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Effect of pH and Calcium Concentration on Some Textural and Functional Properties of Mozzarella Cheese

T. P. Guinee^*, E. P. Feeney, M. A. E. Auty^* and P. F. Fox

^* Dairy Products Research Centre, Teagasc Moorepark, Fermoy, Co. Cork, Ireland
Food Chemistry, Department of Food Science, Technology and Nutrition, University College, Cork, Ireland

Corresponding author:
Timothy P. Guinee; e-mail:
tguinee{at}moorepark.teagasc.ie.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
The effects of Ca concentration and pH on the composition, microstructural, and functional properties of Mozzarella cheese were studied. Cheeses were made using a starter culture (control) or by direct acidification of the milk with lactic acid or lactic acid and glucono--lactone. In each of three trials, four cheeses were produced: a control, CL, and three directly-acidified cheeses, DA1, DA2, and DA3. The cheeses were stored at 4°C for 70 d. The Ca content and pH were varied by altering the pH at setting, pitching, and plasticization. The mean pH at 1 d and the Ca content (mg/g of protein) of the various cheeses were: CL, 5.42 and 27.7; DA1, 5.96 and 21.8; DA2, 5.93 and 29.6; DA3, 5.58 and 28.7. For cheeses with a high pH (i.e., 5.9), reducing the Ca content from 29.6 to 21.8 mg/g of protein resulted in a significant decrease in the protein level and increases in the moisture content and mean level of nonexpressible serum (g/g of protein). Reducing the Ca concentration also resulted in a more swollen, hydrated para-casein matrix at 1 d. The decrease in Ca content in the high-pH cheeses coincided with increases in the mean stretchability and flowability of the melted cheese over the 70-d storage period. The fluidity of the melted cheese also increased when the Ca content was reduced, as reflected by a lower elastic shear modulus and a higher value for the phase angle, , of the melted cheese, especially after storage for <12 d. The melt time, flowability, and stretchability of the low-Ca, high-pH DA1 cheese at 1 d were similar to those for the CL cheese after storage for 12 d. In contrast, the mean values for flowability and stretchability of the high-pH, high-Ca DA2 cheese over the 70-d period were significantly lower than those of the CL cheese. Reducing the pH of high-Ca cheese (27.7 to 29.6 mg/g of protein) from 5.95 to 5.58 resulted in higher flowability, stretchability, and fluidity of the melted cheese. For cheeses with similar pH and Ca concentration, the method of acidification had little effect on composition, microstructure, flowability, stretchability, and fluidity of the melted cheese.

Abbreviation key: AV = maximum apparent viscosity, , CL = control, , CP = curd particle, , CPM = curd particle membrane, , CSLM = confocal scanning laser microscopy, , = phase angle, , DA = directly-acidified, , FDM = fat-in-dry matter, , G' = elastic shear modulus, , GDL = glucono--lactone, , MNFS = moisture-in-nonfat-substance, , NES = nonexpressible serum, , NESC = nonexpressible serum in g/100 g of cheese, , NESP = nonexpressible serum in g/g of protein in cheese, WHC = water-holding capacity

Key Words: (pH • calcium (Ca) • Mozzarella • texture • and functionality)

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Global production of Mozzarella cheese has grown dramatically during the period 1985 to 2000 (Rowney et al., 1999), especially in the United States, where annual production in 1996 was 1.03 million tonne (Sørensen, 1997). The main impetus for the growth of Mozzarella has been the ubiquitous increase in the popularity of pizza, in which Mozzarella is the main cheese used. The functional attributes of importance for pizza include the desired degrees of flow and stringiness on baking. The cheeses best endowed with these characteristics, especially stretchability, are members of the pasta filata group and include Mozzarella, Provolone, and Kashkaval (Fox et al., 2000). In the manufacture of these cheeses, the fermented curd (pH 5.1 to 5.4) is subjected to a plasticization step towards the end of manufacture during which it is scalded (to 57°C), kneaded, and stretched in hot (80°C) water or dilute brine. The conditions of low pH and high temperature are conducive to limited aggregation of para-casein and the formation of para-casein fibers of relatively high tensile strength (Kimura et al., 1992; Taneya et al., 1992). These fibers impart stretchability, stringiness, and chewiness to the melted cheese topping when the cheeses are subsequently cooked on pizza (Fox et al., 2000).

It is generally accepted that pH and Ca concentration influence the ability of curd to plasticize in hot water or hot dilute brine. Hence, in the typical conventional manufacture of pasta filata cheeses, the curd pH is allowed to decrease, via the conversion of lactose to lactic acid, to a value typically in the range 5.4 to 5.1 before plasticization. The reduction in curd pH promotes a number of physico-chemical conditions, which are conducive to the flow of the curd during plasticization: solubilization of micellar calcium phosphate (van Hooydonk et al., 1986), an increase in the ratio of soluble to colloidal Ca (Guinee et al., 2000a), and an increase in para-casein hydration (Creamer, 1985). Generally, at pH values > 5.4, it is more difficult to plasticize the curd (Kimura et al., 1992); the curd becomes progressively less smooth and more lumpy with increasing pH. However, curd may be plasticized successfully at a higher pH (e.g., 5.6) and higher colloidal to soluble Ca ratio if the total concentration of Ca is lower than that in curd made by the traditional procedure, e.g., as in directly acidified Mozzarella, where the milk is setting at pH 5.6 (Keller et al., 1974; Kindstedt and Guo, 1997a).

The functional attributes of Mozzarella cheese are influenced by many factors, including milk pretreatment, make procedure, composition, and proteolysis (Lawrence, 1989; McMahon et al., 1993; Kindstedt, 1995; Madsen and Qvist, 1998). Comparatively little information is available on the effects of pH, and Ca content, and their interaction on the functional attributes of plasticized cheese (e.g., Mozzarella) when cooked on pizza (McMahon et al., 1993; McMahon and Oberg, 1999; Rowney et al., 1999). Keller et al. (1974) reported a significant negative correlation between the Ca concentration in DM and the flowability of directly acidified Mozzarella. Kimura et al. (1992) found that increasing the pH at curd plasticization from 5.2 to 5.5 resulted in a decrease in the stringiness of unheated string cheese. Increasing the Ca content of Mozzarella (from 27.2 to 29.6 mg/g of protein), by increasing the pH at whey drainage from 6.15 to 6.4 resulted in a slight decrease in moisture content from 46.3 to 45.7% (wt/wt) but did not significantly affect the final pH or the apparent viscosity, level of free oil, or flowability during storage (Yun et al., 1995).

The objective of the current study was to investigate the effects of Ca content and pH and their interaction on the texture and heat-induced functionality of Mozzarella cheese.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Cheese Manufacture
In each of three trials, four types of Mozzarella cheese were manufactured, using either a starter culture, as in conventional manufacture of the control cheese (CL), or by direct acidification (DA), using lactic acid (directly acidified cheese, DA1), or a combination of lactic acid and glucono--lactone (GDL; directly acidified cheeses DA2 and DA3). The four cheeses differed with respect to pH of milk at rennet addition (setting), pH of the curd at whey drainage, and pH of the curd at milling and plasticization (Table 1).

After whey drainage, the curds (52 kg) were cheddared in the finishing vat and milled at pH 5.15 (to a chip size of 2.8 x 1.25 x 1.25 cm). The pH of the curd (39°C) at milling was measured by inserting the pH probe (calibrated at 39°C) directly into the curd. The milled curd was dry salted at a level of 4.6% (wt/wt), mellowed for 20 min, and transferred to the stretching unit (Automatic Stretching Machine, Model d; CMT, S. Lorenzo di Peveragno CN, Italy). The curd was heated to 58°C by dousing with hot water at 78°C (used at a level of 1.4 kg/kg of curd and added over a period of 8 min) and batch plasticized mechanically. The molten plastic curd mass was then conveyed by auger to the molding unit where it was formed into a cylindrical mass which was guillotined into 2.3-kg portions (24-cm long). The portions were deposited, in a horizontal orientation, into rectangular molds of the same length; after 1 min, the hot curd in the mold was manually turned to facilitate the molding of the curd to the rectangular shape. This filling procedure ensured that the orientation of the cheese fibers in of the molded curd was similar to that in curd on discharge from the molding unit. The molds containing the curds were then placed in dilute brine (10 g of NaCl/100 g, 0.2 g of Ca/100 g, pH 5.1) at –4.0°C for 30 min to cool the block of curd to a surface temperature of 24°C and a core temperature of <45°C. The cheeses, 14 per vat, were then removed from the molds, allowed to drip dry for 10 min at room temperature, and vacuum wrapped and stored at 4°C.

The details for manufacture of the different cheeses are shown in Table 1. Apart from differences in the pH at setting, whey drainage and plasticization, the manufacturing procedure for the DA cheese was similar to that for the control cheese in terms of cutting (firmness at initiation of cutting, speed, and duration), stirring (speed and duration), cooking rate, cheddaring time, mellowing time, plasticization, molding, and cooling (Table 1). For the manufacture of DA1 Mozzarella, the milk (36°C) was acidified directly to pH 5.6 with lactic acid (5 g/100 ml; (Wardle Chemicals, Calveley, Nr. Tarporley, Cheshire, UK). The cheese-making procedure was otherwise similar to that for the CL cheese, apart from the differences in pH at setting, whey drainage, and salting (Table 1).

In the manufacture of DA2 and DA3 cheeses, milk was acidified with lactic acid (5%, wt/vol), which was added at a rate designed to simulate the pH profile of the CL Mozzarella to the point of whey drainage. After whey drainage, GDL powder (ADM, Ringaskiddy, Cork, Ireland) was mixed with the salt and added to the curd to reduce the pH to the desired value, at levels of 1.6 g/100 g or 3.6 g/100 g for the DA2 and DA3 cheeses, respectively. Treatment of the curd after salting in the manufacture of the DA2 and DA3 cheeses was similar to that for the control. Preliminary bench-scale cheese-making studies were undertaken to establish the levels of lactic acid and GDL required to reduce the pH of the DA curds to the appropriate values at the different stages of manufacture.

Sampling of Cheese
The cheeses were sampled and prepared, as described by Guinee et al. (2000c), to give grated particles (<1 mm), shreds (25-mm long and 4-mm diam), discs (6.5 mm thick and 45.5-mm diam or 2.0-mm thick and 41-mm diam), and cylinders (29-mm high and 29-mm diam).

Composition and Proteolysis
Grated cheeses were analyzed in duplicate for salt, fat, protein, moisture, and pH (at 22°C), Ca, phosphorus, and pH 4.6-soluble N, as described by Guinee et al. (2000c). The level of intact casein (g/100 g) in the cheese was calculated using the formula:

It was assumed that all the protein in the cheese was casein.

Confocal Scanning Laser Microscopy (CSLM)
Cheeses from the three trials were examined by confocal scanning laser microscopy (CSLM) at 1 and 20 d, as described by Guinee et al. (1999). Cheese was cut into 4-cm cubes. One edge of each cube was pulled apart manually. This revealed the fibrous nature of the cheese and enabled sections to be cut parallel to the fiber direction. Sections of cheese, approximately 10 x 10 x 2 mm thick, were cut from a freshly cut cube with a razor blade (at 4°C). This sampling technique ensured that the fibers in the sections had a longitudinal orientation.

The sections were stained with 0.1% (wt/vol) aqueous Nile blue A (C.I. 51180 sulphate salt; Sigma Chemical Co., Poole, Dorset, UK), and examined with a Zeiss LSM310 confocal scanning laser microscope (Carl Zeiss, Ltd., Welwyn Garden City, Herts., UK). Representative images were acquired at both 488 and 633 nm to enable direct comparison of the distribution and microstructure of the fat and protein, respectively; stained regions appear bright against a dark background.

Cheese Firmness
Four to six cylindrical samples were taken from each cheese, placed in an airtight plastic bag, held at 8°C overnight and compressed to 30% of original height on a model 112 Universal Instron Testing Machine (Instron, Ltd., High Wycomb, UK), as described by Guinee et al. (2000c). Firmness was the force required to compress the samples of cheese to 30% of the original height.

Expressible Serum
Cheese serum was expressed by centrifugation of grated cheese at 12,500 x g at 20°C, as described by Kindstedt and Guo (1997a). The expressed serum and fat were collected in a preweighed graduated cylinder that was held at 4° C until the surface layer of fat had solidified. The solidified fat layer was punctured to release the subnatant serum, which was poured off and weighed.

Evaluation of Cheese Functionality on Cooking
Melt time was defined as the time required for a fixed weight of shredded cheese (1.73 kg/m²) to melt and fuse into a molten mass, free of shred identity, on heating at 280°C (Guinee et al., 2000a, 2000c). Flowability was defined as the percentage increase in the diameter of a disk of cheese (45 mm diameter, 6.5 mm thick) on melting at 280°C for 4 min (Guinee et al., 2000a, 2000c). The stretchability of the molten cheese on baked pizza was measured by uniaxial extension at a velocity of 0.066 m/s (Guinee et al., 2000a, 2000c); before heating, the shredded cheese was distributed uniformly at a fixed loading (2.5 kg/m²) onto a pizza base that was precut in half. The maximum apparent viscosity (AV) of the molten cheese (at 70°C) was measured using a helipath viscometry (Brookfield Model DV-II Visometer; Brookfield, Engineering Labs., Stoughton, MA) as described by Guinee et al. (2000a, 2000c).

Viscoelastic Changes during Heating
Changes in the viscoelasticity of the cheese on heating from 20 to 80°C were measured by low amplitude strain oscillation in a controlled stress rheometer (CSL²₅₀₀ Carri-Med; TA Instruments, Inc., New Castle, DE), as described by Guinee et al. (1999). Disks (41-mm diameter, 2 mm thick) of cheese were tempered at 20°C for 15 min, placed between the two parallel, serrated plates of the rheometer cell, equilibrated at 20°C for 3 min and subjected to a harmonic low amplitude shear strain () of 0.005, at an angular frequency of 1 Hz. The temperature was increased to 82°C at a rate of 3°C/min. The elastic shear modulus (G') and phase angle () of the melting cheese were computed continuously during heating.

Statistical Analysis
A randomized complete block design, which incorporated the four treatments (CL, DA1, DA2, and DA3) and three blocks (replicate trials), were used to analyze the response variables relating to the composition of the cheese (Table 2). Analysis of variance (ANOVA) was carried out using a SAS procedure (SAS, 1995) where the effects of treatment and replicates were estimated for all response variables. Duncan’s multiple-comparison test was used as a guide for pair comparisons of the treatment means. The level of significance was determined at P < 0.05.

The four cheeses from each trial were analyzed for microstructure at 1 and 14 d, and for heat-induced changes in viscoelasticity at 1, 5, 12, 20, 35, and 44 d. The latter data are presented as supportive data but were not statistically analyzed.

Where firmness, melt time, flowability, and AV of the melted cheese were plotted as a function of intact casein, linear regression of data from all cheeses at all storage times was performed. The significance of the regressions was determined by applying Student’s t test to r² with n-2 degrees of freedom (df), where n is the actual number of data points.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Cheese-making
The DA1 milk coagulated quite rapidly, forming a gel that was sufficiently firm to cut in 8.7 min, compared with 39 to 44 min for the CL, DA2, and DA3 milks (Table 1). A similar effect has also been reported in other studies where the pH at setting was 6.0 during the manufacture of Mozzarella cheese using DA or a combination of DA and starter cultures (Keller et al., 1974; Kindstedt and Guo, 1997a; Metzger et al., 2000). The rapid curd-forming properties of the DA1 milk are expected as the reduction in set pH leads to a reduction in the net charge on the casein micelles, an increase in the concentration of ionic calcium, and an increase in rennet activity (Walstra and Jenness, 1985; Fox et al., 2000).

Apart from the shorter coagulation time, the manufacture of the DA1 cheese was similar to that of the other cheeses, viz., the time and temperature profiles from setting of the milk to the cooling of the finished cheese (Table 1).

Gross Composition
Data on the composition of the various cheeses are summarized in Table 2. Compared to the other cheeses, the DA1 cheese had a significantly lower level of protein, ash, and Ca, and a higher level of moisture and moisture-in-nonfat substances (MNFS). This effect concurs with the results of others (Keller et al., 1974; Metzger et al., 2000), who reported a reduction in the Ca content of cheese made from milk that had been preacidified before setting, to a degree depending on the extent of pH reduction at setting and whey drainage, and the type of acid used.

The low concentration of Ca in the DA1 cheese compared to the other cheeses, especially DA2, which was plasticized at the same pH, may be attributed to the lower pH at setting and at whey drainage, which results in increased solubilization of micellar calcium phosphate (van Hooydonk et al., 1986) and a concomitant increase in the concentration of soluble Ca in the whey while in contact with the curds. The whey acts a vehicle in which the soluble Ca is removed from the curds at whey drainage. Moreover, for a given pH at whey drainage, a more rapid (all-at-once) acidification and solubilization of Ca (e.g., by preacidification) led to a greater loss of Ca in the whey and a lower concentration in the cheese (Czulak et al., 1969). After cutting, the curd particles (CP) develop a membrane (skin) which has a higher casein-to-fat ratio than the interior (Kalab, 1977) owing to loss of the fat from the surface layer (Fox et al., 2000). Presumably, the permeability of the CP as a whole, and especially the curd particle membrane (CPM), decreases with time between cutting and draining as a consequence of casein dehydration and/or the continuous loss of fat. The movement of NaCl in cheese moisture is a pseudo-diffusion process whereby movement is limited by, among other things, the sieving effect of the protein matrix through which the moisture permeates (Geurts et al., 1974a). It may be assumed that the movement of Ca through the curd particle to the bulk phase whey is similarly impeded by the sieving effect exerted by the protein matrix of the CP, especially at the CPM. Thus, the extensive loss of Ca from the CP before reduced permeability is a limiting factor to its migration to the whey, and may in part account for the low concentration of Ca in rapidly acidified cheese. Hence, a reduction of pH before rennet coagulation, as for DA1, probably effects a more efficient removal of Ca from the curd than the slow reduction of pH after gelation and gel cutting, as in CL, DA2, and DA3. The similar concentration of Ca in the latter cheeses, despite the differences in plasticization pH and final pH, was expected because of the similar pH at setting and at whey drainage, factors that are the major determinants of the total Ca content of cheese (Lawrence et al., 1984).

The higher moisture content of DA1 may be due to its relatively low Ca-to-casein ratio, which would be conducive to a greater degree of casein hydration. This result concurs with those of Shehata et al. (1967) and Keller et al. (1974), each of which showed a linear increase in the moisture content of DA Mozzarella as the Ca level in the cheese decreased. The inverse relationship between the moisture content of Mozzarella and Ca content is also consistent with the findings of model studies involving dilute suspensions of casein or para-casein, which have shown that casein hydration increases as the level of bound Ca decreases (Sood et al., 1979). Hence, as discussed below, the level of nonexpressible serum per gram of protein, which is an index of the degree of protein hydration, was significantly higher in DA1 than in the other cheeses throughout storage. Similarly, sequestration of Ca during the manufacture of processed cheese products, as effected by the addition of emulsifying salts, such as sodium phosphates, is accompanied by an increase in casein hydration (Fox et al., 2000). It is noteworthy that the moisture content of cheeses with similar Ca concentrations did not differ significantly.

In contrast to the results of the current study and those of Shehata et al. (1967) and Keller et al. (1974), Metzger et al. (2000) reported that the moisture content of reduced-fat Mozzarella was not significantly affected by varying the Ca level in the range 17.5 to 30.2 mg/g of protein. The discrepancy between the results of Metzger et al. (2000) and those of the other studies may be related to differences in the pH of the curd at stretching, which, in conjunction with Ca content, probably have a major effect on the hydration of the para-casein. Model studies on rennet-treated skim milk have shown that the hydration of casein micelles increases markedly as the pH is reduced from 6.0 to 5.4 but decreases markedly as the pH is reduced further toward the isoelectric pH (Creamer, 1985). The decrease in hydration at pH values <5.4 is probably a consequence of increased hydrophobic interactions, which are effected by a decrease in the net micellar charge due to increased concentrations of H⁺ and Ca²⁺ (van Hooydonk et al., 1987; Guinee et al., 2000a). In the study of Metzger (2000), the control (high Ca, 30.2 mg/g of protein) cheese was salted at pH 5.5, while the cheeses made using a combination of acid and starter culture (low Ca, 25.7 to 17.8 mg/g of protein) were salted at pH 5.3. The pH at plasticization was probably somewhat lower as the pH usually decreases somewhat (typically 0.05 to 0.1/U over 20 min) during mellowing, since salt diffusion to the center of curd chips is not instantaneous (Guinee and Fox, 1993), and hence, the growth of the starter culture is not immediately inhibited. Therefore, the pH of all cheeses at 1 d were similar at 5.23, and at 15 d ranged from 5.15 to 5.2 in the low-Ca cheeses to 5.3 in the CL cheese (Metzger et al., 2001). In the current study, the CL cheese (high Ca, 27.7 mg/g of protein) was salted at pH 5.3, while the DA1 cheese (low Ca, 21.7 mg/g of protein) was salted at pH 5.6; the pH of the CL and the DA1 at d 1 was 5.42 and 5.96, respectively. Hence, the reduction in Ca level in the studies of Metzger et al. (2000 Metzger et al. (2001) was not accompanied by an increase in moisture content probably because of the low pH of the curd during plasticization, which would offset the effects of lower Ca content on para-casein hydration.

The fat content of the CL cheese was significantly higher than that of the DA cheeses, which had a similar fat content. Moreover, the FDM level of the DA2 and DA3 cheeses was lower than that of the CL and DA1 cheeses, suggesting higher losses of fat to the cheese whey and/or the stretch water during the manufacture of the former cheeses. The fat level in the whey and stretch water was not measured in the current study; hence, no good explanation can be offered for the above trends for fat and FDM. The results of Metzger et al. (2000) showed that preacidification of milk may lead to an increase or a decrease in fat retention in DA cheeses, depending on the type of acid used and the degree of acidification. Further studies are needed to understand the means by which preacidification influences fat retention during the manufacture of DA cheese.

There was a decrease in the level of intact casein in all cheeses during storage (Figure 1); this trend coincided with an increase in the degree of proteolysis, which was described by Feeney et al. (2002). There was a significant effect of the interaction between make procedure and storage time on the level of intact casein (Table 3), with the levels for CL and DA1 being significantly lower than those for DA2 and DA3 at storage times 12 d. The lower levels of intact casein in the former cheeses reflect the lower protein content and the higher degree of proteolysis (Feeney et al., 2002) in the DA1 cheeses.

View larger version (18K): [in this window] [in a new window]   Figure 1. Age-related changes in the concentration of intact casein in Mozzarella cheese made using conventional acidification by starter culture (CL, ) or direct acidification (DA1, ; DA2, ; DA3, ). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g protein): 5.42 and 27.7, (•); 5.96 and 21.8, (); 5.93 and 29.6, (); and 5.58 and 28.7, (). Details of make procedure and composition are given in the text. Values presented are the means from three replicate trials.

The relatively large increase in pH between milling and d 1 is noteworthy. This trend has been observed repeatedly in Mozzarella cheesemaking trials at this laboratory and has also been reported by Guo et al. (1997). This increase in pH may be associated with a reduction in the lactate-to-protein ratio (Huffman and Kristofferson, 1984; Fox and Wallace, 1997) and a loss of buffering capacity of the curd (Czulak et al., 1969) due to removal of lactic acid, soluble Ca, and phosphate in the stretch water. Calcium phosphate is a major determinant of the buffering capacity of cheese, which increases as the pH is reduced in the region 6.0 to 5.1 (Lucey and Fox, 1993). Hence, reductions in the lactate-to-protein ratio and in the concentration of calcium phosphate, as occurs during plasticization, are conducive to an increase in pH. Moreover, on subsequent cooling of the plasticized curd, micellar calcium phosphate dissolves and may result in an increase in pH due to inactivation of H⁺ by the phosphate anion. Differences in the temperature of the curd at which the pH is measured probably also contributed to the increase in pH noted between milling (curd at 39°C) and 1 d (curd at 21°C), as it would affect the equilibrium concentrations of soluble and colloidal phosphate (Walstra and Jenness, 1985; Dagleish and Law, 1989). The pH of cheese curd at 21°C increases also as a result of salt addition, from 5.4 at 0% wt/wt salt to 5.53 at 2.4% wt/wt salt (Guinee, unpublished results).

In contrast to the above, the results of Metzger et al. (2001) indicate that the pH of plasticized curd at 1 d was lower (0.1 to 0.2 U) than that at salting. The discrepancies between the results of the current study and those of the latter may be attributable to differences in make procedure (i.e., differences in the ratio of stretch water to curd, salt content of the curd) that could affect the ratio of buffering substances (e.g., phosphate and protein) to lactate (Lucey and Fox, 1993). Further study is required on the factors that affect the pH of cheese curd.

The pH of the cheese increased gradually during storage, typically by 0.1 to 0.16 U at 70 d (Figure 1). There was a significant effect of the interaction between make procedure and storage time on pH, with the rate of increase during storage being highest for the DA3 and DA4 cheeses and lowest for DA2 (Table 3, Figure 2). The increase in pH during storage was noted by Metzger et al. (2001) for low fat, and by Guo et al. (1997) for full fat in Mozzarella cheeses made using a starter culture. The increase in pH may be associated with the gradual increase in para-casein hydration and the increased availability of various protein residues (e.g., - and -carboxyl groups of aspartic and glutamatic acids), which combine with H+ during storage and thereby reduce the hydrogen ion activity of the moisture phase of the cheese. In turn, the increase in para-casein hydration may be affected by changes in the equilibrium concentrations of soluble and colloidal calcium phosphate (Guo et al., 1997), migration of salt in moisture in contact with the protein phase (e.g., ice structure water, imbibed water; Geurts et al., 1974a, b), and proteolysis.

View larger version (18K): [in this window] [in a new window]   Figure 2. Age-related changes in pH of Mozzarella cheese made using conventional acidification by starter culture (CL, ) or direct acidification (DA1, ; DA2, ; DA3, ). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g protein): 5.42 and 27.7, (•); 5.96 and 21.8, (); 5.93 and 29.6, (); and 5.58 and 28.7, (). Details of make procedure and composition are given in the text. Values presented are the means from three replicate trials.

The NESC was significantly affected by the interaction between make procedure and storage time (Figure 3a; Table 3), with the rate of increase being highest for CL and DA3. At most storage times 15 d, the NESC of DA2 was significantly lower than that of DA3 and numerically lower than that of CL or DA1. At first approximation, it might be expected that the NESC would be highest in DA1, owing to its low calcium concentration and expected higher level of casein hydration (Sood et al., 1979). However, the protein content of the DA1 cheese was significantly lower than that of the other cheeses, a factor that would contribute to the relatively low level of NESC; the protein matrix is the main structural element responsible for holding the water (McMahon et al., 1999). The level of NESP in the DA1 was significantly higher than that in the other cheeses throughout storage, suggesting a higher WHC by its protein phase (Figure 3b). The NESP was significantly affected by the interaction between make procedure and storage time (Table 3), with the differences between DA1 and other cheeses decreasing during storage. While the NESP of DA2 did not differ significantly from that of CL or DA3, the level in DA2 was numerically lower at times 12 d.

View larger version (20K): [in this window] [in a new window]   Figure 3. Age-related changes in the level of nonexpressible serum, expressed as g/100 g of cheese (a) or as g/g of protein (b) in Mozzarella cheese made using conventional acidification by starter culture (CL, •) or direct acidification (DA1, ; DA2, ; DA3, ). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g protein): 5.42 and 27.7, (•); 5.96 and 21.8, (); 5.93 and 29.6, (); and 5.58 and 28.7, (). Details of make procedure and composition are given in the text.

View larger version (147K): [in this window] [in a new window]   Figure 4. Confocal scanning laser micrographs showing protein (a to d) and fat (e to h) as light areas against a dark background in 1-d-old, unheated Mozzarella cheeses made using conventional acidification by starter culture (a, e) or direct acidification (b to d, f to g). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g of protein): 5.42 and 27.7, (CL; a, e); 5.96 and 21.8, (DA1; b, f); 5.93 and 29.6, (DA2; c, g); and 5.58 and 28.7, (DA3; d, h). Details of make procedure and composition are given in the text. Bar corresponds to 25 µm.

The microstructure of the 20-d-old CL, DA2, and DA3 cheeses indicated that during storage at 4°C there was a loss of identity of the protein fibers and a swelling of the protein matrix, which became more continuous. Concomitant with the change in the protein matrix, the fat was in the form of uniformly dispersed droplets and pools, which were generally smaller than those present at 1 d. The swelling of the protein phase suggests an increase in protein hydration (Kindstedt and Guo, 1997b), and is consistent with increase in the NESP. The age-related transition to a more uniform distribution of fat droplets and pools suggests the swelling of the protein phase into regions previously occupied by fat and serum (Kindstedt and Guo, 1997b; McMahon et al., 1999; McMahon and Oberg, 1999).

At 1 d, the microstructure of the DA1 cheese differed markedly from that of the other cheeses. The protein phase of the former was less fibrous and more swollen, with an appearance similar to that of the CL, DA2, and DA3 cheeses at 20 d. Similarly, the fat particles in the DA1 cheese at 1 d appeared to be less elongated than those in the other cheeses at this time. The greater degree of protein swelling in the 1-d-old DA1 cheese is consistent with a greater degree of casein hydration, as reflected by its significantly higher NESP at 1 d (Figure 3). These observations are consistent with those of McMahon and Oberg (1999), who showed a reduction in the calcium content of Mozzarella led to a decrease in the number of pools of expressible serum and an increase in the volume fraction of the protein matrix.

After aging for 20 d, the microstructural differences between the DA1 and other cheeses diminished, a trend which is consistent with the reduction in the differences in the magnitude of the NESP between the cheeses at times >6 d.

Cheese Firmness
While firmness was not significantly affected by storage time, there was a numerical decrease in the firmness of all cheeses during storage (Figure 6). The decrease, which concurs with the results of previous studies on Mozzarella (Yun et al., 1993; Guinee et al., 2000b), is consistent with the decrease in the content of intact casein (Guinee et al., 2000c).

View larger version (14K): [in this window] [in a new window]   Figure 6. Age-related changes in the firmness of Mozzarella cheese made using conventional acidification by starter culture (CL, •) or direct acidification (DA1, ; DA2, ; DA3, ). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g of protein): 5.42 and 27.7, (•); 5.96 and 21.8, (); 5.93 and 29.6, (), and 5.58 and 28.7, (). Details of make procedure and composition are given in the text. Values presented are the means from three replicate trials.

Functionality of Heated Cheese
The functional characteristics of the different cheeses changed during storage, the extent of the change being dependent on the make procedure (Figures 7 and 8). Storage of the CL, DA1, and DA3 cheeses was paralleled by increases in flowability and stretchability and a reduction in melt time. In agreement with previous studies (Yun et al., 1993, 1995; Kindstedt, 1995; Guinee et al., 2000b), the AV of the CL, DA1, and DA3 cheeses decreased during storage. The AV of the DA2 cheeses exceeded the limit of the test method (1000 Pa•s) at all times and was markedly higher than that of the other cheeses at times 35 d. The age-related decrease in AV may be attributed in part to the decrease in the concentration of intact para-casein (Figure 2) and the increased WHC of the casein (Kindstedt and Guo, 1997b; McMahon et al., 1999), as reflected by the increase in the NESP. Regression analysis of the data revealed significant linear relationships (P < 0.05) between the various functional parameters and the levels of NESP (Figure 9) and intact casein (data not shown). The above trends also concur with those of others (Yun et al., 1993; Madsen and Qvist, 1998) who reported that increased proteolysis in Mozzarella cheese, via the use of rennet with strong proteolytic activity (e.g., Cryphonectria parasitica) or the addition of an exogenous proteinase (e.g., from Bacillus licheniformis or Bacillus subtilis), results in higher flowability and lower AV. The improvement in functionality that accompanies the increases in protein degradation and hydration, and the consequent decrease in protein aggregation, are expected as the flowability, stretchability, and AV of heated cheese may be considered as displacement of contiguous planes of the para-casein matrix.

View larger version (19K): [in this window] [in a new window]   Figure 7. Age-related changes in the melt time (a), flowability (b), and stretchability (c) of Mozzarella cheese made using conventional acidification by starter culture (CL, •) or direct acidification (DA1, ; DA2, ; DA3, ). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g protein): 5.42 and 27.7, (•); 5.96 and 21.8, (); 5.93 and 29.6, (), and 5.58 and 28.7, (). Details of make procedure and composition are given in the text. Values presented are the means from three replicate trials.

View larger version (87K): [in this window] [in a new window]   Figure 8. Age-related changes in the apparent viscosity of Mozzarella cheeses made using conventional acidification by starter culture (CL, ) or direct acidification (DA1, striped; DA2, hatched; DA3, diagonal). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g protein): 5.42 and 27.7, (); 5.96 and 21.8, (striped); 5.93 and 29.6, (hatched); and 5.58 and 28.7, (diagonal). Values > 1000 Pa are indicated by an asterisk (*) at the top of the bar. Details of make procedure and composition are given in the text. Values presented are the means from three replicate trials.

View larger version (15K): [in this window] [in a new window]   Figure 9. Relationship between levels of nonexpressible serum, expressed as g/g of protein and melt time (a), flowability (b), and stretchability (c). The data were obtained from Mozzarella cheeses with different pH and Ca level made using conventional acidification by starter culture or direct acidification and stored at 4°C over a 70-d ripening period; regression lines with intercept shown. Regression was significant (P < 0.05) in all cases.

Changes in Viscoelasticity on Heating
Changes in G' and for the four cheeses, from trial 1, at 1 d are shown in Figure 10; similar trends were noted at different storage times and for the cheeses from trials 2 and 3 (results not shown). Increasing the temperature of the cheese resulted in a decrease in G' and an increase in for all cheeses, to an extent dependent on the make procedure. G' decreased steeply, from a high at 20°C (G'₂₀), as the temperature was raised to 45°C, and thereafter more gradually, to a low at 80°C (G'₈₀). The initial decrease in G' indicates a softening of the cheese, an effect which was probably due mainly to liquefaction of the fat phase, which is fully liquid at 40°C. In contrast, increased gradually from 15 to 20 to 25° as the temperature was raised from 20 to 45°C, and thereafter more steeply to a high value, ₈₀, ranging from 40 to 70°, depending on the make procedure. The changes in G' and marked a phase transition from an unheated cheese, which had a largely elastic rheological response ( 13 to 15° at 20°C), to a melted cheese, which is more fluid in character ( 40 to 60° at 80°C). These changes in viscoelasticity are similar to those previously reported for Cheddar and Mozzarella (Guinee et al., 1999).

View larger version (17K): [in this window] [in a new window]   Figure 10. Storage modulus, G', (a) and phase angle, , (b) as a function of temperature for 1-d-old Mozzarella cheese made using conventional acidification by starter culture (CL, •) or direct acidification (DA1, ; DA2, ; DA3, ). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g of protein): 5.42 and 27.7, (•); 5.96 and 21.8, (); 5.93 and 29.6, (); and 5.58 and 28.7, (). Details of make procedure and composition are given in the text.

The G' versus time and versus time curves for the CL and DA3 cheeses were generally similar. The latter cheeses had values of G'₂₀ intermediate between those of the DA1 and DA2 cheeses, G'₈₀ values which were of similar magnitude (2 to 3 kPa) to those of the DA1, and ₈₀ values which were similar to, or slightly lower than, those of DA1 cheese.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Mozzarella cheeses, which differed in Ca content and pH, were made using a starter culture for the control cheese, CL, or by direct acidification of the milk with lactic acid (DA1 cheese), or by lactic acid and glucono--lactone (DA2 and DA3 cheeses). The study indicated that the pH at setting, whey drainage, and curd plasticization had a marked influence on cheese composition (pH, Ca content, moisture level), level of NESP, WHC of the para-casein matrix, heat-induced changes in viscoelasticity, and storage-related changes in the functionality of the heated cheese. The DA1 cheese, which had a high pH (5.9) and a low Ca concentration (22 mg/g protein), had low levels of total protein (and, therefore, low levels of intact casein), and high levels of moisture, MNFS, and NESP. These characteristics of the DA1 cheese resulted in it having high initial values of flowability and stretchability at 1 d that were comparable to those attained by the control cheese that had been aged for 15 d.

In contrast, raising the pH of Mozzarella with typical levels of Ca (29 mg/g protein) and moisture (49%, wt/wt), from 5.42 or 5.58, as in CL and DA3, respectively, to 5.93, as in DA2, resulted in significantly lower values of flowability and stretchability at storage times 1 d.

The results of a recent survey (Guinee et al., 2000a) indicated that each of the different functional attributes of commercial Mozzarella fall within a range of values, e.g., flowability, 43 to 68%; stretchability, 56 to 134 cm. The results of the current study showed that Ca content and pH markedly influenced the time required for each functional attribute to attain values typical of commercial Mozzarella, i.e., CL, DA1, and DA2 acquired a flowability of 43% after storage for 19, 1, and 79 d, respectively. Thus, alteration of pH and Ca concentration is a convenient means by which the functional shelf life of Mozzarella cheese can be varied.

View larger version (154K): [in this window] [in a new window]   Figure 5. Confocal scanning laser micrographs showing protein (a to d) and fat (e to h) as light areas against a dark background in 20-d-old unheated Mozzarella cheeses made using conventional acidification by starter culture (a, e) or direct acidification (b to d, f to g). The cheese-making procedure was altered to give cheeses of different pH and Ca level (mg/g protein): 5.42 and 27.7, (CL; a, e); 5.96 and 21.8, (DA1; b, f); 5.93 and 29.6, (DA2; c, g); and 5.58 and 28.7, (DA3; d, h). Details of make procedure and composition are given in the text. Bar corresponds to 25 µm.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

Received for publication May 21, 2001. Accepted for publication January 28, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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