualization, writing-review and editing. The authors have not stated any conflicts of interest.

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