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Effect of On-Farm Heating and Storage of Milk on Cottage Cheese Yield
Effect of On-Farm Heating and Storage of Milk on Cottage Cheese Yield D. J. D Z U R E C and R. R. ZALL Department of Food Science Cornell University Ithaca, NY 14853
not studied. Although high temperature heat treatment of milk increases cheese yield, it is generally kept to a minimum (8) because of its negative effect on textural quality (20). This study shows effects of subpasteurization heat treatment and milk storage on yield of cottage cheese.
ABSTRACT
Effect of on-farm heat treatment of milk on cottage cheese yields was studied. Fresh, raw milk heated to 74°C for 10 s was cooled and stored for 7 days at 3°C. Control and experimental lots of milk were separated and pasteurized at 72°C for 15 s and were used to make cottage cheese. Microbiological, shelf-life, flavor, and texture studies showed the experimental lots of cheese were as good as or better than control lots. Yield of cottage cheese was significantly higher when made from heated milk.
MATERIALS AND METHODS Milk Source, Heat Treatment, and Storage
INTRODUCTION
Spoilage of milk and milk products by psychrotrophic bacteria is a quality control problem of great concern to the dairy industry (11, 18). The use of farm bulk tanks instead of cans has changed milk handling: raw milk now is cooled more quickly and to a lower temperature; it also is held longer prior to processing (12). This extended cold storage provides a selective environment in which psychrotrophic bacteria can thrive and predominate (7). Psychrotrophs produce proteolytic and lipolytic enzymes resistant to pasteurization (9). Alternative methods to those currently used are needed to control psychrotrophic bacteria. Zall and Chen (21) reported that by heating fresh raw milk to 74°C for 10 s, psychrotrophic counts could be reduced to almost zero. Heat treatment improved the quality of raw milk and made it feasible to store it at 3°C for up to 7 days with no deleterious effects. This milk was satisfactory for use in the fluid markets and acceptable for the manufacture of buttermilk and yogurt. The effect of an on-farm heat treatment of milk on yield, flavor, and texture of cheese was
Received July 24, 1981. 1982 J Dairy Sci 65:2296-2300
Approximately 450 liters of raw milk were collected from the Cornell University dairy herd where about 400 cows are milked twice daily in a double ten Herringbone DeLaval milking parlor. Uncooled milk was collected from the pipeline as the milk flowed from the milking parlor to the storage tanks. The milk was divided into two portions, control milk and experimental milk. Experimental milk was immediately heated to 74°C for 10 s (thermalization), then cooled to approximately 22°C by passage through an Alfa Laval Misther (now called Microtherm) plate heat exchanger (Figure 1). The milk was further cooled to 3 + I°C and stored in an ll00-1iter refrigerated bulk tank. Control milk was placed p r o m p t l y into stainless steel cans, cooled to 3 -+ I°C in ice water, and made into cottage cheese within 24 h at the food science laboratory. Experimental milk was kept in cold storage on the farm for 5 days. It then was placed in stainless steel cans and transported to the laboratory where it was kept for an additional 2 days at 3 + I°C. Total storage time after the thermalization was 7 days. Cottage Cheese Manufacture
Control and experimental milks were separated with a DeLaval separator, Model 100. Skim milk was pasteurized at 72°C for 15 s in an APV plate heat exchanger, Type JR.H.E., cooled, placed in sanitized stainless steel cans, and stored overnight at 3 + 1°C.
2296
ON-FARM HEATING OF MILK
(~
2297
(PRECOOLED MILK TO TANK)
:~:ROM END UNIT
® 700
.,RTABIUZED MILK
IPREC~OOLING WATER
F Figure 1. Misther plate heat exchanger diagram. The heat exchanger section (# 1) is the heart of the plant. There the milk is heated to the desired temperature (usually in the range 70 to 74°C). After heating, the milk passes a coil (# 2) where it is kept at this temperature for a certain time (approx. 10 s). The heating medium in the heat exchanger (# 1) is recirculating hot water from a heater (# 3). A pump (# 4) circulates the hot water.
The following morning, 19.5-kg portions of control and experimental skim milk were placed into 23-liter cheese vats and made into cottage cheese curd according to the short set method described by Emmons and Tuckey (8) and Kosikowski (10). Hand paddles were used to agitate contents of the vats. Specially designed 6.3-ram wire cheese knives were used to cut the coagulum when appropriate. Starter cultures were obtained f r o m Chr. Hansen's Laboratory, Milwaukee, WI, and Marschall Products, Miles Laboratories, Inc., Madison, WI, Commercial single strength rennet obtained from Pfizer, Inc. was diluted 1:40 with water and added to vats at the rate of 1 ml/454 kg of milk. The coagulum was cut at pH 4.6, and the curd was cooked over 2 h to a final temperature of 55°C. The curd was washed three times with 10°C water that had been chlorinated to 5
ppm. Most of the wash water was drained from the vats. The curd was placed on stainless steel trays inclined at 5° to promote drainage and was allowed to drain for 1 h at 3 + I°C. It then was stored in 2.5 kg sized plastic containers at 3 + I°C. Twenty-one vats of curd from control milk and 29 vats of curd from experimental milk were produced during a 1980 to 1981 test period. Analytical Procedures
Samples of raw milk were monitored for microbial content by the standard plate c o u n t method (SPC) (3) and the rapid psychrotrophic count method (PBC) of Oliveria and Parmelee (13). All samples were tested for inhibitory substances by the Delvotest (G. B. Fermentation Industries, Inc., Des Plaines, IL). Phosphatase activity was checked for the Journal of Dairy Science Vol. 65, No. 12, 1982
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DZUREC AND ZALL
TABLE 1. Yield and moisture content of cottage cheese curd made from control and experimental milks during a 1980 to 1981 test period.
Control 2 Mean Standard Deviation Experimental 3 Mean Standard Deviation
Yield ~
Moisture
(Kg)
(%)
16.00 1.18
82 2
16.85 1.45
82 2
Kg cheese at 80% moisture per 100 kg skim milk. 2Cheese curd from untreated milk; 21 vats made.
Fresh and 1-wk-old curd were analyzed for coliform counts according to Standard Metbods for tbe Examination of Dairy Products (3). The keeping quality of the curd was monitored by testing the curd for microbial content by standard plate count, yeast and mold count (3), and rapid psychrotrophic count (13) on fresh and 3-wk-old samples. Organoleptic quality of the curd was evaluated by a trained taste panel. Samples of curd were judged either acceptable or unacceptable in appearance, texture, and flavor. Yield data comparing control and experimental lots of cheese were evaluated statistically by Student's t test (17).
aCheese curd from heat treated (74°C/10 s), stored milk (7 days); 29 vats made.
RESULTS A N D DISCUSSION
experimental milk that had been heated prior to pasteurization; it also was tested in all the pasteurized skim milk used for cheese making (5). A Corning Model 5 pH meter was used to measure pH. Curd yields were determined gravimetrically from curd drained for 1 h. Weights were read to .02 kg. Moisture percentage of the curd was determined according to the Association of Official Analytical Chemists (4). Yields were calculated with moisture contents adjusted to 80% to allow comparison between the two treatments.
The yield of cottage cheese curd from thermalized, stored, and pasteurized milk was higher (P<.05) than that of cottage cheese curd made from fresh, pasteurized milk (Table 1). Experimental and control lots of cottage cheese curd did n o t have significantly different moisture contents (Table 1). The microbial quality of the heated and stored milks (Table 2) supports the data presented by Zall and Chen (21). After 7 days storage, bacterial counts of thermalized milk complied with microbial standards prescribed by the Pasteurized Milk Ordinance for Grade A milk. Standard plate counts of 7-day-old milk were less than 100,000 CFU(colony forming units)/ml.
TABLE 2. Standard plate count (SPC) and psychrotrophic bacteria count (PBC) in heated/stored milk during a 1980 to 1981 test period. Mean
Standard deviation
(CFU 1/ml milk) sPc Raw Heat treated Day 12 Heat treated Day 73
4,400 88 40,000
2,500 49 24,000
PBC Raw Heat treated Day 1 Heat treated Day 7
700 10 40,000
550 4 24,000
1CFU = Colony forming units. 2Milk heat treated, 74°C/10 s, 1st day after heat treatment. aMilk heat treated, 74°C/10 s, 7th day after heat treatment. Journal of Dairy Science Vol. 65, No. 12, 1982
ON-FARM HEATING OF MILK In all cases, raw milk was negative for inhibitory substances by the Delvotest. Thermalized mitk was positive for phosphatase (.1 Scharer unit). Pasteurized skim used for cheese making was negative for phosphatase. The on-farm heat treatment of milk is not in compliance with the Pasteurized Milk Ordinance (1), which forbids heating milk above 52°C except for pasteurization. However, new regulations generally are adapted to reflect potential improvements suggested by research. All lots of cottage cheese were acceptable in flavor, texture, and appearance by a trained taste panel. For some lots, cottage cheese curd made from heat-treated, stored milk were superior in flavor and texture. In no instance was the experimental curd inferior to the control curd. Microbial numbers in all lots of cheese were low. No coliforms were in the cheese curd at any time. Cheese curd retained low microbial counts through 3 wk of cold storage at 3 -+ 1°C (Table 3). High heat treatment (> 90°C) of cheese milk denatures more whey proteins and results in a greater increase in cottage cheese yield (19) in comparison with milk subjected to only minimum legal pasteurization (72°C, 15 s). Therefore, it was unexpected that milk subjected to a relatively mild heat treatment (74 C, 10 s), stored for 7 days, and then subjected to normal
2299
pasteurization would give higher yields of satisfactory cottage cheese (8, 10). Aylward et al. (6) indicated that growth of psychrotrophic bacteria in cold stored raw milk leads to a decrease in cottage cheese yield. The proteolytic activity of psychrotrophs decreases the a m o u n t of casein available for curd production. Use of highest quality milk would maximize yield in cottage cheese production. High quality milk is obtained by minimizing both the initial bacterial count and the time of storage prior to cheese manufacture. Acceptable cottage cheese was made from experimental heated milk. In addition, storing milk 7 days prior to cheese manufacture also increased cheese yield. T h i s finding was contrary to expectation that a minimum storage period would offer maximum yield. Studies (15, 19) indicated a need to alter manufacturing procedures for making cottage cheese from milk pasteurized at higher than conventional temperatures. Such changes included cutting the curd at higher pH, using longer cooking times, and adding stabilizers and other additives. In this study, no changes in the manufacture of cottage cheese curd were required. The mechanism by which cottage cheese yields are increased by heating cheese milk is a matter of speculation. Studies (14, 16) showed that heat treatments of milk induce an as-
TABLE 3. Standard plate count (SPC), psychrotrophic bacteria count (PBC), and yeast and mold count in fresh and stored samples of cottage cheese curd during a 1980 to 1981 test period. Mean
Standard deviation
(CFUt/g cottage cheese) SPC Fresh 2 3 Weeksa PBC Fresh 3 Weeks Yeast and molds Fresh 3 Weeks
70 1300
52 540
<1 1100
300
<10 <10
l CFU, Colony forming units. 2Cottage cheese curd analyzed within 24 h of manufacture. 3Cottage cheese curd analyzed after 3 wk of storage at 3 +- 1°C. Journal of Dairy Science Vol. 65, No. 12, 1982
2300
DZUREC AND ZALL
s o c i a t i o n b e t w e e n casein a n d w h e y p r o t e i n s . O u r h e a t t r e a t m e n t s were relatively mild. It is not known whether these heat treatments can cause such an association. T h e m e c h a n i s m o f i n c r e a s e d yields is u n d e r i n v e s t i g a t i o n . A n i n c r e a s e d yield o f a p p r o x i m a t e l y 5% was o b t a i n e d in c o t t a g e cheese o P r o d u c t i o n b y h e a t i n g t h e c h e e s e m i l k t o 74 C f o r 10 s a n d s t o r i n g at 3 -+ 1°C for 7 days. T h e U n i t e d S t a t e s d a i r y i n d u s t r y m a y have m u c h to gain f r o m s u c h a process in its m a n u f a c t u r e o f m o r e t h a n 4 m kg o f c o t t a g e cheese each y e a r (2). ACKNOWLEDGMENTS
This research was s p o n s o r e d in p a r t b y a g r a n t f r o m t h e Alfa Laval C o m p a n y o f T u m b a , Sweden. Personnel from Cornell's Teaching a n d R e s e a r c h F a r m at H a r f o r d , NY, p r o v i d e d valuable d a y - t o - d a y assistance in t h e m i l k handling study. Joseph Chen and Hung-Chang C h e n c o n t r i b u t e d t e c h n i c a l s u p p o r t to t h e project. REFERENCES
1 Anonymous. 1978. Grade A Pasteurized Milk Ordinance. US Publ. Health Serv., US Govt. Printing Office, Washington, DC. 2 Anonymous. 1980. Milk facts. Milk Ind. Found., Washington, DC. 3 American Public Health Assoc. 1978. Standard methods for the examination of dairy products. Marth, E. H., ed. Washington, DC. 4 Association of Official Analytical Chemists. 1975. Official methods of analysis of the Association of Official Analytical Chemists. 12th ed. Horwitz, W., ed. Washington, DC. 5 Atherton, H. V., and J. A. Newlander. 1977. Chemistry and testing of dairy products. 4th ed. Avi Publ. Co., Inc., Westport, CT. 6 Aylward, E. B., J. O'Leary, and B. E. Langlois. 1980. Effect of milk storage in cottage cheese yield. J. Dairy Sci. 63:1819.
Journal of Dairy Science Vol. 65, No. 12, 1982
7 Cousins, C. M., M. E. Sharpe, and B. A. Law. 1977. The bacteriological quality of milk for Cheddar cheesemaking. Dairy Ind. 42:12. 8 Emmons, D. B., and S. L. Tuckey. 1967. Cottage Cheese and Other Cultured Milk Products. Chas. Pfizer and Co., NY. 9 Griffiths, M. W., J. D. Phillips, and D. D. Muir. 1981. Thermostability of proteases and lipases from a number of species of psychrotrophic bacteria of dairy origin. J. Appl. Bacteriol. 50:289. 10 Kosikowski, F. V. 1977. Cheese and fermented milk foods. 2nd ed. Edwards Brothers, Inc., Ann Arbor, MI. 1I Mikolajcik, E. M. 1979. Psychrotrophic bacteria and dairy product quality. I. Major organisms involved and defects produced. Cult. Dairy Prod. J. 14:6. 12 Mohammed, F. O., and R. Bassette. 1979. Quality and yield of cottage cheese influenced by psychrotrophic microorganisms in milk. J. Dairy Sci. 62:222. 13 Oliveria, J. S., and C. E. Parmelee. 1976. Rapid enumeration of psychrotrophic bacteria in raw and pasteurized milk. J. Milk Food Technol. 39:269. 14 Sawyer, W. H. 1969. Complex between 134actoglobulin and g-casein. A review. J. Dairy Sci. 52:1347. 15 Smith, J. 1980. Sir-Jay process for increasing cottage cheese yields. Stabilized Products, Inc., St. Louis, MO. 16 Smits, P., and J. H. Van Brouwershaven. 1980. Heat-induced association of ~3-1actoglobulin and casein micelles. J. Dairy Res. 47:313. 17 Snedecor, G. W., and W. G. Cochran. 1967. Statistical methods. 6th ed. Iowa State Univ. Press, Ames. 18 Thomas, S. B., and B. F. Thomas. 1973. Psychrotrophic bacteria in refrigerated bulk-collected raw milk, Part I. Dairy Ind. 38:11. 19 Vakaleris, D. G. 1962. A method for making cottage cheese. US Pat. 3,039,879. 20 White, C. H., and B. W. Ray. 1977. Influence of heat treatment and methods of acidification of milk on manufacture and properties of cottage cheese. J. Dairy Sci. 60:1236. 21 Zall, R. R., and J. H. Chen. 1981. Heating and storing milk on dairy farms before pasteurization in milk plants. J. Dairy Sci. 64:1540.
not studied. Although high temperature heat treatment of milk increases cheese yield, it is generally kept to a minimum (8) because of its negative effect on textural quality (20). This study shows effects of subpasteurization heat treatment and milk storage on yield of cottage cheese.
ABSTRACT
Effect of on-farm heat treatment of milk on cottage cheese yields was studied. Fresh, raw milk heated to 74°C for 10 s was cooled and stored for 7 days at 3°C. Control and experimental lots of milk were separated and pasteurized at 72°C for 15 s and were used to make cottage cheese. Microbiological, shelf-life, flavor, and texture studies showed the experimental lots of cheese were as good as or better than control lots. Yield of cottage cheese was significantly higher when made from heated milk.
MATERIALS AND METHODS Milk Source, Heat Treatment, and Storage
INTRODUCTION
Spoilage of milk and milk products by psychrotrophic bacteria is a quality control problem of great concern to the dairy industry (11, 18). The use of farm bulk tanks instead of cans has changed milk handling: raw milk now is cooled more quickly and to a lower temperature; it also is held longer prior to processing (12). This extended cold storage provides a selective environment in which psychrotrophic bacteria can thrive and predominate (7). Psychrotrophs produce proteolytic and lipolytic enzymes resistant to pasteurization (9). Alternative methods to those currently used are needed to control psychrotrophic bacteria. Zall and Chen (21) reported that by heating fresh raw milk to 74°C for 10 s, psychrotrophic counts could be reduced to almost zero. Heat treatment improved the quality of raw milk and made it feasible to store it at 3°C for up to 7 days with no deleterious effects. This milk was satisfactory for use in the fluid markets and acceptable for the manufacture of buttermilk and yogurt. The effect of an on-farm heat treatment of milk on yield, flavor, and texture of cheese was
Received July 24, 1981. 1982 J Dairy Sci 65:2296-2300
Approximately 450 liters of raw milk were collected from the Cornell University dairy herd where about 400 cows are milked twice daily in a double ten Herringbone DeLaval milking parlor. Uncooled milk was collected from the pipeline as the milk flowed from the milking parlor to the storage tanks. The milk was divided into two portions, control milk and experimental milk. Experimental milk was immediately heated to 74°C for 10 s (thermalization), then cooled to approximately 22°C by passage through an Alfa Laval Misther (now called Microtherm) plate heat exchanger (Figure 1). The milk was further cooled to 3 + I°C and stored in an ll00-1iter refrigerated bulk tank. Control milk was placed p r o m p t l y into stainless steel cans, cooled to 3 -+ I°C in ice water, and made into cottage cheese within 24 h at the food science laboratory. Experimental milk was kept in cold storage on the farm for 5 days. It then was placed in stainless steel cans and transported to the laboratory where it was kept for an additional 2 days at 3 + I°C. Total storage time after the thermalization was 7 days. Cottage Cheese Manufacture
Control and experimental milks were separated with a DeLaval separator, Model 100. Skim milk was pasteurized at 72°C for 15 s in an APV plate heat exchanger, Type JR.H.E., cooled, placed in sanitized stainless steel cans, and stored overnight at 3 + 1°C.
2296
ON-FARM HEATING OF MILK
(~
2297
(PRECOOLED MILK TO TANK)
:~:ROM END UNIT
® 700
.,RTABIUZED MILK
IPREC~OOLING WATER
F Figure 1. Misther plate heat exchanger diagram. The heat exchanger section (# 1) is the heart of the plant. There the milk is heated to the desired temperature (usually in the range 70 to 74°C). After heating, the milk passes a coil (# 2) where it is kept at this temperature for a certain time (approx. 10 s). The heating medium in the heat exchanger (# 1) is recirculating hot water from a heater (# 3). A pump (# 4) circulates the hot water.
The following morning, 19.5-kg portions of control and experimental skim milk were placed into 23-liter cheese vats and made into cottage cheese curd according to the short set method described by Emmons and Tuckey (8) and Kosikowski (10). Hand paddles were used to agitate contents of the vats. Specially designed 6.3-ram wire cheese knives were used to cut the coagulum when appropriate. Starter cultures were obtained f r o m Chr. Hansen's Laboratory, Milwaukee, WI, and Marschall Products, Miles Laboratories, Inc., Madison, WI, Commercial single strength rennet obtained from Pfizer, Inc. was diluted 1:40 with water and added to vats at the rate of 1 ml/454 kg of milk. The coagulum was cut at pH 4.6, and the curd was cooked over 2 h to a final temperature of 55°C. The curd was washed three times with 10°C water that had been chlorinated to 5
ppm. Most of the wash water was drained from the vats. The curd was placed on stainless steel trays inclined at 5° to promote drainage and was allowed to drain for 1 h at 3 + I°C. It then was stored in 2.5 kg sized plastic containers at 3 + I°C. Twenty-one vats of curd from control milk and 29 vats of curd from experimental milk were produced during a 1980 to 1981 test period. Analytical Procedures
Samples of raw milk were monitored for microbial content by the standard plate c o u n t method (SPC) (3) and the rapid psychrotrophic count method (PBC) of Oliveria and Parmelee (13). All samples were tested for inhibitory substances by the Delvotest (G. B. Fermentation Industries, Inc., Des Plaines, IL). Phosphatase activity was checked for the Journal of Dairy Science Vol. 65, No. 12, 1982
2298
DZUREC AND ZALL
TABLE 1. Yield and moisture content of cottage cheese curd made from control and experimental milks during a 1980 to 1981 test period.
Control 2 Mean Standard Deviation Experimental 3 Mean Standard Deviation
Yield ~
Moisture
(Kg)
(%)
16.00 1.18
82 2
16.85 1.45
82 2
Kg cheese at 80% moisture per 100 kg skim milk. 2Cheese curd from untreated milk; 21 vats made.
Fresh and 1-wk-old curd were analyzed for coliform counts according to Standard Metbods for tbe Examination of Dairy Products (3). The keeping quality of the curd was monitored by testing the curd for microbial content by standard plate count, yeast and mold count (3), and rapid psychrotrophic count (13) on fresh and 3-wk-old samples. Organoleptic quality of the curd was evaluated by a trained taste panel. Samples of curd were judged either acceptable or unacceptable in appearance, texture, and flavor. Yield data comparing control and experimental lots of cheese were evaluated statistically by Student's t test (17).
aCheese curd from heat treated (74°C/10 s), stored milk (7 days); 29 vats made.
RESULTS A N D DISCUSSION
experimental milk that had been heated prior to pasteurization; it also was tested in all the pasteurized skim milk used for cheese making (5). A Corning Model 5 pH meter was used to measure pH. Curd yields were determined gravimetrically from curd drained for 1 h. Weights were read to .02 kg. Moisture percentage of the curd was determined according to the Association of Official Analytical Chemists (4). Yields were calculated with moisture contents adjusted to 80% to allow comparison between the two treatments.
The yield of cottage cheese curd from thermalized, stored, and pasteurized milk was higher (P<.05) than that of cottage cheese curd made from fresh, pasteurized milk (Table 1). Experimental and control lots of cottage cheese curd did n o t have significantly different moisture contents (Table 1). The microbial quality of the heated and stored milks (Table 2) supports the data presented by Zall and Chen (21). After 7 days storage, bacterial counts of thermalized milk complied with microbial standards prescribed by the Pasteurized Milk Ordinance for Grade A milk. Standard plate counts of 7-day-old milk were less than 100,000 CFU(colony forming units)/ml.
TABLE 2. Standard plate count (SPC) and psychrotrophic bacteria count (PBC) in heated/stored milk during a 1980 to 1981 test period. Mean
Standard deviation
(CFU 1/ml milk) sPc Raw Heat treated Day 12 Heat treated Day 73
4,400 88 40,000
2,500 49 24,000
PBC Raw Heat treated Day 1 Heat treated Day 7
700 10 40,000
550 4 24,000
1CFU = Colony forming units. 2Milk heat treated, 74°C/10 s, 1st day after heat treatment. aMilk heat treated, 74°C/10 s, 7th day after heat treatment. Journal of Dairy Science Vol. 65, No. 12, 1982
ON-FARM HEATING OF MILK In all cases, raw milk was negative for inhibitory substances by the Delvotest. Thermalized mitk was positive for phosphatase (.1 Scharer unit). Pasteurized skim used for cheese making was negative for phosphatase. The on-farm heat treatment of milk is not in compliance with the Pasteurized Milk Ordinance (1), which forbids heating milk above 52°C except for pasteurization. However, new regulations generally are adapted to reflect potential improvements suggested by research. All lots of cottage cheese were acceptable in flavor, texture, and appearance by a trained taste panel. For some lots, cottage cheese curd made from heat-treated, stored milk were superior in flavor and texture. In no instance was the experimental curd inferior to the control curd. Microbial numbers in all lots of cheese were low. No coliforms were in the cheese curd at any time. Cheese curd retained low microbial counts through 3 wk of cold storage at 3 -+ 1°C (Table 3). High heat treatment (> 90°C) of cheese milk denatures more whey proteins and results in a greater increase in cottage cheese yield (19) in comparison with milk subjected to only minimum legal pasteurization (72°C, 15 s). Therefore, it was unexpected that milk subjected to a relatively mild heat treatment (74 C, 10 s), stored for 7 days, and then subjected to normal
2299
pasteurization would give higher yields of satisfactory cottage cheese (8, 10). Aylward et al. (6) indicated that growth of psychrotrophic bacteria in cold stored raw milk leads to a decrease in cottage cheese yield. The proteolytic activity of psychrotrophs decreases the a m o u n t of casein available for curd production. Use of highest quality milk would maximize yield in cottage cheese production. High quality milk is obtained by minimizing both the initial bacterial count and the time of storage prior to cheese manufacture. Acceptable cottage cheese was made from experimental heated milk. In addition, storing milk 7 days prior to cheese manufacture also increased cheese yield. T h i s finding was contrary to expectation that a minimum storage period would offer maximum yield. Studies (15, 19) indicated a need to alter manufacturing procedures for making cottage cheese from milk pasteurized at higher than conventional temperatures. Such changes included cutting the curd at higher pH, using longer cooking times, and adding stabilizers and other additives. In this study, no changes in the manufacture of cottage cheese curd were required. The mechanism by which cottage cheese yields are increased by heating cheese milk is a matter of speculation. Studies (14, 16) showed that heat treatments of milk induce an as-
TABLE 3. Standard plate count (SPC), psychrotrophic bacteria count (PBC), and yeast and mold count in fresh and stored samples of cottage cheese curd during a 1980 to 1981 test period. Mean
Standard deviation
(CFUt/g cottage cheese) SPC Fresh 2 3 Weeksa PBC Fresh 3 Weeks Yeast and molds Fresh 3 Weeks
70 1300
52 540
<1 1100
300
<10 <10
l CFU, Colony forming units. 2Cottage cheese curd analyzed within 24 h of manufacture. 3Cottage cheese curd analyzed after 3 wk of storage at 3 +- 1°C. Journal of Dairy Science Vol. 65, No. 12, 1982
2300
DZUREC AND ZALL
s o c i a t i o n b e t w e e n casein a n d w h e y p r o t e i n s . O u r h e a t t r e a t m e n t s were relatively mild. It is not known whether these heat treatments can cause such an association. T h e m e c h a n i s m o f i n c r e a s e d yields is u n d e r i n v e s t i g a t i o n . A n i n c r e a s e d yield o f a p p r o x i m a t e l y 5% was o b t a i n e d in c o t t a g e cheese o P r o d u c t i o n b y h e a t i n g t h e c h e e s e m i l k t o 74 C f o r 10 s a n d s t o r i n g at 3 -+ 1°C for 7 days. T h e U n i t e d S t a t e s d a i r y i n d u s t r y m a y have m u c h to gain f r o m s u c h a process in its m a n u f a c t u r e o f m o r e t h a n 4 m kg o f c o t t a g e cheese each y e a r (2). ACKNOWLEDGMENTS
This research was s p o n s o r e d in p a r t b y a g r a n t f r o m t h e Alfa Laval C o m p a n y o f T u m b a , Sweden. Personnel from Cornell's Teaching a n d R e s e a r c h F a r m at H a r f o r d , NY, p r o v i d e d valuable d a y - t o - d a y assistance in t h e m i l k handling study. Joseph Chen and Hung-Chang C h e n c o n t r i b u t e d t e c h n i c a l s u p p o r t to t h e project. REFERENCES
1 Anonymous. 1978. Grade A Pasteurized Milk Ordinance. US Publ. Health Serv., US Govt. Printing Office, Washington, DC. 2 Anonymous. 1980. Milk facts. Milk Ind. Found., Washington, DC. 3 American Public Health Assoc. 1978. Standard methods for the examination of dairy products. Marth, E. H., ed. Washington, DC. 4 Association of Official Analytical Chemists. 1975. Official methods of analysis of the Association of Official Analytical Chemists. 12th ed. Horwitz, W., ed. Washington, DC. 5 Atherton, H. V., and J. A. Newlander. 1977. Chemistry and testing of dairy products. 4th ed. Avi Publ. Co., Inc., Westport, CT. 6 Aylward, E. B., J. O'Leary, and B. E. Langlois. 1980. Effect of milk storage in cottage cheese yield. J. Dairy Sci. 63:1819.
Journal of Dairy Science Vol. 65, No. 12, 1982
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