Physicochemical characterization of Mozzarella cheese wheys and stretchwaters in comparison with several other sweet wheys

Article Outline

• Abstract
• Introduction
• Materials and Methods
  • Reagents
  • Collection of Whey and Stretchwater Samples
  • Units
  • Statistics
• Results and Discussion
  • pH
  • DM
  • Carbohydrates
  • Organic Acids
  • Minerals
  • Nitrogen Fractions
• Conclusions
• Acknowledgements
• References
• Copyright

Abstract 

To better understand the origins of the problems occurring during Mozzarella cheese whey concentration, lactose crystallization, and spray-drying steps, a physicochemical characterization was achieved. For this purpose, Mozzarella cheese wheys were sampled and their content in different compounds such as total nitrogen, noncasein nitrogen, nonprotein nitrogen, lactate, citrate, chloride, sulfate, phosphate anions, calcium, magnesium, potassium, sodium cations, and the sugars glucose and galactose were measured. In a second step, the results were compared with the corresponding content in Cheddar cheese wheys, Raclette cheese wheys, soft cheese wheys, and Swiss-type cheese wheys. At the end of this survey, it was shown that Mozzarella cheese wheys were more concentrated in lactate and in minerals—especially phosphate, calcium, and magnesium—than the other cheese wheys and that they contained galactose. These constituents are known to be hygroscopic. Complementary surveys are now necessary to compare the hygroscopicity of galactose and lactate and discover whether the amounts of these compounds found in Mozzarella cheese wheys are a factor in the problems encountered during the concentration, lactose crystallization, and spray-drying steps.

Key words: Mozzarella cheese whey, stretchwater, lactate, galactose

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Introduction 

Mozzarella cheese has achieved spectacular growth during the last century, first in the United States and then worldwide. In the United States, Mozzarella cheese production increased from 200,000 to 1,200,000 t between 1975 and 2000 (Kindstedt, 2002). In France, Mozzarella cheese production has achieved a 5-fold growth over the last decade (CNIEL, 1998–2007), reaching 25,000 t in 2005. In fact, the production of Mozzarella cheese will probably meet or exceed that of Cheddar cheese in the next decade.

Such production results in the unavoidable production of large amounts of byproducts. Whey produced equals approximately 9-fold the cheese tonnage produced (Schuck et al., 2004). In the case of Mozzarella, cheese production also results in the production of stretchwater, which is the hot water used in the stretching step specific to pasta filata cheeses.

It is necessary to enhance the value of these byproducts, which have a high nutritional value (whey proteins, lactose, and so on) but also a strong potential to pollute. Kosikowski (1979) and Smithers (2008) reviewed the history of the utilization and valorization of whey over the past decades. This valorization often begins with concentration and spray-drying.

In the case of Mozzarella cheese wheys and stretchwaters, the concentration, lactose crystallization, and spray-drying steps are often problematic, leading to insufficient dry matter at the end of the evaporation step, poor crystallization rates, and stickiness and caking of the powders. In fact, manufacturers are often forced to mix Mozzarella cheese wheys with other wheys to achieve successful drying. Above a certain rate of incorporation of Mozzarella cheese whey, the aforementioned problems occur. Solving these problems requires knowledge of the manufacturing processes, characterization of whey composition, and knowledge of water dynamics and how water interacts with the different compounds present in Mozzarella cheese wheys and stretchwaters.

Although numerous research studies have been conducted on Mozzarella cheesemaking and cheese functionality (e.g., melting behavior), to our knowledge very little research has been conducted specifically on the characterization of Mozzarella cheese whey and stretchwater composition. Solving the problems of Mozzarella cheese whey and stretchwater valorization requires a good characterization of Mozzarella cheese whey and stretchwater composition to correctly identify the compounds that can be a factor in these problems. This characterization should be made keeping in mind that, over the last century, the number of the different types of Mozzarella cheese manufacturing processes has multiplied as a result of demand from pizza manufacturers, producers, and consumers (Kindstedt, 2004).

To highlight the differences between Mozzarella cheese whey and other sweet wheys, a comparison between 6 samples of Mozzarella cheese wheys and samples of wheys originating from different cheesemaking technologies—soft cheese, Cheddar cheese, Raclette cheese, and Swiss-type cheese—was made thereafter. Stretchwaters were also analyzed and characterized.

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Materials and Methods 

Reagents 

Acetic acid, hydrochloric acid, and sodium acetate were from Panreac (Lyon, France); standard solutions of sodium, potassium, calcium, and magnesium were from Merck (Fontenay-sous-bois, France); sodium azide was from Riedel-de-Haën (Seelze, Germany); and sulfuric acid, trichloroacetic acid, cesium chloride, and lanthanum oxide were from VWR (Fontenay-sous-bois, France).

Collection of Whey and Stretchwater Samples 

Three Mozzarella cheese whey samples were collected in each of 2 different factories. Three samples of soft cheese whey were collected in 1 factory. Three samples of Cheddar cheese whey were collected in 1 factory. Four samples of Raclette cheese whey were collected in 1 factory. Three samples of Swiss-type cheese whey were collected in 1 factory. For each technology, the whey samples were taken at the entrance of the concentrator unit.

Stretchwaters were sampled after skimming in each of 2 different factories. Cheese wheys and stretchwaters were preserved in sodium azide (0.2g/L) to prevent microbial growth before analysis and were refrigerated at 4°C.

Total N was determined following the Kjeldahl method (FIL-IDF, 1993). Nonprotein N was determined following FIL-IDF (1993) method. Noncasein N was determined following FIL-IDF (1964) method. Dry matter was measured following FIL-IDF (1987) method. The amounts of Ca, K, Na, and Mg in whey were determined, following the method of Brulé et al. (1974), with an atomic absorption spectrometer SpectrAA 220 FS (Varian, Les Ulis, France).

The anions lactate, chloride, sulfate, phosphate, and citrate were quantified, following the method described by Gaucheron et al. (1996), with a Dionex ICS 3000 system (Dionex, Sunnyvale, CA) that included a carbonate trap (ATC3), a guard AG11 and a column (AS11 IonPac, 4×250mm), an ionization suppressor (ASRS, 4mm), and a conductivity detector for anions.

Carbohydrates were quantified using a Dionex ICS 3000 system that included a column (CarboPac PA1, 4 × 250mm) and an amperometric detector. The following elution conditions were used (at 1 mL/min and at 30°C): isocratic elution at 1 mL/min with 8mM NaOH for 16min followed by gradient elution with 8 to 34mM NaOH for 10min. The system was then reequilibrated for 5min with an 8mM NaOH solution before the subsequent injection. The signal was analyzed with Chromeleon software (version 6.80 SP1 Build 2238, 1996–2006, Dionex).

Before the analysis, standards of different concentrations of a reference solution were measured. The amount of lactose was determined by difference, knowing the DM and the content of wheys in all major components.

Units 

Dry matter values, carbohydrate contents, and nitrogen fractions were expressed in grams per 100 grams of product. Contents of anions, cations, and organic acids were expressed in millimoles, which is the more pertinent unit for determining in which forms the minerals are associated and reasoning on their possible interactions.

Statistics 

Data analysis was performed using R software (version 2.6.2, 2008, www.r-project.org) and the packages Rcmdr (version 1.3–15, 2008, www.r-project.org) and FactoMineR (version 1.08, 2008, www.r-project.org). Mineral equilibriums were simulated with the Milksalts GLM software (Mekmene et al., 2009).

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Results and Discussion 

Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7 present the mean values found for each whey and stretchwater. In this section, we present and discuss the different results, focusing first on value comparison among wheys and second on the composition of stretchwaters and on value comparison between stretchwaters and wheys.

Table 1. pH values of Mozzarella cheese whey and stretchwater and several sweet wheys

Table 2. Dry matter values (g/100g of product) of Mozzarella cheese whey and stretchwater and several sweet wheys

Item	DM (g/100g)
	Mean	SD
Mozzarella cheese whey	6.06a	0.17
Mozzarella cheese stretchwater	6.62a	0.01
Soft cheese whey	6.15a	0.13
Swiss-type cheese whey	5.92a	0.01
Cheddar cheese whey	5.91a	0.18
Raclette cheese whey	5.89a	0.30

Table 3. Carbohydrate content (g/100g of product) of Mozzarella cheese whey and stretchwater and several sweet wheys

Item	Galactose (g/100g)		Glucose (g/100g)		Lactose (g/100g)
	Mean	SD	Mean	SD	Mean	SD
Mozzarella cheese whey	0.12b	0.02	0.01a	0.01	4.13b	0.06
Mozzarella cheese stretchwater	0.65a	0.14	0.01a	0.01	3.54c	0.14
Soft cheese whey	0.08bcde	0.06	0.01a	0.01	4.61a	0.20
Swiss-type cheese whey	0.01e	0.00	0.01a	0.01	4.48a	0.01
Cheddar cheese whey	0.02de	0.01	0.00a	0.00	4.34a	0.12
Raclette cheese whey	0.04c	0.02	0.01a	0.01	4.21ab	0.23

Table 4. Organic acids content (mM) of Mozzarella cheese whey and stretchwater and several sweet wheys

Table 5. Anions content (mM) of Mozzarella cheese whey and stretchwater and several sweet wheys

Table 6. Cations content (mM) of Mozzarella cheese whey and stretchwater and several sweet wheys

Table 7. Nitrogen fractions (g/100g) of Mozzarella cheese whey and stretchwater and several sweet wheys

pH 

The pH results are shown in Table 1. Our data were higher than those found in the literature for Cheddar cheese wheys. In fact, Hill et al. (1985) as well as Hargrove et al. (1976) found a pH close to 6.10 for Cheddar cheese wheys whereas our value was 6.35. Hill et al. (1985) found a value higher than ours (6.67) for Mozzarella cheese wheys. However, the pH data for wheys in the literature are very scarce because pH depends too strongly on the time of sampling and the individual process of the factory.

Swiss-type cheese wheys had the highest pH (6.52); all other wheys had a pH close to 6.40. Thus, Mozzarella cheese wheys can be considered sweet wheys, if wheys with a pH higher than 6.2 are considered to be sweet wheys. The pH differences among the wheys were not statistically significant (P=0.87).

Stretchwaters had a pH that was more acid than that of wheys (about 5.13). In fact, stretchwaters originate from the final treatment of the curd (the stretching step, which is necessary for attaining a sufficient demineralization of the casein micelle) and, as a result, their pH is representative of the degree of acidification achieved in this last step.

DM 

Table 2 shows the DM values of the different wheys analyzed. The highest DM values were found in Mozzarella cheese wheys and soft cheese wheys (6.1 g/100g); the other wheys had a DM value of 5.9. However, these DM values are close to each other and did not show much difference (P=0.53). Dry matter values can vary greatly, even for a given type of whey. For example, for Cheddar cheese wheys, Hill et al. (1985) found a DM value of 5.90 whereas Hargrove et al. (1976) give a value of 6.70.

Stretchwaters had a DM value higher than those of wheys (6.62). This is not in agreement with Ardisson-Korat and Rizvi (2004), who found a lower DM content for stretchwaters compared with the corresponding Mozzarella cheese wheys.

Carbohydrates 

Table 3 shows the carbohydrates content of each whey. The determination of monosaccharides (galactose and glucose) by ionic chromatography showed that Mozzarella cheese wheys had a galactose content (about 0.12 g/100g) higher than the other wheys (P < 0.0001). Mozzarella cheese is manufactured using thermophilic starters such as Streptococcus thermophilus and Lactobacillus bulgaricus, which do not ferment galactose and therefore lead to the presence of galactose in whey. However, soft cheese wheys and Raclette cheese wheys also contain a small amount of galactose (about 0.07 g/100g). The highest amount of galactose was found in stretchwaters (about 0.6 g/100g). In this study we did not analyze pure citric Mozzarella cheese wheys but it is assumed that these wheys do not contain any galactose because there is no hydrolysis by starters. We once analyzed such a whey sample (unpublished data) that confirmed this fact. Cheddar cheese wheys and Swiss-type cheese wheys contained a very small amount of galactose (<0.02 g/100g). Glucose amounts were less than 0.03 g/100g for all wheys analyzed in this study.

Like many monosaccharides, galactose is known to be hygroscopic. Rheinländer (1982) showed that lactose-hydrolyzed whey was more difficult to dry than native whey because of the hygroscopicity of its constituents. Moreover, San Jose et al. (1977) made sorption isotherms showing that lactose-hydrolyzed dry milk adsorbed more water than nonhydrolyzed milk at water activity (a_w) higher than 0.5. However, neither study separates the respective roles of glucose and galactose in this hygroscopicity.

In a recent study about solvation of monosaccharides in water, Kraütler et al. (2007) showed that galactose, having fewer intramolecular hydrogen bonds than glucose, has a greater ability to form hydrogen bonds with water than glucose does. However, galactose is less stable in aqueous solution, which could explain its lower solubility.

Mozzarella cheese wheys had the lowest amount of lactose (4.13%; P=0.0006974), which was in accordance with the fact that they had the greatest amounts of galactose and lactate, which are products resulting from bioconversion of lactose. Conversely, the highest amounts of lactose (4.48%) were found in Swiss-type cheese wheys, which contain no lactate at all and almost no galactose.

The low lactose content of Mozzarella cheese wheys was similar to the low lactose content shown by De Wit (2001) for lactic wheys and by Imbert-Pondaven (1977) for acid wheys (4.00%). The lower amount of lactose could explain the different crystallization behavior of Mozzarella cheese wheys. In fact, lower amounts of lactose will result in decreased supersaturation. Mozzarella cheese stretchwaters had a lower lactose content (3.54 g/100g) than wheys.

Organic Acids 

Table 4 shows the average lactate and citrate content of each whey (P=0.0012 for lactate and 0.0017 for citrate). Mozzarella cheese wheys had an average content of 12mM lactate and the same content of citrate. The proportions of these 2 compounds varied greatly depending on the process used. “Citric” wheys also had greater amounts of phosphate and calcium ions because citrate solubilizes colloidal calcium phosphate. The lactate content of soft cheese wheys was equal to that of Mozzarella cheese wheys, Cheddar cheese wheys had half the lactate content of Mozzarella cheese wheys, and Swiss-type cheese wheys had no lactate content.

The amounts of lactate measured by ionic chromatography for Mozzarella cheese wheys and soft cheese wheys were close to the values found by Saulnier et al. (1996) for fresh cheese whey (13.43mM). However, there is great variability of lactate values in the literature, which makes it difficult to compare these values. For example, for Cheddar cheese wheys, Mullin and Emmons (1997) found values close to 4mM, which is in accordance with our values, whereas Hargrove et al. (1976) found a mean value of 15.72mM, which is higher than the value we found for Mozzarella cheese wheys. The amounts of lactate found in stretchwaters were very high. In fact, they were close to 80mM, which is 6-fold the amounts found in the more concentrated wheys.

To complete this study, the different forms of lactate in wheys were simulated using the Milksalts GLM software (Mekmene et al., 2009). At the pH of sweet wheys, lactate is mainly present in its free basic form. However, stretchwaters, which have pH of 5.13, contain a small proportion of lactic acid; they also contain high amounts of lactate and calcium, which makes the presence of calcium lactate very likely.

As lactate is found in higher amounts in the most-difficult-to-dry wheys (soft cheese wheys, Mozzarella cheese wheys, acid wheys), higher lactate content is the most likely explanation for said difficulties. In fact, lactate has a very low glass transition temperature (T_g). Mimouni et al. (2007) recently demonstrated that calcium lactate crystallization in concentrator units leads to concentrated acid whey thickening. However, very few data are available on the interactions between lactate and water in concentrated solutions. The hygroscopicity of lactate thus needs to be confirmed and correctly characterized.

Citrate amounts were the same among the tested wheys (about 8mM), with Mozzarella cheese wheys having the highest amounts (>10mM). The presence of citric Mozzarella cheese whey in the analyzed wheys is doubtless the reason for the higher amounts. The values for citrate were in accordance with those found for Cheddar cheese by Mullin and Emmons (1997) and for sweet and acid wheys by Cataldi et al. (2003) and Saulnier et al. (1996).

Minerals 

Table 5 shows the anion contents of all the different wheys. Soft cheese wheys had a chloride content of 38.43mM, by far the highest value of all the tested wheys (P=0.0002). This was in accordance with the results of Cataldi et al. (2003) and Saulnier et al. (1996), who found values of 34.69 and 39.42mM, respectively, for soft cheese wheys. The chloride values for all of the other wheys were close to 25mM; Mozzarella cheese wheys and Swiss-type cheese wheys had slightly higher values (29mM).

Mozzarella cheese wheys and soft cheese wheys had the highest phosphate values (P=0.0004), which were close to 12mM. These values were in accordance with those found by Cataldi et al. (2003) for soft cheese whey (12.11mM). They were higher than those found by Hill et al. (1985) for Mozzarella cheese whey (4mM) and by De Wit (2001) and Adrian and Bourlier (1980) for soft cheese wheys (7mM), but lower by far than those found by Saulnier et al. (1996) for soft cheese wheys (26mM).

The amounts of sulfate were in the 4.4 to 5.5mM range. Again, Mozzarella cheese wheys and soft cheese wheys had the highest amounts (5.5mM). However, the differences were not significant (P=0.0905). Few data are available in the literature about the sulfate content of cheese wheys. The values found by Saulnier et al. (1996) by capillary electrophoresis for soft cheese whey are lower than ours.

The cation content of the different wheys analyzed is presented in Table 6. The amounts of calcium ranged from 9.50mM (Swiss-type cheese wheys) to 16mM (Mozzarella cheese wheys and soft cheese wheys; P < 0.0001). The calcium values were in accordance with those presented by Imbert-Pondaven (1977) for Swiss-type cheese wheys, with those found by De Wit (2001) for soft cheese whey, and with those found by Hill et al. (1985) for Cheddar cheese wheys, but not for Mozzarella cheese wheys, for which we found a far higher value. However, in their chromatographic study on the mineral composition of wheys, Cataldi et al. (2003) recorded a high calcium content for Mozzarella cheese wheys.

Given the fact that Mozzarella cheese wheys and soft cheese wheys both had higher amounts of phosphate and calcium, we conclude that these cheese processes lead to a higher rate of casein micelle demineralization, which results in atypically sweet wheys from the mineral content point of view. That can be one explanation for the fact that these 2 types of cheese wheys are more difficult to dry than the others. The demineralization of the casein micelle is part of Mozzarella processing because it prepares the curd for the stretching step.

Similarly, Mozzarella cheese wheys and soft cheese wheys had the highest amounts of magnesium (more than 3.6mM) whereas Swiss-type cheese wheys had the lowest amounts (3.26mM; P < 0.0001). This shows again the higher casein micelle demineralization in the case of these 2 technologies.

The amounts of potassium ranged from 36.14mM (Raclette cheese wheys) to 42.45mM (soft cheese wheys; P < 0.0001). These values were in accordance with all the references except for Saulnier et al. (1996), who found higher values by capillary electrophoresis and Hargrove et al. (1976), who found lower values by atomic absorption spectrometry.

Concerning the amounts of sodium, Cheddar cheese wheys had the highest value (22.38mM) whereas soft cheese wheys and Raclette cheese wheys had the lowest values (P=0.0003). Because sodium content depends greatly on the technique used to salt the cheese, it was difficult to make an efficient comparison. However, our values were in the range of those found in the literature.

The mineral content of Mozzarella cheese stretchwaters was also very high. The calcium content exceeded 40mM, the magnesium content exceeded 5mM, the sodium and chloride contents exceeded 40mM, and the phosphate content exceeded 18mM. Only the potassium content is close to that found in wheys. The chloride and sodium values found by Ardisson-Korat and Rizvi (2004) in stretchwater (>500mM NaCl) were far higher than ours.

To conclude, Mozzarella and soft cheese wheys can be considered to be the most concentrated in minerals, especially in calcium, phosphate, and magnesium, because of higher rates of casein micelle demineralization during dripping. This higher mineral content could explain the fact that Mozzarella cheese wheys contain high amounts of lactate despite their pH, which is close to that of a sweet whey. It is therefore likely that Mozzarella cheese wheys have a high buffer capacity. However, the only survey on the buffer capacity of cheese wheys (Hill et al., 1985) does not show a high buffer capacity for Mozzarella cheese wheys.

This high mineral content is a factor that limits the capacity of these wheys to be properly concentrated and spray-dried. Indeed, wheys that are concentrated in minerals are known to be very hygroscopic. Additionally, in a recent study about the water sorption of delactosed permeate, Liang et al. (2009), using the Peleg model, showed that total sugar, lactic acid, and mineral contents can have a significant influence on sorption.

Nitrogen Fractions 

Table 7 shows the amounts of each nitrogen fraction. These fractions are nonprotein N, calculated caseins (total N content − NCN), and calculated whey proteins (NCN − NPN). Mozzarella cheese wheys had one of the highest N contents of all wheys of this study. Raclette wheys had the highest N content, being particularly rich in whey proteins.

The amounts of N we found for Mozzarella cheese wheys and Raclette cheese wheys were higher than those found by Hill et al. (1985) for Mozzarella cheese wheys. For the other cheese wheys, however, our values were in accordance with the literature. Nitrogen content can be strongly influenced by the standardization of milk. Higher or lower values can therefore be correlated with the standardization of milk more than with the cheesemaking process. It is therefore difficult to make unbiased comparisons in this area.

Our values for NPN content were slightly lower than those found by De Wit (2001) and Hargrove et al. (1976). However, the compositions given by De Wit are standardized to a DM value of 6.5, which is higher than those that we found.

Stretchwaters had a lower total N content than wheys, except for soft cheese wheys. Stretchwaters had a similar NPN content (0.2 g/100g) and a lower whey protein content (0.32 g/100g) than wheys, but a much higher casein content (0.21 g/100g). These values were different from those of Ardisson-Korat and Rizvi (2004), who found lower amounts of each fraction in stretchwaters.

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Conclusions 

Mozzarella cheese whey is a sweet whey (pH=6.38) that, in some aspects (high mineral and lactate content), is closer to an acid whey than to a conventional sweet whey (e.g., Swiss-type cheese whey). Moreover, its relatively high galactose content is a unique feature, linked to the bacteria used in the manufacture of Mozzarella cheese. Stretchwaters, which are a byproduct of Mozzarella cheese production and are sometimes mixed with whey before drying, have the same features at a higher magnitude, with very high amounts of lactate and galactose. Therefore, their incorporation in whey is very questionable because this incorporation enriches hygroscopic compounds. We need to conduct further research to confirm and characterize the hygroscopicity of lactate and galactose. It is necessary, for example, to know the threshold values above which concentrating and drying lactate and galactose is difficult.

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Acknowledgements 

We are grateful to GEA Process Engineering (St. Quentin en Yvelines, France) for financial support and fruitful scientific discussions. We thank Frédéric Gaucheron for his collaboration.

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