Pasteurized Process Cheese Research Articles

The effect of emulsifying salts on the turbidity of a diluted milk system with varying pH and protein concentration

Solutions of 10 commonly used emulsifying salts (ES) listed in the Code of Federal Regulations (21CFR133.179) for pasteurized process cheese were tested for their effect on the turbidity of a diluted milk system at different pH and protein concentrations to characterize the conditions that affect micellar structure. Emulsifying salt solutions were made by mixing the ES in a 1-in-20 dilution of water in skim milk ultrafiltrate (3 kDa molecular weight cut-off) to obtain ES concentrations from 0 to 248 mM. Skim milk was added to solutions containing nanopure water, skim milk ultrafiltrate, and a specific ES ranging in concentration from 0 to 248 mM and pH 5, 5.8, 6.8, 7.8, and 8.8. The turbidity of the samples was measured as the optical density at 400 nm immediately after mixing (time, t = 0), after 30 s (t = 30s), and after 30 min (t = 30min). Emulsifying salts were found to cause a decrease in the turbidity of the system, which was modeled using an exponential decay model, where C* represents a threshold salt concentration at which rapid dissociation occurs. At pH values 5.8 and 6.8, the ES caused the greatest decrease in turbidity of the diluted milk system. At pH 5, the ES had the least effect on the turbidity of the system. Sodium hexametaphosphate was found to have the strongest dissociative effect, with a C* value of 0.33 mM for t = 0 at pH 6.8. In contrast, the largest C* value calculated at pH 6.8 was monosodium phosphate at 278.22 mM. Increased time resulted in lower C* values. The model established for this study can be used to predict the dissociation of casein micelles in the presence of various types of ES.

Effect of sodium hexametaphosphate concentration and cooking time on the physicochemical properties of pasteurized process cheese

Sodium hexametaphosphate (SHMP) is commonly used as an emulsifying salt (ES) in process cheese, although rarely as the sole ES. It appears that no published studies exist on the effect of SHMP concentration on the properties of process cheese when pH is kept constant; pH is well known to affect process cheese functionality. The detailed interactions between the added phosphate, casein (CN), and indigenous Ca phosphate are poorly understood. We studied the effect of the concentration of SHMP (0.25–2.75%) and holding time (0–20min) on the textural and rheological properties of pasteurized process Cheddar cheese using a central composite rotatable design. All cheeses were adjusted to pH 5.6. The meltability of process cheese (as indicated by the decrease in loss tangent parameter from small amplitude oscillatory rheology, degree of flow, and melt area from the Schreiber test) decreased with an increase in the concentration of SHMP. Holding time also led to a slight reduction in meltability. Hardness of process cheese increased as the concentration of SHMP increased. Acid-base titration curves indicated that the buffering peak at pH 4.8, which is attributable to residual colloidal Ca phosphate, was shifted to lower pH values with increasing concentration of SHMP. The insoluble Ca and total and insoluble P contents increased as concentration of SHMP increased. The proportion of insoluble P as a percentage of total (indigenous and added) P decreased with an increase in ES concentration because of some of the (added) SHMP formed soluble salts. The results of this study suggest that SHMP chelated the residual colloidal Ca phosphate content and dispersed CN; the newly formed Ca-phosphate complex remained trapped within the process cheese matrix, probably by cross-linking CN. Increasing the concentration of SHMP helped to improve fat emulsification and CN dispersion during cooking, both of which probably helped to reinforce the structure of process cheese.

Effects of pH on the Textural Properties and Meltability of Pasteurized Process Cheese Made with Different Types of Emulsifying Salts

Functional properties of pasteurized process cheese (PPC) made with different types of emulsifying salts (ES) (2%, wt/wt) were investigated as a function of different pH values (from 5.3 to approximately 5.9). The ES investigated were trisodium citrate (TSC), disodium phosphate (DSP), sodium hexametaphosphate (SHMP), and tetrasodium pyrophosphate (TSPP). Meltability and textural properties were determined using UW-MeltProfiler and uniaxial compression, respectively. All PPC samples exhibited an increase in degree of flow (DOF) determined at 45 degrees C when the pH was increased from 5.3 to 5.6, presumably reflecting greater Ca binding by the ES, increased charge repulsion and therefore greater casein dispersion. When the pH of PPC was increased from 5.6 to approximately 5.9, 2 types of ES (DSP and SHMP) exhibited no further increase in DOF at 45 degrees C; while DOF increased in 1 type of PPC (made with TSC) but decreased in another (made with TSPP). TSPP is able to form crosslinks with casein especially in the vicinity of pH 6, which likely restricted melt; in contrast TSC does not crosslink caseins and the increase in pH helped cause greater casein dispersion. Low pH samples (5.3) were not significantly harder than higher pH samples for all ES types but exhibited fracture. The PPC with the highest hardness values at pHs 5.3 and 5.6 were made with TSPP and TSC, respectively. The pH-dependent functional behavior of PPC was strongly influenced by the type of ES and its physicochemical properties including its ability to bind Ca, the possible creation of crosslinks with casein and casein dispersion during cooking.

Rheological, textural and melting properties of commercial samples of some of the different types of pasteurized processed cheese

The detailed rheological and textural properties of commercially available samples of the three major categories of pasteurized processed cheese in the USA, that is, pasteurized process American cheese (PPA), pasteurized process cheese product (PPP) and pasteurized process cheese food (PPF), were analysed. All samples had similar pH values, ranging from 5.65 to 5.87, but most were ∼5.7. Even within each of the categories considerable differences were observed in textural and melting behaviour. The samples with the lowest melt and the lowest loss tangent parameter from the rheological tests were two PPF and one PPA samples. These two PPF samples also had the highest hardness and adhesiveness force. Significant correlations were observed between several textural, rheological and melting properties.

Microstructure of pasteurized process cheese manufactured from vacuum condensed and ultrafiltered milk

Milk standardized to 45 g·kg-1 protein (UF1 and CM1) and 60 g·kg-1 protein (UF2 and CM2) using ultrafiltered milk (150 g·kg-1 protein) or vacuum condensed milk (120 g·kg-1 protein) was used for manufacturing Cheddar cheese. Pasteurized Process cheeses were manufactured using a 1:1 blend of 18-week and 30-week Cheddar cheese. The moisture content of the Process cheeses ranged from 393 to 402 g·kg-1. Fat content was the highest in the control cheese (350 g·kg-1) and the lowest in UF2 (316 g·kg-1). Microstructure of cheeses was observed using cryo-scanning elec- tron microscopy. Fat globules of different sizes embedded in the continuous protein network were observed. Whereas, a porous structure with relatively large pores was noted in the control cheese, more compact protein masses were observed in cheeses made from concentrated milk. Fat globules in all cheeses were surrounded by cavities. Firmness of cheese was associated with less porous (compact) protein network. Large areas of dense highly fused protein were observed in UF2 chee- ses, which showed the highest resistance to compression (highest firmness). Fractures in the protein network were observed as the firmness of cheese increased. Such fractures reduced the ability of protein network to entrap fat and increased the level of free oil. Appendages connecting fat globules to protein network were seen in cheese containing low amounts of free oil which indicated good emulsifying properties. The continuous less rigid protein structure with good emulsifying properties (the presence of appendages connecting protein network to fat globules) produced cheese with increased meltability. This study shows that the application of concentrated milks for Cheddar cheese-making influences Process cheese functionality and structure.

Linear Viscoelastic Properties of Regular- and Reduced-Fat Pasteurized Process Cheese During Heating and Cooling

The rheology of process cheese during heating and cooling was examined by measuring the transient and dynamic linear viscoelastic properties of regular fat, lower moisture and an 80% reduced-fat, higher moisture pasteurized process cheese from 10 to 50°C. The dynamic (stress and frequency sweep) and transient (creep and recovery) rheological properties of the reduced-fat process cheese were found to be higher than that of regular-fat process cheese, indicating that fat content changed rheological properties more than moisture content. The temperature-dependent frequency dispersions of storage and loss moduli (dynamic mechanical spectra) were fitted with a power-law model, and master curves (at a reference temperature of 30°C) and shift factors were obtained by shifting the temperature-dependent frequency dispersion of dynamic mechanical spectra. The relaxation spectra (moduli, viscosities and relaxation times) of both cheeses were obtained from the master curves using the generalized Maxwell model and nonlinear regression. The viscosity distribution of corresponding Maxwell model elements were higher for the reduced-fat cheese by a factor of 1.6–4.7 compared to the regular-fat cheese, indicating that the higher moisture content in the reduced-fat process cheese did not loosen the protein matrix or soften the cheese even though higher moisture is recommended to cheese manufacturers in order to compensate for some textural defects in reduced-fat cheeses.

Effect of Trisodium Citrate Concentration and Cooking Time on the Physicochemical Properties of Pasteurized Process Cheese

The effects of the concentration of trisodium citrate (TSC) emulsifying salt (0.25 to 2.75%) and holding time (0 to 20min) on the textural, rheological, and microstructural properties of pasteurized process Cheddar cheese were studied using a central composite rotatable design. The loss tangent parameter (from small amplitude oscillatory rheology), extent of flow (derived from the University of Wisconsin Meltprofiler), and melt area (from the Schreiber test) all indicated that the meltability of process cheese decreased with increased concentration of TSC and that holding time led to a slight reduction in meltability. Hardness increased as the concentration of TSC increased. Fluorescence micrographs indicated that the size of fat droplets decreased with an increase in the concentration of TSC and with longer holding times. Acid-base titration curves indicated that the buffering peak at pH 4.8, which is due to residual colloidal calcium phosphate, decreased as the concentration of TSC increased. The soluble phosphate content increased as concentration of TSC increased. However, the insoluble Ca decreased with increasing concentration of TSC. The results of this study suggest that TSC chelated Ca from colloidal calcium phosphate and dispersed casein; the citrate-Ca complex remained trapped within the process cheese matrix. Increasing the concentration of TSC helped to improve fat emulsification and casein dispersion during cooking, both of which probably helped to reinforce the structure of process cheese.

Effect of Vacuum-Condensed or Ultrafiltered Milk on Pasteurized Process Cheese

Milk was concentrated by ultrafiltration (UF) or vacuum condensing (CM) and milks with 2 levels of protein: 4.5% (UF1 and CM1) and 6.0% (UF2 and CM2) for concentrates and a control with 3.2% protein were used for manufacturing 6 replicates of Cheddar cheese. For manufacturing pasteurized process cheese, a 1:1 blend of shredded 18- and 30-wk Cheddar cheese, butter oil, and disodium phosphate (3%) was heated and pasteurized at 74°C for 2min with direct steam injection. The moisture content of the resulting process cheeses was 39.4 (control), 39.3 (UF1), 39.4 (UF2), 39.4 (CM1), and 40.2% (CM2). Fat and protein contents were influenced by level and method of concentration of cheese milk. Fat content was the highest in control (35.0%) and the lowest in UF2 (31.6%), whereas protein content was the lowest in control (19.6%) and the highest in UF2 (22.46%). Ash content increased with increase in level of concentration of cheese milk with no effect of method of concentration. Meltability of process cheeses decreased with increase in level of concentration and was higher in control than in the cheeses made with concentrated milk. Hardness was highest in UF cheeses (8.45 and 9.90kg for UF1 and UF2) followed by CM cheeses (6.27 and 9.13kg, for CM1 and CM2) and controls (3.94kg). Apparent viscosity of molten cheese at 80°C was higher in the 6.0% protein treatments (1043 and 1208 cp, UF2 and CM2) than in 4.5% protein treatments (855 and 867 cp, UF1 and CM1) and in control (557 cp). Free oil in process cheeses was influenced by both level and method of concentration with control (14.3%) being the lowest and CM2 (18.9%) the highest. Overall flavor, body and texture, and acceptability were higher for process cheeses made with the concentrates compared with control. This study demonstrated that the application of concentrated milks (UF or CM) for Cheddar cheese making has an impact on pasteurized process cheese characteristics.

Comparison of Pilot-Scale and Rapid Visco Analyzer Process Cheese Manufacture

Numerous formulation and processing parameters influence the functionality of process cheese. A small-scale manufacturing and analysis method could be used to evaluate the influence of formulation parameters and processing conditions on the functionality of process cheese. The objective of this study was to compare process cheese produced on a small-scale (25g) in a Rapid Visco Analyzer (RVA) to process cheese produced on a pilot-scale (4.5kg) in a Blentech twin-screw pilot-scale cooker (BTS). Three replicates of pasteurized process cheese (PC) and pasteurized process cheese food (PCF) were produced in an RVA and in a BTS. Texture profile analysis (TPA) and the RVA melt test were performed on all PC and PCF produced. There was a significant replication effect on TPA-hardness and hot apparent viscosity of the PC and PCF produced in the RVA and the BTS. The PC and PCF produced in the RVA had significantly higher TPA-hardness and hot apparent viscosity compared with PC and PCF produced in the BTS. The RVA manufacturing time (short vs. long) did not have a significant effect on TPA-hardness values for PC or PCF. However, the long manufacturing time significantly increased hot apparent viscosity for PC and PCF. The RVA was successfully used to manufacture process cheeses; however, differences in the manufacturing profiles and type of cooker influenced the functional properties of the process cheese.

Factors That Contribute to the Botulinal Safety of Reduced-Fat and Fat-Free Process Cheese Products

The effects of fat, type of natural cheese, and adjunct process cheese ingredients were evaluated to determine factors that contribute to the botulinal safety of reduced-fat (RF) process cheese products stored at 30°C. In the first set of experiments, pasteurized process cheese products (PPCPs) were formulated using full-fat (FF) Cheddar, 30% RF Cheddar, or skim milk (SM) cheese as cheese-base types and were standardized to 59% moisture, pH 5.75, 2.8 or 3.2% total salts, and 15 to 19% fat. Subsequent trials evaluated the effect of fat levels and adjunct ingredients in PPCPs made with SM, RF, and FF cheese (final fat levels, less than 1, 13, and 24%, respectively). When fat levels of PPCPs were comparable (15.1, 19.1, and 16.2 for product manufactured with SC, RF, and FF cheese, respectively), botulinal toxin production was delayed for up to 2 days in PPCPs formulated with SM compared with RF or FF cheese; however, the effect was not statistically significant. When fat levels were reduced to less than 1% in SM PPCPs, toxin production was delayed 2 weeks in products made with SM compared with RF or FF cheese manufactured with 13 or 24% fat, respectively. The antibotulinal effect of adjunct ingredients varied among the products manufactured with different fat levels. Sodium lactate significantly delayed toxin production (P < 0.05) for all fat levels tested, whereas β-glucan fat replacer did not delay toxin production. An enzyme-modified cheese used as a flavor enhancer significantly delayed toxin production (P < 0.05) in SM (less than 1% fat) products but had little to no inhibitory effect in RF (13% fat) and FF (24% fat) cheese products. Similarly, monolaurin increased the time to detectable toxin in SM products but was ineffective in RF or FF cheese products. These results verify that RF PPCPs exhibit greater safety than FF products and that safety may be enhanced by using certain adjunct ingredients as antimicrobials.

Estimation and Fortification of Vitamin D3 in Pasteurized Process Cheese

The objective of this study was to develop methods for the estimation and fortification of vitamin D3 in pasteurized Process cheese. Vitamin D3 was estimated using alkaline saponification at 70°C for 30min, followed by extraction with petroleum ether:diethyl ether (90:10vol.vol) and HPLC. The retention time for vitamin D3 was approximately 9min. A standard curve with a correlation coefficient of 0.972 was prepared for quantification of vitamin D3 in unknown samples. In the second phase of the study, pasteurized Process cheeses fortified with commercial water- or fat-dispersible forms of vitamin D3 at a level of 100 IU per serving (28g) were manufactured. There was no loss of vitamin D3 during Process cheese manufacture, and the vitamin was uniformly distributed. No losses of the vitamin occurred during storage of the fortified cheeses over a 9-mo period at 21 to 29°C and 4 to 6°C. There was an approximately 25 to 30% loss of the vitamin when cheeses were heated for 5min in an oven maintained at 232°C. Added vitamin D3 did not impart any off flavors to the Process cheeses as determined by sensory analysis. There were no differences between the water- and fat-dispersible forms of the vitamin in the parameters measured in fortified cheeses.

Factors Affecting Pink Discoloration in Annatto‐colored Pasteurized Process Cheese

ABSTRACTThe effects of cooking temperatures, cooling rates, pH, types of emulsifying salts, age of cheese, and sources of colorant were evaluated. Annatto emulsions showed less stability during continuous heating than annatto solutions or suspensions. Increased cooking temperatures resulted in slight decreases in Hunter a and b values, and overall decreases in hue angles. Emulsifying salt blends with increased amounts of sodium citrate resulted in decreased hue angles. Process cheeses made with more than 25% colored cheese showed lower hue angles than those samples made with uncolored cheese only. Whey powder as a source of added solids in process cheese food promoted discoloration, with acid whey resulting in the most pinking.

Survival of Bacterial Pathogens in Pasteurized Process Cheese Slices Stored at 30°C

Six lots of commercial pasteurized process cheese slices were evaluated for the ability to support the growth of four foodborne pathogens, Listeria monocytogenes, Staphylococcus aureus, Salmonella serotypes, and Escherichia coli O157:H7, during 4 days of storage at 30°C. Individual cheese slices were inoculated separately with each pathogen to yield ca. 103 CFU/g. Slices were packaged in sterile plastic sample bags and stored at 30°C for up to 96 h. Populations of Salmonella serotypes and Escherichia coli O157:H7 decreased an average of 1.3 and 2.1 log10 CFU/g, respectively, by 36 h and Salmonella serotypes decreased an additional 0.6 logi0 CFU/g during the remaining 60 h. Populations of Listeria monocytogenes also decreased, although to a lesser extent, exhibiting approximately a 0.6-log10 CFU/g reduction in 96 h. Staphylococcus aureus levels remained relatively constant during the testing period, and were below levels that support detectable enterotoxin production. The process cheese slices tested allowed survival but did not support rapid growth of S. aureus, whereas populations of L. monocytogenes, E. coli O157:H7, and Salmonella serotypes decreased during the 96-h storage at 30°C.

Growth of Proteolytic Clostridium botulinum in Process Cheese Products: I. Data Acquisition for Modeling the Influence of pH, Sodium Chloride, Emulsifying Salts, Fat Dry Basis, and Temperature

Outgrowth of proteolytic Clostridium botulinum type A and B spores in pasteurized process cheese products was assessed to acquire data for improved models of botulinum stability. High-moisture (58.5%) products were made with different levels of pH (5.45 to 5.9), sodium chloride (1.1 to 2.8%, wt/wt) and citrates or phosphates as emulsifying salts (1.5 to 2%, wt/wt), and held at 15 to 30°C. Supplemental experiments were carried out to address the effect of lactic acid concentration originating from the nonfat and 50% fat dry basis (FDB) cheese raw materials, of moisture (50 to 69%), and of total fat (0.1 to 41%, wt/wt). Colony counts were recorded as substitutes for the traditional times to toxin formation. In the last experimental series a polyclonal ELISA against type A and B toxins was carried out as an alternative to the mouse challenge test. Very low spore levels could lead to detectable toxin formation. Temperature strongly influenced outgrowth. At 18°C outgrowth only occurred in 3 months at favorable aw (0.966) and pH (5.9). At 25°C, outgrowth occurred within one week under favorable conditions. No growth occurred within 3 months when aw and pH were 0.95 and 5.55 respectively. Polyphosphate appeared to be more inhibitory than citrate. Moisture is a frequently used indicator of botulinum stability, but when the FDB deviates from 50%, moisture is actually a poor indicator. Components such as NaCl, emulsifying salts, and lactic acid determine stability. Fat does not contribute to stability. Increased fat levels can reduce moisture without a concomitant increase in stability.

Utilization of cheddar cheese containing nisin as an antimicrobial agent in other foods

Cheddar cheese made with nisin-producing lactococci contained between 400 and 1200 IU of nisin per gram of cheese. Cultures used were Lactococcus lactis ssp. cremoris JS102, a nisin-producing transconjugant developed in the laboratories of Dr. L.L. McKay and Lactococcus lactis ssp. lactis NCDO 1404 obtained from the National Collection of Food Bacteria, Reading, England. Pasteurized process cheese spreads with 53% and 60% moisture and 0, 301 and 387 IU nisin/g were manufactured and inoculated with 2000 spores of Clostridium sporogenes PA 3679 during manufacture. The heat process did not reduce nisin activity in the cheese spreads. The spreads were incubated at 22° and 37°C for 90 days. Spoilage was detected by the presence of gas and/or odor in the packages. The shelf-life of the nisin-containing cheese spreads was significantly greater than that of the control cheese spreads at the lower temperature at both moisture levels, whereas the keeping quality of the higher moisture cheeses at the higher temperature was not significantly different. Club cheese or cold pack cheese spreads with moisture levels of 44% and 60% and 0, 100 and 300 IU nisin/g were made. These cold processed cheese spreads were inoculated with 1000 cfu per g of Listeria monocytogenes V7, Staphylococcus aureus 196E and spores of C. sporogenes PA 3679. Heat shocked spores of PA 3769 at the same number were added to separate lots of the cheese spread. The cold pack cheese spreads were incubated at 23° and 37°C for up to 8 weeks. Samples were taken weekly and analyzed for surviving organisms. Significant reductions in numbers of the non-sporeforming test microbes were noted at both temperatures, at both moisture levels and both levels of nisin. Heat shocking the spores was needed to show reduction in numbers during the storage of the cold pack cheese spreads. The data obtained in this study suggest that the use of nisin-containing cheese as an ingredient in pasteurized process cheese or cold pack cheese spreads could be an effective method of controlling the growth of undesirable microorganisms in these processed foods.

Contribution of Composition, Physicochemical Characteristics and Polyphosphates to the Microbial Safety of Pasteurized Cheese Spreads

Pasteurized process cheese spread was manufactured with moisture contents of 52, 54, 56 and 60%. Three different types of phosphate emulsifier were used, disodium ortho-phosphate and two commercially-available polyphosphates, S9 and S9H. Pasteurized, processed cheese spreads were inoculated with approximately 1 × 104Clostridium botulinum spores/gram cheese in the cook kettle, held 3 min at 80°C, hot-filled into glass containers, and incubated at 30°C. Samples were analyzed over 30 weeks for growth of C. botulinum and toxigenesis. Toxin was first detected in 60% moisture cheese with disodium ortho-phosphate as the emulsifier at 8 weeks and in 60% moisture cheese with the test polyphosphates as the emulsifier when tested at 20 weeks. None of the other cheese formulations were toxic at 20 weeks. Toxin production correlated statistically to time, moisture, pH and phosphate type.

Shelf-Life of Pasteurized Process Cheese Spreads Made from Cheddar Cheese Manufactured with a Nisin-Producing Starter Culture

Cheddar cheese made with a nisin-producing starter culture and Cheddar cheese made with a commercially available starter culture were used to manufacture pasteurized process cheese spreads at low and high moisture percentages (53 and 60%, respectively). Composition did not differ between spreads of similar moisture content with and without nisin. The nisin contents of cheese spreads were 301 and 387 IU/g at the high and low moisture percentages, respectively. Nisin was not inactivated by the thermal process used during cheese spread manufacture. Shelf-life of pasteurized process cheese spreads was determined during storage at 22 and 37°C. Low moisture cheese spreads with nisin had a longer shelf-life than corresponding cheese spreads without nisin when cheeses were incubated at either temperature. High moisture cheese spreads with nisin had a longer shelf-life than control spreads when cheeses were incubated at 22°C. However, shelf-life did not differ between high moisture spread with nisin and cheese spreads without nisin when cheeses were incubated at 37°C.

A Linear Programming Model Integrating Resource Allocation and Product Acceptability for Processed Cheese Products

A linear programming model integrates cheese manufacturing, blending, and aging and flavor components for a processed cheese manufacturing operation. The input resources for the process cheese product are idenified and defined. The constraints define the desired product using legal, quality, and management guidelines. The objective function maximizes the net returns of a pasteurized process cheese batch. Several scenarios are analyzed and compared. The critical factors that have high impact on costs and profits are identified and their economic influence quantified.

Antibotulinal Effectiveness of Nisin in Pasteurized Process Cheese Spreads

Pasteurized process cheese spreads were prepared at moisture levels ranging from 52 to 57% with added sodium chloride at levels from 0 to 2.0%, with disodium phosphate levels ranging from 1.4 to 2.5%, and with nisin levels of 0 to 250 ppm. Clostridium botulinum spores were added at a level of approximately 1000 spores per gram of cheese spread except for control batches and one experiment where the spore levels were varied (10–1000 spores/g). The cheese spreads were incubated at 30°C for up to 48 weeks. Nisin is an effective antibotulinal agent in pasteurized process cheese spreads. Addition of nisin allows formulation of pasteurized process cheese spreads with reduced sodium levels (addition of 1.4% disodium phosphate and no added sodium chloride) or slightly higher moisture levels (55–57%) by comparison to typical commercial pasteurized process cheese spreads. Higher levels of nisin (100 and 250 ppm) were required to prevent outgrowth of botulinal spores in cheese spreads with highest moisture levels or most greatly reduced sodium levels. However, in a cheese spread of 52% moisture prepared with 2.5% disodium phosphate but no added sodium chloride, a nisin level of 12.5 ppm was able to prevent completely outgrowth and toxin production by C. botulinum.

Quantitation of Growth of Mold on Cheese

Earlier work by others indicated that a mold colony grows radially at a constant rate on solid media. This concept was used in our study to develop a method for quantifying growth of mold on cheese. The ability of molds to grow on cheeses or pasteurized process cheese made with or without addition of sorbate was compared. Cheeses tested were mild Cheddar, aged Cheddar, aged-smoked Cheddar, brick and pasteurized process cheese. Pasteurized process cheeses were made from the natural cheeses by addition of water and a phosphate salt, then the mixture was heated. Some pasteurized process cheese from mild Cheddar was made to contain 0–500 ppm sorbic acid. Natural cheeses were sliced under aseptic conditions and were placed in sterile petri-plates. The hot and molten pasteurized process cheeses were poured into petri-plates. A spore suspension of Aspergillus parasiticus or Penicillium camemberti was inoculated onto the center of the cheese slice or pasteurized process cheese, and plates were covered and incubated at 22°C. The radius of mold colonies was measured at 24-h intervals. Data were analyzed by linear regression and lag period and rate of radial growth were calculated. Mold colonies grew radially at constant rates on cheeses and pasteurized process cheese. Lag in growth of each mold was longest on aged Cheddar cheese and pasteurized process cheese made from it, whereas it was shortest on mild Cheddar, brick and pasteurized process cheeses made therefrom. A. parasiticus grew faster on all cheeses and pasteurized process cheeses than did P. camemberti. Aged Cheddar cheese and pasteurized process cheese made from it effectively slowed the growth of both molds that were studied. Pasteurized process cheese containing sorbic acid inhibited growth of both molds. Generally, the higher the concentration of sorbic acid in the pasteurized process cheese, the slower was mold growth and the longer was the lag period.