Use of high-pressure processing and low-temperature storage to extend the performance shelf-life of two types of String cheese.

The manufacturing method of String cheese is similar to Mozzarella, but the hot curd is extruded through narrow tubes or pipes, which align the protein fibers that provides the characteristic ability for consumers to pull strings from this cheese. Firmness is another important performance attribute for consumers who just bite into the String cheese without peeling off strings. There have only been a few studies on String cheese, but it is known that stringiness and firmness decrease during prolonged storage, which is a particular challenge for exporting String cheese. We explored 2 treatments to try to retain the stringiness and firmness of String cheese for longer storage periods. The techniques used were high pressure processing (HPP; 600 MPa for 3 min) and reduced storage temperature (0°C). In other cheese varieties, these techniques have helped extend the performance shelf-life. We tested these techniques using the 2 main types of commercial String cheese: direct acid (DASC) and cultured String cheese (CSC), that were obtained from 2 different manufacturing facilities. The DASC had higher fat (∼2.2%) and higher pH values (∼0.2 units) compared with the CSC. The CSC had higher protein content (∼3.4%), higher insoluble calcium content (∼8 mg insoluble Ca/g protein) and higher hardness values (∼4 N) compared with the DASC. Due to the compositional differences, the 2 varieties were statistically analyzed separately for all other attributes. In both cheese types, HPP caused an immediate reduction in stringiness, some solubilization of insoluble calcium, and a slight increase in the cheese pH values. HPP also caused a slight increase in the TPA hardness of the CSC samples until 14 d (possibly due to a slight increase in cheese pH). The use of the 0°C storage temperature reduced proteolysis and helped retain firmness during storage. Low temperature storage could help extend the performance shelf-life of String cheese by a couple of months, but HPP was not suitable as the process caused an immediate reduction in stringiness due to the disruption of the matrix induced by the HPP treatment.

Functionality of process cheese made from Cheddar cheese with various rennet levels and high-pressure processing treatments.

Due to its versatility and shelf stability, process cheese is gaining interest in many developing countries. The main structural component (base) of most processed cheese formulations is young Cheddar cheese that has high levels of intact casein. Exporting natural Cheddar cheese base from the United States to distant overseas markets would require the aging process to be slowed or reduced. As Cheddar cheese ripens, the original structure is broken down by proteolysis and solubilization of insoluble calcium phosphate. We explored the effect of varying rennet levels (we also used a less proteolytic rennet) and application of high-pressure processing (HPP) to Cheddar cheese, as we hoped these treatments might limit proteolysis and concomitant loss of intact casein. To try to retain high levels of insoluble Ca, all experimental cheeses were made with a high-draining pH and from concentrated milk. To compare our intact casein results with current practices, we manufactured a Cheddar cheese that was prepared according to typical industry methods (i.e., use of unconcentrated milk, calf chymosin [higher levels], and low draining pH value [∼6.2]). All experimental cheeses were made from ultrafiltered milk with protein and casein contents of ∼5.15% and 4.30%, respectively. Three (low) rennet levels were used: control (38 international milk clotting units/mL of rennet per 250 kg of milk), and 25% and 50% reduced from this level. All experimental cheeses had similar moisture contents (∼37%) and total Ca levels. Four days after cheese was made, half of the experimental samples from each vat underwent HPP at 600 MPa for 3 min. Cheddar cheese functionality was monitored during aging for 240 d at 4°C. Cheddar cheese base was used to prepare process cheese after aging for 14, 60, 120, 180, and 240 d. Loss tangent (LT) values of cheese during heating were measured by small strain oscillatory rheology. Intact casein levels were measured using the Kjeldahl method. Acid or base titrations were used to determine the buffering capacity and insoluble Ca levels as a percentage of total Ca. The LTmax values (an index of meltability) in process cheese increased with aging for all the cheese bases; the HPP treatment significantly decreased LTmax values of both base (natural) and process cheeses. All experimental cheeses had much higher levels of intact casein compared with typical industry-make samples. Process cheese made from the experimental treatments had visually higher stretching properties than process cheese made from Cheddar with the typical industry-make procedure. Residual rennet activity was not affected by rennet level, but the rate of proteolysis was slightly slower with lower rennet levels. The HPP treatment of Cheddar cheese reduced residual rennet activity and decreased the reduction of intact casein levels. The HPP treatment of Cheddar cheese resulted in process cheeses that had slightly higher hardness values, lower LTmax values, and retained higher storage modulus values at 70°C. We also observed that the other make procedures we used in all experimental treatments (i.e., using a less proteolytic chymosin, using a concentrated cheese milk, and maintaining a high draining pH value) had a major effect on retaining high levels of intact casein.

Partial enrichment of phospholipids by enzymatic hydrolysis and membrane filtration of whey protein phospholipid concentrate.

Whey protein phospholipid concentrate (WPPC) contains high amounts of phospholipids (PL; 4.5 ± 1%) but there is interest in further enriching the PL content for nutritional and functional applications. Chemical methods were unsuccessful in separating PL from proteins due to the presence of protein-fat aggregates. Instead, we explored hydrolysis of the proteins to peptides with the objective of removing peptides, thereby concentrating the PL fraction. We used microfiltration (MF) with a pore size of 0.1 µm to help reduce protein/peptide retention. Hydrolyzing proteins should facilitate passage of low molecular weight peptides through the MF membrane, while concentrating fat and PL in the MF retentate. Bench-top experiments were performed to select the proteolytic enzyme that resulted in the most extensive hydrolysis of proteins in WPPC from among 5 different commercial proteases. Sodium dodecyl sulfate-PAGE analysis was performed to measure the extent of protein hydrolysis over a period of 4 h. Alcalase enzyme was found to exhibit the highest proteolytic activity at conditions of pH 8 and temperature 55°C. The intensity of major protein bands (milkfat globule membrane proteins, caseins, ß-lactoglobulin) in WPPC decreased in sodium dodecyl sulfate-PAGE profiles as hydrolysis progressed, along with the appearance of low molecular weight bands. Pilot-scale MF production, coupled with diafiltration (DF), of the hydrolyzed sample aided in the removal of peptides that caused an ~18% reduction in protein content with the final retentate having a total PL content of 9.3% dry basis (db) with protein and fat contents at approximately 43.8 ± 0.4% (db) and 48.9 ± 1.2% (db), respectively. The MF permeate had minimal fat content, indicating that there was no transmission of lipids or PL through the membrane during the MF/DF process. Confocal laser scanning microscopy and particle size analysis of enzyme hydrolyzed solution revealed that protein aggregates were still present after 1 h of hydrolysis. Complete removal of proteins and peptides was not achieved by this process, suggesting that a combination of enzymes would be needed for further hydrolysis of protein aggregates in WPPC solution to further enrich the PL content.

Comparison of micellar casein isolate and nonfat dry milk for use in the production of high-protein cultured milk products.

High protein levels in yogurt, as well as the presence of denatured whey proteins in the milk, lead to the development of firm gels that can make it difficult to formulate a fluid beverage. We wanted to prepare high-protein yogurts and explore the effects of using micellar casein isolate (MCI), which was significantly depleted in whey protein by microfiltration. Little is known about the use of whey protein-depleted milk protein powders for high-protein yogurt products. Microfiltration also depletes soluble ions, in addition to whey proteins, and so alterations to the ionic strength of rehydrated MCI dispersions were also explored, to understand their effects on a high-protein yogurt gel system. Yogurts were prepared at 8% protein (wt/wt) from MCI or nonfat dry milk (NDM). The NDM was dispersed in water, and MCI powders were dispersed in water (with either low levels of added lactose to allow fermentation to achieve the target pH, or a high level to match the lactose content of the NDM sample) or in ultrafiltered (UF) milk permeate to align its ionic strength with that of the NDM dispersion. Dispersions were then heated at 85°C for 30 min while stirring, cooled to 40°C in an ice bath, and fermented with yogurt cultures to a final pH of 4.3. The stiffness of set-style yogurt gels, as determined by the storage modulus, was lowest in whey protein-depleted milk (i.e., MCI) prepared with a high ionic strength (UF permeate). Confocal laser scanning microscopy and permeability measurements revealed no large differences in the gel microstructure of MCI samples prepared in various dispersants. Stirred yogurt made from MCI that was prepared with low ionic strength showed slow rates of elastic bond reformation after stirring, as well as slower increases in cluster particle size throughout the ambient storage period. Both the presence of denatured whey proteins and the ionic strength of milk dispersions significantly affected the properties of set and stirred-style yogurt gels. Results from this study showed that the ionic strength of the heated milk dispersion before fermentation had a large influence on the gelation pH and strength of acid milk gels, but only when prepared at high (8%) protein levels. Results also showed that depleting milk of whey proteins before fermentation led to the development of weak yogurt gels, which were slow to rebody and may be better suited for preparing cultured milk beverages where low viscosities are desirable.

Slower proteolysis in Cheddar cheese made from high-protein cheese milk is due to an elevated whey protein content.

A growing number of companies within the cheese-making industry are now using high-protein (e.g., 4-5%) milks to increase cheese yield. Previous studies have suggested that cheeses made from high-protein (both casein and whey protein; WP) milks may ripen more slowly; one suggested explanation is inhibition of residual rennet activity due to elevated WP levels. We explored the use of microfiltration (MF) to concentrate milk for cheese-making, as that would allow us to concentrate the casein while varying the WP content. Our objective was to determine if reducing the level of WP in concentrated cheese milk had any impact on cheese characteristics, including ripening, texture, and nutritional profile. Three types of 5% casein standardized and pasteurized cheese milks were prepared that had various casein:true protein (CN:TP) ratios: (a) control with CN:TP 83:100, (b) 35% WP reduced, 89:100 CN:TP, and (c) 70% WP reduced, 95:100 CN:TP. Standardized milks were preacidified to pH 6.2 with dilute lactic acid during cheese-making. Composition, proteolysis, textural, rheological, and sensory properties of cheeses were monitored over a 9-mo ripening period. The lactose, total solids, total protein, and WP contents in the 5% casein concentrated milks were reduced with increasing levels of WP removal. All milks had similar casein and total calcium levels. Cheeses had similar compositions, but, as expected, lower WP levels were observed in the cheeses where WP depletion by MF was performed on the cheese milks. Cheese yield and nitrogen recoveries were highest in cheese made with the 95:100 CN:TP milk. These enhanced recoveries were due to the higher fraction of nitrogen being casein-based solids. Microfiltration depletion of WP did not affect pH, sensory attributes, or insoluble calcium content of cheese. Proteolysis (the amount of pH 4.6 soluble nitrogen) was lower in control cheeses compared with WP-reduced cheeses. During ripening, the hardness values and the temperature of the crossover point, an indicator of the melting point of the cheese, were higher in the control cheese. It was thus likely that the higher residual WP content in the control cheese inhibited proteolysis during ripening, and the lower breakdown rate resulted in its higher hardness and melting point. There were no major differences in the concentrations of key nutrients with this WP depletion method. Cheese milk concentration by MF provides the benefit of more typical ripening rates.

Effects of varying casein and pectin concentrations on the rheology of high-protein cultured milk beverages stored at ambient temperature.

Shelf-stable cultured milk beverages that have high protein levels can be difficult to successfully manufacture. With increasing protein level, rapid phase separation and gel formation occur in cultured beverages, which may not be prevented even with the inclusion of stabilizers such as high methoxy (HM) pectin. To limit protein aggregation in cultured milk beverages we investigated micellar casein as an interesting alternative to milk, due to the absence of whey proteins, which can contribute to increased gel strength in cultured products. In this study, micellar casein dispersed in ultrafiltered milk permeate was fermented to pH 4.1, blended with HM pectin, homogenized, thermally processed, and bottled for storage at ambient temperature for 6 mo. Utilizing response surface methodology with a central composite rotatable design, the protein and pectin contents were varied between 5 and 9% and 0.0 and 1.0%, respectively. The elastic modulus, loss tangent, and yield stress of these beverages were measured during storage to observe the extent of bond restructuring, whereas particle size and visual phase separation were measured to determine stability. Response variables were measured initially after thermally processing the beverages, and after 1 and 6 mo of storage at ambient temperature. All samples quickly formed gels after homogenizing, regardless of the pectin level. The stiffness (elastic modulus) of all samples increased throughout storage and was determined mainly by the protein content; however, the growth of elastic bonds over time was slowed with high levels of pectin. At 6 mo of storage, yield stress values were significantly lower for beverages with <7.5% protein when they were stabilized with ≥0.85% pectin. Prediction models for visual phase separation in beverages stored for 6 mo were significantly affected by the protein content, with increasing instability at lower protein levels. Models were used to identify optimal protein (<7.5%) and pectin (≥0.85%) concentrations to minimize the stiffness of gels during ambient storage. Samples in this optimized region were predicted to have low yield stress values and were easily fluidized by gentle shaking of the bottle at 6 mo.

A study of various chemical pretreatments to fractionate lipids from whey protein phospholipid concentrate.

Dairy-derived lipids such as phospholipids (PL) have been gaining interest due to their functional and nutritional properties. Our research goal was to develop a separation process (nonsolvent based) to produce an enriched dairy lipid fraction from whey protein phospholipid concentrate (WPPC). Various chemical pretreatments (i.e., adjustment of pH, calcium, or temperature) were applied to rehydrated commercial WPPC solutions. These treatments were done on a bench-top scale to aid in the precipitation of proteins or PL. The chemically treated solutions were centrifuged and fractionated into the following 3 layers: (1) top fat layer, (2) supernatant in the middle zone, and (3) sediment at the bottom of the centrifuge tubes. The thickness and size of the layers varied with the treatment parameters. Compositional analysis of each layer showed that the proteins, fat, and PL always appeared to fractionate in similar proportions. The proteins in each layer were characterized using sodium dodecyl sulfate-PAGE under reducing and nonreducing conditions. Different proteins including whey proteins, caseins, and milk fat globule membrane proteins and lipoproteins were identified, and no specific type of protein had an affinity for either the top or bottom layer. All types of proteins were present in each of the layers after centrifugation, and there were no major differences in fractionation of the proteins between layers with respect to the chemical treatment applied. The microstructure of protein and fat in WPPC was investigated using confocal laser scanning microscopy. Dual staining of the rehydrated WPPC solution with Fast Green FCF (proteins) and Nile Red (lipids) showed the presence of very large protein aggregates that varied in size from 20 to 150 µm, with fat trapped within these aggregates. The confocal laser scanning microscopy images of liquid WPPC revealed fine strands of a weak protein network surrounding the fat globules. This indicated that there were specific interactions between the proteins, as well as between the fat and proteins in WPPC. Sodium dodecyl sulfate treatment was performed to understand the nature of the interactions between protein and fat. We found that about 35% of the fat present in WPPC was in the form of free fat, which was only physically entrapped within the protein aggregates. The remaining fat had some form of association with the proteins in WPPC. Other fractionation techniques would be needed to obtain an enriched dairy lipid fraction.

Effect of substituting whey cream for sweet cream on the textural and rheological properties of cream cheese.

In the manufacture of cream cheese, sweet cream and milk are blended to prepare the cream cheese mix, although other ingredients such as condensed skim milk and skim milk powder may also be included. Whey cream (WC) is an underutilized fat source, which has smaller fat droplets and slightly different chemical composition than sweet cream. This study investigated the rheological and textural properties of cream cheeses manufactured by substituting sweet cream with various levels of WC. Three different cream cheese mixes were prepared: control mix (CC; 0% WC), cream cheese mixes containing 25% WC (25WC; i.e., 75% sweet cream), and cream cheese mixes with 75% WC (75WC; i.e., 25% sweet cream). The CC, 25WC, and 75WC mixes were then used to manufacture cream cheeses. We also studied the effect of WC on the initial step in cream cheese manufacture (i.e., the acid gelation process monitored using dynamic small amplitude rheology). Acid gels were also prepared with added denatured whey proteins or membrane proteins/phospholipids (PL) to evaluate how these components affected gel properties. The rheological, textural, and sensory properties of cream cheeses were also measured. The WC samples had significantly higher levels of PL and insoluble protein compared with sweet cream. An increase in the level of WC reduced the rate of acid gel development, similar to the effect of whey phospholipid concentrate added to mixes. In cream cheese, an increase in the level of added WC resulted in significantly lower storage modulus values at temperatures <20°C. Texture results, obtained from instrumental and sensory analyses, showed that high level of WC resulted in significantly lower firmness or hardness values and higher stickiness compared with cream cheeses made with 25WC or CC cream cheeses. The softer, less elastic gels or cheeses resulting from the use of high levels of WC are likely due to the presence of components such as PL and proteins from the native milk fat globule membrane. The use of low levels of WC in cream cheese did not alter the texture, whereas high levels of WC could be used if manufacturers want to produce more spreadable products.

Effects of the depletion of whey proteins from unconcentrated milk using microfiltration on the yield, functionality, and nutritional profile of Cheddar cheese.

Some European dairies use low concentration factor microfiltration (MF) in their cheese plants. Removal of whey protein (WP) from milk before cheesemaking using microfiltration without concentration provides the opportunity to produce a value-added by-product, milk-derived whey. However, few studies have focused on the effects on cheese properties caused by the depletion of WP from cheese milk. Most studies have concentrated cheese milk using MF in addition to depletion of WP. In our approach, cheese milk was not concentrated during WP depletion using MF. We wanted to quantify residual WP levels in cheese made from MF milk and to explore whether WP depletion from milk would influence functionality, nutritional profile, and cheese quality during ripening. Casein (CN) contents for all milks were kept at ∼2.5%, to eliminate the confounding factor of concentration of CN, which was observed in some previous MF studies. Cheese milks had similar ratios of CN to fat. Three standardized milks were produced with various CN:true protein (TP) ratios: (a) control with a CN:TP ratio of 83:100, (b) 35% WP depletion, 89:100 CN:TP, and (c) 70% WP depletion, 95:100 CN:TP. Cheddar cheeses were made from MF milk with various WP depletion levels and aged for 9 mo, and their functionality was evaluated during ripening. We found no major differences in cheese composition or pH values between samples. Cheese yield, solids recovery, and nitrogen recovery were slightly higher in the 95:100 CN:TP cheeses compared with the control. These enhanced recoveries reflect that MF-treated milk started with a higher fraction of CN-based protein solids, rather than WP solids. The standardized milk from the 95:100 CN:TP treatment also had a slightly higher fat content compared with the control, likely helping to increase cheese yield. Rheological properties of cheeses during heating were similar between treatments. Hardness initially decreased with age for all cheeses due to proteolysis or solubilization, or both, of calcium phosphate. Maximum loss tangent (LT), an index of cheese meltability, was slightly lower for the control cheese until 30 d of ripening, but after 30 d, all treatments exhibited similar maximum LT values. The temperature where LT = 1 (crossover temperature), an index of softening point during heating, was slightly lower for MF cheese compared with the control cheeses during ripening. Microfiltration treatment had no significant influence on proteolysis. Sensory properties were similar between the cheeses, except for bitterness. Bitterness intensity was slightly lower in the MF cheeses than in the control cheeses and increased in all cheeses during ripening. We detected no major differences in the concentrations of key nutrients or vitamins between the various cheeses. Depletion of WP in cheese milk by MF did not negatively affect cheese quality, or its nutritional profile, and resulted in similar cheesemaking yields.

Behavior of stabilizers in acidified solutions and their effect on the textural, rheological, and sensory properties of cream cheese.

Stabilizers are routinely added during cream cheese manufacture to help prevent syneresis during storage. We investigated how different types of stabilizers affected the texture, rheology, and sensory properties of cream cheese. Cream cheeses were manufactured with 0.33% xanthan gum (XG), locust bean gum (LBG), guar gum (GG), or a combination (CBN) of these 3 stabilizers (0.11% of each). Rheological properties of solutions of the individual stabilizers and their combination (equal amounts) were also determined under conditions similar to the aqueous phase of cream cheese (0.6% gum, 1.8% NaCl, and pH 5). Dynamic small amplitude rheological properties of the cream cheeses were measured during heating from 5 to 80°C at the rate of 1°C/min and cooling at the same rate (because most cream cheese is hot packed/filled before cooling). Measured rheological parameters included storage modulus (G') and loss tangent. Hardness of cream cheeses was determined by texture profile analysis. Quantitative spectrum descriptive sensory analysis was also performed. Distinct differences were observed between the rheological properties of solutions of the individual stabilizers and the CBN containing all the stabilizers. Results showed that CBN solution formed a strong, thermally reversible gel due to synergistic interaction between stabilizers, whereas XG solution formed a weak gel that was not greatly affected by temperature. Solutions of LBG and GG behaved rheologically as entangled polymer solutions. In the high-temperature (>35°C) region, cream cheeses made with XG and CBN showed higher G' values compared with other cream cheeses. The G' values were higher for XG- and CBN-stabilized cream cheeses than LBG- and GG-stabilized cream cheeses at several temperature regions during the cooling cycle. The CBN-stabilized cream cheeses had higher hardness values than the cream cheeses manufactured with the individual stabilizers. Differences were observed between the sensory attributes of cream cheeses stabilized with CBN and those made with individual stabilizers. At low temperatures, the higher hardness and G' values of CBN-stabilized cream cheeses could be due to synergistic interaction between XG and galactomannans. The higher elasticity of XG-stabilized cream cheeses at high temperatures could be due to its higher thermal stability. This study showed that the stabilizers added during manufacture of cream cheese affected its texture, rheological, and sensory properties.

Low- and reduced-fat milled curd, direct-salted Gouda cheese: Comparison of lactose standardization of cheesemilk and whey dilution techniques.

Control of acidity is critical for cheese quality, as high acidity can be associated with poor flavor and textural attributes. We investigated an alternative method to control cheese acidity, specifically in low-fat (LF) and reduced-fat (RF) milled curd, direct-salted Gouda cheese, which involved altering the initial lactose content of cheesemilk. In traditional Gouda cheese manufacture, a critical technique to control acidity is whey dilution (WD); that is, partial removal of whey and its replacement with water. Direct standardization of the lactose content of milk during the ultrafiltration process could be a simpler and more effective technique to control cheese acidity. This study compared the effect of traditional WD at 2 different levels, 15 and 30% (WD15 and WD30), with the alternative approach of adjustment of the lactose content of milk using low-concentration-factor ultrafiltration (LCF-UF). The composition, texture, functionality, and sensory properties of these LF and RF Gouda cheeses were evaluated. A milled curd, direct-salted cheese manufacturing protocol was used. Milks used for cheesemaking had a lactose-to-casein (L:CN) ratio of approximately 1.8, which is the typical ratio found in milk, whereas milks prepared with lactose standardization (LS) were made from UF concentrated milks with water added during filtration to achieve a L:CN ratio of approximately 1.1. Cheeses made with LS exhibited lower lactose and lactic acid contents than WD30 and WD15, leading to significantly higher pH values in the cheese. Dynamic small-amplitude oscillatory rheology indicated that use of LS led to cheeses with a lower crossover temperature (melting point) than the cheeses made with WD. Cheeses made with LS had lower insoluble Ca contents, likely caused by the addition of water required to achieve the lower L:CN ratio in these milks. Sensory analysis also indicated that LS cheeses had lower acidity and softer texture. These results suggest that standardization of the L:CN ratio of milk could be a useful alternative to WD (or a curd rinse step) to reduce acidity in cheeses. In addition, LS could be used to help soften texture and increase meltability, if desired in lower-fat cheese types.

Effect of processing methods and protein content of the concentrate on the properties of milk protein concentrate with 80% protein.

In recent years, a large increase in the production of milk protein concentrates (MPC) has occurred. However, compared with other types of milk powders, few studies exist on the effect of key processing parameters on powder properties. In particular, it is important to understand if key processing parameters contribute to the poor solubility observed during storage of high-protein MPC powders. Ultrafiltration (UF) and diafiltration (DF) are processing steps needed to reduce the lactose content of concentrates in the preparation of MPC with a protein content of 80% (MPC80). Evaporation is sometimes used to increase the TS content of concentrates before spray drying, and some indications exist that inclusion of this processing step may affect protein properties. In this study, MPC80 powders were manufactured by 2 types of concentration methods: membrane filtration with and without the inclusion of an evaporation step. Different concentration methods could affect the mineral content of MPC powders, as soluble salts can permeate the UF membrane, whereas no mineral loss occurs during evaporation, although a shift in calcium equilibrium toward insoluble forms may occur at high protein concentration levels. It is more desirable from an energy efficiency perspective to use higher total solids in concentrates before drying, but concerns exist about whether a higher protein content would negatively affect powder functionality. Thus, MPC80 powders were also manufactured from concentrates that had 3 different final protein concentrations (19, 21, and 23%; made from 1 UF retentate using batch recirculation evaporation, a similar concentration method). After manufacture, powders were stored for 6 mo at 30°C to help understand changes in MPC80 properties that might occur during shelf-life. Solubility and foaming properties were determined at various time points during high-temperature powder storage. Inclusion of an evaporation step, as a concentration method, resulted in MPC80 that had higher ash, total calcium, and bound calcium (of rehydrated powder) contents compared to concentration with only membrane filtration. Concentration method did not significantly affect the bulk (tapped) density, solubility, or foaming properties of the MPC powders. Powder produced from concentrate with 23% protein content exhibited a higher bulk density and powder particle size than powder produced from concentrate that had 19% protein. The solubility of MPC80 powder was not influenced by the protein content of the concentrate. The solubility of all powders significantly decreased during storage at 30°C. Higher protein concentrations in concentrates resulted in rehydrated powders that had higher viscosities (even when tested at a constant protein concentration). The protein content of the concentrate did not significantly affect foaming properties. Significant changes in the mineral content are used commercially to improve MPC80 solubility. However, although the concentration method did produce a small change in the total calcium content of experimental MPC80 samples, this modification was not sufficiently large enough (<7%) to influence powder solubility.

Effects of processing conditions on the texture and rheological properties of model acid gels and cream cheese.

Manufacture of cream cheese involves the formation of an initial acid-induced gel made from high-fat milk, followed by a series of processing steps including shearing, heating, and dewatering that complete the conversion of the acid gel into a complex cheese product. We investigated 2 critical parameters for their effect on the initial gel: homogenization pressure (HP) of the high-fat cheese milk, and fermentation temperature (FT). The impact of a low (10 MPa) and high (25 MPa) HP, and low (20°C) and high (26°C) FT were investigated for their effects on rheological and textural properties of acid-induced gels. Intact acid gels were sheared and heated to 80°C, and then their rheological properties were analyzed to help understand the effect of shearing/heating processes on the gel characteristics. The effect of HP on fat globule size distribution and the amount of protein not involved in emulsion droplets (i.e., in the bulk phase) were also studied. For cream cheese trials, a central composite experimental design was used to explore the effect of these 2 parameters (HP and FT) on the texture, rheology, and sensory properties of experimentally manufactured cream cheese. Storage modulus (G') and hardness values of cream cheeses were obtained from small amplitude oscillatory rheology tests and texture profile analysis, respectively. Quantitative spectrum descriptive sensory analysis was also performed. Consistency of acid gels (measured using a penetration test) increased with an increase in FT and with an increase in HP. Although stiffer acid-induced gels were formed at high FT, after the heating and shearing processes the apparent viscosity of the samples formed at high FT was lower than those formed at low FT. For the cream cheeses, significant prediction models were obtained for several rheological and textural attributes. The G' values at 8°C, instrumental hardness, and sensory firmness attributes were significantly correlated (r > 0.84); all these attributes significantly decreased with an increase in FT, and HP was not a significant parameter in the prediction models developed for these attributes. Significant interactions were observed between the HP and FT terms for these prediction models. Higher HP increased the amount of protein adsorbed at interface of fat globules but decreased bulk phase protein content (which may be important for crosslinking this gelled emulsion system). At higher FT temperature, coarser gel networks were likely formed. The combined effect of a coarser acid gel network at high FT, and less bulk phase casein available for crosslinking the acidified emulsion gel with an increase in HP, could have contributed to the lower stiffness/firmness observed in cream cheese made under conditions of both high FT and high HP. Stickiness of cream cheese greatly increased under conditions of high FT and high HP, whereas the sensory attributes cohesiveness of mass and difficulty to dissolve decreased. This study helped to better understand the complex relationships between the initial acid-induced gel phase and properties of the (final) cream cheese.

Investigating the properties of high-pressure-treated, reduced-sodium, low-moisture, part-skim Mozzarella cheese during refrigerated storage.

We proposed that the performance and sensory properties of reduced-Na, low-moisture, part-skim (LMPS) Mozzarella cheese could be extended by the application of high hydrostatic pressure (HHP) to cheese postmanufacture and thereby decrease microbial and enzymatic activity. Fermentation-produced camel chymosin was also used as a coagulant to help reduce proteolysis during storage. Average composition of the LMPS Mozzarella cheeses was 48.6 ± 0.6% moisture, 22.5 ± 0.4% fat, 24.5 ± 0.6% protein, and 1.0 ± 0.1% NaCl. Blocks of cheeses were divided into 3 groups randomly after manufacture and stored at approximately 4°C for 20 wk. The control group was not HHP treated. Two weeks after manufacture, 2 groups of cheese samples were treated with HHP at 500 or 600 MPa for 3 min and then returned to storage at approximately 4°C. Analysis was performed during 20 wk of storage after cheese manufacture. Texture profile analysis (TPA) and dynamic low-amplitude oscillatory rheology were used to monitor cheese functionality. Quantitative descriptive analysis was conducted with 9 trained panelists using a 15-point scale to evaluate texture and flavor attributes of unmelted cheese as well as cheeses melted on pizzas. Pressure treatments at 500 and 600 MPa resulted in approximately 1 and 2 log reduction in the numbers of starter culture, respectively, compared with the control when measured 1 d after HHP treatment. Starter numbers continued to decrease in all cheeses over the 20 wk of storage, but the decrease was larger in the HHP-treated cheeses. Even though the initial numbers of nonstarter lactic acid bacteria were the same in all cheeses, the numbers of these bacteria increased faster in the control cheeses. High-pressure treatment of LMPS Mozzarella cheese resulted in an initial (1 d after HHP treatment) increase in pH, but by 2 wk after HHP treatment there was no statistical difference in pH values between control and HHP-treated samples. Immediately after treatment, HHP-treated cheeses exhibited significantly lower TPA and sensory (unmelted) hardness. However, by 14 wk after pressure treatment, the 600-MPa HHP-treated cheese had significantly higher TPA compared with control or 500-MPa HHP-treated cheeses. Sensory panels also indicated that by 14 wk after HHP treatment, the 600-MPa treated samples were significantly firmer than the control or 500-MPa treated cheeses. Compared with control cheese, cheeses treated at 600 or 500 MPa exhibited lower water-soluble nitrogen values at 6 and 10 wk after pressure treatment, respectively. By 10 wk after pressure treatment, the levels of intact αS1-casein were significantly higher in all HHP-treated cheeses compared with the control. Pizza sensory panels indicated that 600-MPa treated cheese was significantly chewier and exhibited lower blister quantity and higher strand thickness compared with control cheeses. High hydrostatic pressure treatment of low-Na, LMPS Mozzarella cheese could result in the extension of its desired baking characteristics when the cheese is stored at refrigerated temperature.

Characterization of the chemical structures and physical properties of exopolysaccharides produced by various Streptococcus thermophilus strains.

Exopolysaccharides (EPS) produced by some lactic acid bacteria are often used by the dairy industry to improve the rheological and physical properties of yogurt, but the relationship between their structure and functional effect is still unclear. The EPS from different species, or different strains from the same species, may differ in terms of molar mass, repeating unit structure, and EPS yield during fermentation of milk. This study aimed to characterize the detailed properties of EPS produced from 7 strains of Streptococcus thermophilus, which is one of the key cultures used for yogurt manufacture. Milk was fermented with strains DGCC 7698, DGCC 7710, DGCC 7785, ST-10255y, St-143, STCth-9204, and ST4239. These strains were selected because they have been used in previous studies on yogurt texture, but a complete description of their EPS structural properties has not yet been reported. All strains were fermented under a similar acidification rate by adjusting the level of supplementation with peptone or the inoculation level, which allowed for a comparison of EPS yields under similar growth conditions (reconstituted skim milk at 40°C). The EPS from each strain was isolated and the weight-average molar mass and z-average root mean square radius determined using size-exclusion chromatography multiangle laser light scattering. The monosaccharide composition of EPS was determined using gas chromatography-mass spectrometry, and repeating unit structure was determined using nuclear magnetic resonance spectroscopy. The weight-average molar mass values of EPS ranged from 0.14 to 1.61 × 106 g/mol. All 7 EPS samples were uncharged. The strains ST-10255y and ST4239 had EPS with the same repeating unit structure. The monosaccharide compositions of the various EPS were mainly composed of glucose and galactose, with low levels of rhamnose in the EPS isolated from DGCC 7698, and N-acetylgalactosamine in the EPS from DGCC 7785, ST-10255y, and ST4239. The yields of EPS (measured when fermented milks reached pH 4.6) ranged from 8.0 to 76.4 mg of glucose equivalents/kg. In addition to (free) EPS, some strains were also able to produce capsular polysaccharide (associated with the bacterial cells) when observed with negative staining technique. The results of our study will help the dairy industry to better understand the mechanism by which different strains of Streptococcus thermophilus affect yogurt texture.

Effect of standardizing the lactose content of cheesemilk on the properties of low-moisture, part-skim Mozzarella cheese.

The texture, functionality, and quality of Mozzarella cheese are affected by critical parameters such as pH and the rate of acidification. Acidification is typically controlled by the selection of starter culture and temperature used during cheesemaking, as well as techniques such as curd washing or whey dilution, to reduce the residual curd lactose content and decrease the potential for developed acidity. In this study, we explored an alternative approach: adjusting the initial lactose concentration in the milk before cheesemaking. We adjusted the concentration of substrate available to form lactic acid. We added water to decrease the lactose content of the milk, but this also decreased the protein content, so we used ultrafiltration to help maintain a constant protein concentration. We used 3 milks with different lactose-to-casein ratios: one at a high level, 1.8 (HLC, the normal level in milk); one at a medium level, 1.3 (MLC); and one at a low level, 1.0 (LLC). All milks had similar total casein (2.5%) and fat (2.5%) content. We investigated the composition, texture, and functional and sensory properties of low-moisture, part-skim Mozzarella manufactured from these milks when the cheeses were ripened at 4°C for 84d. All cheeses had similar pH values at draining and salting, resulting in cheeses with similar total calcium contents. Cheeses made with LLC milk had higher pH values than the other cheeses throughout ripening. Cheeses had similar moisture contents. The LLC and MLC cheeses had lower levels of lactose, galactose, lactic acid, and insoluble calcium compared with HLC cheese. The lactose-to-casein ratio had no effect on the levels of proteolysis. The LLC and MLC cheeses were harder than the HLC cheese during ripening. Maximum loss tangent (LT), an index of cheese meltability, was lower for the LLC cheese until 28d of ripening, but after 28d, all treatments exhibited similar maximum LT values. The temperature where LT=1 (crossover temperature), an index of softening point during heating, was higher for MLC and LLC cheese at 56 and 84d of ripening. The LLC cheese also had lower blister color and less stretch than MLC and HLC cheese. Adjusting the lactose content of milk while maintaining a constant casein level was a useful technique for controlling cheese pH, which affected the texture, functionality, and sensory properties of low-moisture, part-skim Mozzarella cheese.

Effect of tetrasodium pyrophosphate concentration and cooking time on the physicochemical properties of process cheese.

Tetrasodium pyrophosphate (TSPP) is widely used as an emulsifying salt (ES) in process cheese. Previous reports have indicated that TSPP exhibits some unusual properties, including the gelation of milk proteins at specific ES concentrations. We studied the effect of various concentrations (0.25-2.75%) of TSPP and cooking times (0-20min) on the rheological, textural, and physical properties of pasteurized process Cheddar cheese using a central composite rotatable experimental design. Cheeses were made with a constant pH value to avoid pH as a confounding factor. Modeling of the textural properties of process cheese made with TSPP exhibited complex behavior, with polynomial models (cubic) giving better predictions (higher coefficient of determination values) than simpler quadratic models. Meltability indices (degree of flow from the UW MeltProfiler (University of Wisconsin-Madison), loss tangent value at 60°C from rheological testing, and Schreiber melt area) initially decreased with increasing TSPP concentrations, but above a critical ES concentration (~1.0%) meltability increased at higher TSPP concentrations. The storage modulus values measured at 70°C for process cheese initially increased with increasing TSPP concentration, but above a concentration of 1% ES, the storage modulus values decreased. Cooking time had little effect on the various melting or rheological properties. With an increase in TSPP concentration, the insoluble Ca and P contents increased, suggesting that TSPP addition resulted in the formation of insoluble calcium pyrophosphate complexes; some of which were likely associated with caseins. A portion of the added TSPP remained in the soluble phase. The acid-base buffering profiles also indicated that calcium pyrophosphate complexes were formed in cheese made with TSPP. In milk systems, low levels of TSPP have been shown to induce protein crosslinking and gelation, whereas at higher TSPP concentrations milk gelation was inhibited due to excessive charge repulsion from these calcium pyrophosphate complexes. We hypothesized that a similar phenomenon was occurring in our process cheese, resulting in the initial reduction in meltability with TSPP addition due to protein crosslinking, but at higher TSPP levels meltability increased due to excessive charge repulsion.

Angiotensin-I-converting enzyme-inhibitory peptides in commercial Wisconsin Cheddar cheeses of different ages.

Bioactive peptides, including angiotensin-I-converting enzyme-inhibitory (ACEI) peptides, were investigated in commercially produced Wisconsin Cheddar cheeses that ranged in age from ≤ 6d to more than 2 yr. The ACEI activity of cheese was determined in water-soluble extracts (WSE) that were fractionated for components with molecular weight (MW) ≤ 3,000 Da, and peptides identified using HPLC and tandem mass spectrometry. The number of types of bioactive peptides increased with an increase in ripening time. Six of the identified ACEI peptides, Ile-Pro-Pro (IPP), Val-Pro-Pro (VPP), Glu-Lys-Asp-Glu-Arg-Phe (EKDERF), Val-Arg-Tyr-Leu (VRYL), Tyr-Pro-Phe-Pro-Gly-Pro-Ile-Pro-Asn (YPFPGPIPN), and Phe-Phe-Val-Ala-Pro (FFVAP), with known high ACEI activity (low IC50 values, the concentration needed to inhibit ACE to 50% of its original activity) were synthesized and used to quantify the amounts of these peptides in various cheese extracts. The concentrations of these 6 ACEI peptides increased up to a certain stage of ripening. The maximum contents of IPP, VPP, and EKDERF were 2.8, 7.4, and 5.3mg/100 g of cheese, respectively, and these levels were found in a 1-yr-old Cheddar cheese sample. The maximum content of VRYL (7.5mg/100 g of cheese) was found in a 2-yr-old Cheddar cheese sample, whereas the maximum content of YPFPGPIPN (6.8 mg/100 g of cheese) was found in a 6-mo-old Cheddar cheese sample. Trace amounts of FFVAP were found in these cheeses. Aged Cheddar cheese was found to be a rich source of ACEI peptides even though large differences exist between cheeses from different manufacturers.

Low-sodium Cheddar cheese: Effect of fortification of cheese milk with ultrafiltration retentate and high-hydrostatic pressure treatment of cheese.

Low-sodium cheeses often exhibit an acidic flavor due to excessive acid production during the manufacturing and the initial stage of ripening, which is caused by ongoing starter culture activity facilitated by the low salt-in-moisture levels. We proposed that this excessive starter-induced acidity could be prevented by the fortification of cheese milk with ultrafiltration (UF) retentates (to increase curd buffering), and by decreasing microbial activity using the application of high-hydrostatic pressure (HHP) treatment (that is, to reduce residual starter numbers). Camel chymosin was also used as a coagulant to help reduce bitterness development (a common defect in low-sodium cheeses). Three types of low-Na (0.8% NaCl) Cheddar cheeses were manufactured: non-UF fortified, no HHP applied (L-Na); UF-fortified (cheese milk total solids = 17.2 ± 0.6%), no HHP applied (L-Na-UF); and UF-fortified, HHP-treated (L-Na-UF-HHP; 500 MPa for 3 min applied at 1 d post-cheese manufacture). Regular salt (2% NaCl) non-UF fortified, non-HHP treated (R-Na) cheese was also manufactured for comparison purposes. Analysis was performed at 4 d, 2 wk, and 1, 3, and 6 mo after cheese manufacture. Cheese functionality during ripening was assessed using texture profile analysis and dynamic low-amplitude oscillatory rheology. Sensory Spectrum and quantitative descriptive analysis was conducted with 9 trained panelists to evaluate texture and flavor attributes using a 15-point scale. At 4 d and 2 wk of ripening, L-Na-UF-HHP cheese had ~2 and ~4.5 log lower starter culture numbers, respectively, than all other cheeses. Retentate fortification of cheese milk and HHP treatment resulted in low-Na cheeses having similar insoluble calcium concentrations and pH values compared with R-Na cheese during ripening. The L-Na-UF cheese exhibited significantly higher hardness values (measured by texture profile analysis) compared with L-Na cheese until 1 mo of ripening; however, after 1 mo, all low-Na cheeses exhibited similar hardness values, which were significantly lower than R-Na cheese. Pressure treatment significantly increased maximum loss tangent (meltability) from rheology testing and decreased melt temperature. Sensory results indicated only very slight bitterness (<2.5 out of 15-point scale) was detected in all cheeses during the 6 mo of ripening. The L-Na-UF-HHP cheese did not significantly differ in bitterness and acidity from R-Na cheese during ripening. Pressures treatment of cheese at 500 MPa and cheese milk retentate fortification could be used to improve the quality of low-Na cheese.