Effect of Milk Storage on Cottage Cheese Yield1

Effect of Milk Storage on Cottage Cheese Yield I E. B. A Y L W A R D , J. O ' L E A R Y , and B. E. L A N G L O I S Food Science Section Department of Animal Sciences University of Kentucky Lexington 40506

ty (20). Holding milk in the range 3 to 7°C is selective for psychrotrophic microorganisms (7). More than 50% of the psychrotrophic microflora in raw milk were proteolytic Gramnegative bacteria (14, 18). There was extensive bacterial growth and proteolysis (23) and an increase in amino nitrogen (3) in milk stored at 4 to 5° C. Yield of cottage and other cheeses is dependent upon casein content of milk (19). The degrading effect of psychrotrophic proteases are much greater on casein than on whey proteins (11). Any factor affecting the casein content of raw milk has a potentially great impact on yield of cottage cheese. Manufacturer's profits depend on yield; thus, factors that decrease casein are costly (9). The purpose of this study was to determine the effect of low-temperature storage of skim milk on yield of cottage cheese.

ABSTRACT

The effect o f storage of raw milk for up to 12 days at 5°C on the yield of cottage cheese was investigated. Grade A raw milks were obtained from the University of Kentucky dairy herd and from the bulk fluid milk supply at a local processor. Cottage cheese was manufactured by the short-set (5 h) and long-set (14 h) procedures. Yield of cottage cheese decreased with an increase in time that raw milk was stored. Yield decreases were 2.5 to 3% per day of low temperature storage after the bacterial count of raw milk attained 106/ml. These decreases occurred in the first 2 days of added storage of the bulk fluid milk from the local processor. Noncasein and nonprotein nitrogen increased as time of storage of raw skim milk increased. Bacterial analysis of the raw skim milks showed rapid increases in standard plate, psychrotrophic, and proteolytic counts. Coliform count also exhibited a rapid increase, but the final population was n o t as large as psychrotrophic and proteolytic counts.

MATERIALS AND METHODS Source of Milk

INTRODUCTION

The United States dairy industry has undergone considerable change during this century. Milk originally was delivered to the consumer by the dairy farmer, but now it is collected from the farm every other day by insulated tanker trucks and delivered to central locations for processing (26). The longer cold storage of raw milk has stressed the relationship between psychrotrophic microorganisms and milk quali-

Received October 1, 1979. ~Published with the approval of the Director of the Kentucky Agricultural Experiment Station as

Grade A raw milk was from the University of Kentucky (UK) dairy herd and from the mingled bulk fluid milk supply of a local processor. The raw milk was separated by DeLaval No. 240 centrifugal separator, and skim milk collected into 189-liter portable vats (Cherry-Burrell) was stored at 5°C. Two lots of milk were from each source for a total of four lots. Milk Treatment

Beginning on the day of collection (day 0) two 28-kg lots of skim milk were withdrawn from storage and pasteurized separately (62.8°C, 30 min). This was every 2nd day until the milk was no longer stable to pasteurization. Cheese Manufacture

Cottage

Journal Article No. 79-5-167. 1980 J Dairy Sci 63:1819-1825

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cheese manufacturing procedures

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AYLWARD ET AL.

followed those outlined by Kosikowski (19) and Emmons and Tuckey (12). The longset and short-set methods were used alternately on each 28-kg lot of pasteurized milk. For the short-set (32.2°C) method one lot of milk was inoculated with 5% lactic starter culture and divided among four cylindrical copper vats, each containing 6.8 kg; the vats were placed in a thermostatically controlled water bath (American Instrument Company, Silver Spring, MD). The system was equipped with mechanical agitation and special 6.4-mm cheese knives. Rennet powder (Nutritional Biochemical Corporation, Cleveland, OH) was diluted 250-fold with water. At this concentration it was equivalent to a single strength rennet solution. Before use this solution was diluted further 100-fold, and 1.5 ml were added to each v a t . The curd was cut at pH 4.8 and cooked to a final temperature of 54.5°C. The curd was washed twice with water (pH 6.0) at 21 and 5°C. Washed curd was drained on cheesecloth for 1.5 h before weighing. For the long-set (22.2°C) method the second lot of milk was inoculated with 1% lactic starter and the procedure repeated. Lactic cultures numbers 44 and 70 (Chr. Hansen's Laboratory, Milwaukee, WI) were used. Each vat was treated and analyzed separately. Microbiological Analysis

Raw and pasteurized skim milk samples from each lot were collected aseptically in sterile 7.62 cm × 17.78 cm Whirl-Pak (Scientific Products, McGraw Park, IL). Samples were analyzed immediately for standard plate count (SPC), psychrotrophic count (PBC) and proteolytic, coliform, and yeast and mold counts (16, 27). Spore count was determined by heating samples to 80°C for 10 rain before plating. Acid

A Beckman expanded scale pH meter was used for pH measurement. Titratable acidity (TA), as percent lactic acid, was measured as described by Atherton and Newlander (2). Nitrogen Distribution

Pasteurized milk was analyzed for total nitrogen (TN), nonprotein nitrogen (NPN), and noncasein nitrogen (NCN) and whey for total nitrogen. Casein nitrogen was estimated by Journal of Dairy Science Vol. 63, No. 11, 1980

subtracting NCN from TN. The procedures of Rowland (24, 25) were used to obtain the NPN and NCN fractions. Nitrogen was measured by the Kjeldahl method. Moisture Percentage

Two composite cheese samples from each 28-kg tot were weighed, dried in an oven at 100°C for 24 h, cooled in a desiccator, and the percent moisture was determined by reweighing (19). Statistical Analysis

Mean dry weight of cottage cheese from each copper vat was analyzed statistically by the Duncan multiple range procedure (14). RESULTS AND DISCUSSION

Tiae effect of low-temperature storage of raw skim milks on yield and percent moisture of cottage cheese is in Table 1. The UK skim milk was stable for 10 days storage while mingled skim milk was stable for only 4 days of storage. During 10 days of 5°C storage, the average decrease in cottage cheese yield from UK skim milk was approximately 2% per day. However, yield did not change greatly until after the 4 days of storage. The decrease was greater than the decrease in the Cheddar cheese yield reported by O'Leary et al. (22). A possible reason for the greater decrease is that cottage cheese manufacture allows more opportunity for loss of curd fines than does Cheddar manufacture. In addition, Cheddar cheese curd is not washed, nor is it heated to the high cooking temperature (54.4°C) of cottage cheese curd. Throughout storage, mingled skim milk exhibited a rapid decrease in yield of cottage cheese. The rate of decrease was approximately 2.5% per day of 5°C storage. Cousin and Marth (6) reported decreased cottage cheese yield during 4.4°C storage of milks inoculated with psychrotrophs. A statistical analysis of the yield results from both skim milk sources also is presented in Table 1. Initially, cottage cheese yield from mingled skim milks was different (P<.10) from the yield from UK skim milks. There was a decrease (P<.10) in yield from mingled skim milks during 2 days of 5°C storage. Cottage cheese yield from UK skim milks did not vary significantly during 4 days of storage. Yield

MILK STORAGE AND COTTAGE CHEESE

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TABLE 1. Effect of storage of skim milk on cottage cheese yield and moisture content. Days of 5°C storage Source

0

2

4

6

8

10

167.9 -4.2 84.3

163.7 -1.3 83.9

150.4 -4.1 84.3

UK

Yielda Change b Moisture (%)c

186.4 .... 81.5

188.3 +.5 82.5

183.4 -1.3 83.4

Mingled

Yielda Change b Moisture (%)c

207.5 .... 79.0

191.2 --3.9 79.6

186.8 --1.1 79.6

aDry weight (g) of cottage cheese from each vat; means of 16 observations. b% Change per day from previous measurement. CMeans of 8 observations. e'f'gMeans for yield from each source of milk with different superscripts differ (P<.lO). UK SE = 3.71 with 91 dr, mingled SE = 7.51 with 47 df.

decreases f r o m UK skim milks between 4 days and 6 days of storage and b e t w e e n 6 days and 10 days of storage were different (P<.10). A l t h o u g h percentage decreases in yield f r o m both skim milks b e t w e e n 2 days and 4 days of storage and f r o m UK skim milks b e t w e e n 6 days and 8 days o f storage were not significant, t h e y could be economically i m p o r t a n t to a manufacturer. The m e a n moisture c o n t e n t of cottage cheese p r o d u c e d f r o m UK skim milks increased during the first 6 days of skim milk storage (Table 1). The moisture c o n t e n t of cottage cheese p r o d u c e d from mingled skim milks increased slightly during skim milk storage. Hicks et al. ( 1 7 ) r e p o r t e d that Cheddar cheese moisture increased as t i m e of milk storage increased. Because the m o i s t u r e c o n t e n t of cottage cheese is regulated by law at n o t m o r e

than 80% (12), cottage cheese m a n u f a c t u r e d f r o m stored skim milk could fail to m e e t legal requirements. The resulting soft or weak curd also could be subject to e~cessive shattering (8). Before the widespread use of mechanical refrigeration at the farm, the m i c r o f l o r a o f raw milk rapidly became d o m i n a t e d by lactic acid bacteria (29). Thus, the pH and T A of raw milks provided reasonably g o o d indication of microbiological c o n t a m i n a t i o n . The data in Table 2 indicate that this was n o t true for the skim milks. Today, the microflora of farm t a n k milks consists p r e d o m i n a n t l y of p s y c h r o t r o p h i c microorganisms. Psychrotrophs can cause proteolysis of cold-stored milks while producing little acid or slight alkalinity (20). C h a p m a n et al. (5) r e p o r t e d that the pH of milk stored at 5 ° C either remained constant for 3 days of storage or increased during the first 2 days and

TABLE 2. Effect of storage of skim milk on pH and percent titratable acidity (TA).a Days of 5°C storage Source UK Mingled

0

2

4

pH % TA

6.84 .14

6.81 .15

6.77

pH % TA

6.88 .16

6.86 .17

6.41 .28

.16

6

8

10

6.63 .16

6.52 .16

6.45 .20

aMeans of 8 observations. Journal of Dairy Science Vol. 63, No. 11, 1980

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AYLWARD ET AL.

TABLE 3. Bacterial counts of raw skim milk. Source of milk UK

Mingled

Trial

Trial

Days 5°C storage

1

2

1

2

Standard plate count

0 4

4.43" 7.11

4.00 5.84

6.28 8,72

6.15 8.67

Psychrotrophic count Proteolytic count Coliform count

0 4 0 4 0 4

2.64 7,08 4.23 6.32 1.77 3.45

2.48 6.11

6.18 8,85

6.40 8.83

3.08 5.40 2.45 3.38

3.89 8.28 5.04 7.91

4.54 8,40 4.40 7.84

*Log10 CFU/ml.

then decreased. During 6 days of 5°C storage, UK skim milks developed a distinct odor, but pH and TA remained within the range (.12 to .20 TA) considered normal for fresh raw milk. The pH of mingled skim milks (Table 2) changed only slightly during 2 days of 5°C storage while yield of cottage cheese decreased nearly 8%. Titratable acidity and pH measurements were not good indicators of the microbiological quality of these skim milks. The effect of 5°C storage on counts of microorganisms is in Table 3. Psychrotrophic counts (PBC) of UK skim milks, expressed as a percentage of SPC, increased from 3% or less initially to approximately 100% of SPC at the end of the storage period. Chapman et al. (5) reported a similar rate of psychrotrophic increase during 5°C milk storage. Cousins et al. (7) reported that the greatest damage to milk constituents occurs when bacterial numbers exceed 106/ml. In mingled skim milks, rapid increase of SPC and PBC began with counts in excess of 106/ml and corresponded to the time of greatest decrease in cottage cheese yield. Figure 1 shows that the microbial population of UK skim milks attained nearly 107/mi before substantial decrease of yield occurred. The nature of contaminating microorganisms and their ability to multiply during cold-storage will affect greatly the physical and biochemical changes that take place in milk (7). The higher population of microorganisms attained by UK skim milks before substantial decrease in cheese Journal of Dairy Science Vol. 63, No. 11, 1980

yield could have been due to differences in composition of the contaminating microflora. While there were differences in yield between the long-set and short-set procedures for UK milk (Figure 1), there was no trend favoring one procedure over the other. However, with mingled milk the short-set procedure gave higher yields in all cases, with differences averaging 9%. Since the moisture content of the cheese remained fairly constant, differences in yield could not be accounted for by incorporation of whey protein. These differences may

o

175 ~504°

~ i~o~ 510

60I

70

80

910

~oglO CFU/ml

Figure 1. Relationship between yield and CFU (SPC)/g for curd from milk from two sources made by the short- and long-set procedures; o mingled shortset, ~ mingled long-set, ~ UK short-set, [] UK longset.

MILK STORAGE AND COTTAGE CHEESE TABLE 4. Mean bacterial counts of pasteurized milk. a Bacterial countb

UK Mingled

SPC

PC

PBC

1.73 3.18

.18 1.23

.29 <.00

aAll counts log10 CFU/ml. bSPC = standard plate count; PC = proteolytic count; PBC = psychrotrophic count.

have been due to higher microbial survival rate in pasteurized mingled milks (Table 4) or to more heat resistant proteases (1). Either of these possibilities coupled with a longer setting time would allow for greater proteolysis during cheese manufacture. The coliform count in mingled skim milks exhibited as much as a 3.44 log count increase during 4 days of 5°C storage. Coliform populations of both skim milks did n o t attain the

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same number as did SPC, PBC, and proteolytic counts. There were fewer coliforms in the UK skim milks initially, and they did not increase as rapidly as the coliform counts of mingled skim milks. The yeast and mold and spore counts (Figure 2) did not exceed 10 s/ m l . Some of the fungi are producers of proteolytic enzymes. Some fungal proteases substitute for rennin as milk coagulants during cheese manufacture (19). Proteases often first occur or substantially increase during bacterial sporulation (15). At approximately 7 days of storage, the UK skim milk spore count increased sharply. The spore count of mingled skim milk increased throughout storage. These increases corresponded to the time of most rapid decrease in yield of cottage cheese. Although in comparatively small numbers, yeast and molds and sporeformers could have contributed to proteolysis of the cold-stored skim milks. Nitrogen fractions of mingled and UK skim milks are in Table 5, as well as changes during storage. The casein nitrogen (CN) content of the mingled milks was 13% higher than that of

TABLE 5. Nitrogen distribution in stored skim milks. Nitrogen fractions (mg N/IO0 ml skim milk)

=~-

/o~O~

~3

:::!iii.

Days of 5°C storage

Source

0

4

UK Mingled

446.0 530.5

UK Mingled

79.8 117.5

UK Mingled

33.3 37.8

10

Total nitrogen 456.8 530.5

454.3

Noncasein nitrogen 98.8* 139.3*

111.3"

Nonprotein nitrogen

-

UK Mingled 0

I

I

I

I

I

2

4

6

.8

10

-

366.2 413.0

44.8* 58.0"

50.0

Casein nitrogen a 358.0 391.2

343.0

I

12

*Differ from day 0 (P<.05).

DAYSOF 5C STORAGE

Figure 2. Growth of yeast and mold, and spores in stored milk: X mingled yeast and mold, ~ mingled spore, o UK yeast and mold, ~ UK spore.

**Differ from day 0 (P<.01). aobtained by subtracting noncasein N from total N. Journal of Dairy Science Vol. 63, No. 11, 1980

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AYLWARD ET AL.

TABLE 6. Effect of low temperature skim milk storage on whey nitrogen (WN) as a percentage of total nitrogen (aN). WN/TN X 100 Days of 5°C storage 6

Source

0

2

4

UK Mingled

29.0 27.5

29.8 29.6

30.0" 29.7

30.7"*

8

10

30.9"*

31.8" *

*Differ from day 0 (P<.05). **Differ from day 0 (P<.01).

the UK milk. This higher casein content would account for the 11% greater yield at 0 day. During skim milk storage, NPN and NCN increased and CN decreased. The metabolic activity of the psychrotrophic or proteolytic microorganisms in the skim milks could have caused these changes. Many of the psychrotrophic microorganisms in raw milk produce proteolytic enzymes (29). Proteolysis of casein can reduce yield of cheese from the affected milk. Cousin and Marth (6) found increased NPN and NCN during storage of milks inoculated with psychrotrophic bacteria. Weckbach and Langlois (28) found significant postpasteurization increases of NCN, noncasein protein nitrogen, and proteose-peptone in milks inoculated with a P s e u d o rnonas before heat treatment. The psychrotrophic microflora of cold-stored raw milk have an important impact on its casein content, and, thus, on the yield of cheese from the milk. The rate of CN decrease during 4 days of storage of mingled skim milks was approximately the same as the rate of decrease in yield of cottage cheese. There was approximately 19% less cottage cheese produced from UK skim milks after storage; however, the CN decrease was not this large. One possible explanation is a different species composition of the contaminating microflora of the skim milks obtained from different sources. This could have resulted in differing effects on casein. Increased moisture in the UK curd (Table 1), resulting in curd shattering, would have been responsible for the different yields in relations to the amount of casein destroyed in the two skim milks. Casein nitrogen decreased throughout storage of the skim milks. Casein degradation products, Journal of Dairy Science Vol. 63, No. 11, 1980

soluble at the isoelectric point of casein, should have contributed to increased nitrogen in whey and wash waters during cheese manufacture. Wash water was not collected, but the changes in whey nitrogen during skim milk storage are in Table 6. The nitrogen content of whey increased as time of skim milk storage increased. Noncasein nitrogen, as a percentage of TN, increased 4% in mingled skim milks and nearly 7% in UK skim milks. Whey nitrogen increases were nearer 2 to 3%. These differences were unexplained. The metabolic activity of the psychrotrophic and proteolytic microorganisms in the stored skim milks apparently was responsible for increases in n o n p r o t e i n a n d noncasein nitrogen. The highest quality milk should be used to obtain maximum yield of cottage cheese. This requires minimizing the initial bacterial count in milk and minimizing the time of milk storage before cheese is manufactured. ACKNOWLEDGMENT

The authors thank John Bucy for his assistance during the manufacture of cheese. REFERENCES

1 Adams, D. M., J. T. Barach, and M. L. Speck. 1976. Effect of psychrotrophic bacteria from raw milk on milk proteins and stability of milk proteins to ultrahigh temperature treatment. J. Dairy Sci. 59:823. 2 Atherton, H. V., and J. A. Newlander. 1977. The chemistry and testing of dairy products. 4th ed. AVI Publishing Co., Westport, CT. 3 Babel, F. J. 1953. Activity of bacteria and enzymes in raw milk held at 4.4°C. J. Dairy Sci. 36:562. (Abstr.)

MILK STORAGE AND COTTAGE CHEESE 4 Bliss, C. I. 1967. Statistics in biology. McGraw-Hill Book Company, New York, NY. 5 Chapman, J. R., M. E. Sharpe, and B. A. Law. 1976. Some effects of low-temperature storage of milk on cheese production and cheddar cheese flavour. Dairy Ind. 41(2):42. 6 Cousin, M. A., and E. H. Marth. 1977. Cottage cheese and yoghurt manufactured from milks pre-cultured with psychrotrophic bacteria. Cult. Dairy Prod. J. 12(2):15. 7 Cousins, C. M., M. E. Sharpe, and B. A. Law. 1977. The bacteriological quality of milk for cheddar cheesemaking. Dairy Ind. 42(7) : 12. 8 Cross, S. D., J. M. Henderson, and W. L. Dunkley. 1977. Losses and recovery of curd fines in cottage cheese manufacture. J. Dairy Sci. 60:1820. 9 Custer, E. W. 1977. Manufacturing top-quality cottage cheese. Cult. Dairy Prod. J. 12(4):18. 10 Davis, J. D., and F. J. MacDonald. 1953. Richmond's dairy chemistry. Charles Griffin and Co., Ltd., London. 11 DeBeukelar, N. J., M. A. Cousin, R. L. Bradley, Jr., and E. H. Marth. 1977. Modification of milk proteins by psychrotrophic bacteria. J. Dairy Sci. 60:857. 12 Emmons, D. B., and S. L. Tuckey. 1967. Cottage cheese and other cultured milk products. Chas. Pfizer and Co., Inc., New York, NY. 13 Frazer, W. C., and D. C. Westoff. 1978. Food microbiology. McGraw-Hill Book Company, New York, NY. 14 Hankin, L., G. R. Stephens, and W. F. Dillman. 1975. Quality control significance of special media for the enumeration of microbial groups in cottagetype cheese. J. Milk Food Technol. 38:738. 15 Hanson, R. S., J. A. Peterson, and A. A. Yousten. 1970. Unique biochemical events in bacterial sporulation. Annu. Rev. Microbiol. 24:53. 16 Hausler, W. J., Jr., ed. 1972. Standard methods for the examination of dairy products. 13th ed. Am.

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Publ. Health Assoc., Washington, DC. 17 Hicks, C. L., J. O'Leary, and J. Bucy. 1978. Degradation of protein and lipids during milk storage prior to cheddar cheese manufacture. J. Dairy Sci. 61(Suppl. 1):205. (Abstr.) 18 Kiuru, K., E. Eklund, H. Gyllenberg, and M. Antila. 1970. Psychrotrophic microorganisms in farm tank milk and their proteolytic activity. XVIII. Int. Dairy Congr. 1E: 108. 19 Kosikowski, F. V. 1977. Cheese and fermented milk foods. Edwards Brothers, Inc., Ann Arbor MI. 20 Law, B. A., M. E. Sharpe, and H. R. Chapman. 1976. The effect of lipolytic Gram-negative psychrotrophs in stored milk on the development of rancidity in Cheddar cheese. J. Dairy Res. 43:459. 21 Mohamed, F. O., and R. Bassette. 1979. Quality and yield of cottage cheese influenced by psychrotrophic microorganisms in milk. J. Dairy Sci. 62:222. 22 O'Leary, J., C. L. Hicks, and J. Bucy. 1977. Effect of low temperature storage of milk on cheese yield. J. Dairy Sci. 60(Suppl. 1):55. (Abstr.) 23 Olson, N. F. 1977. Factors affecting cheese yields. Dairy Ind. 42(4): 14. 24 Rowland, S. J. 1938. The precipitation of the proteins in milk. J. Dairy Res. 9: 30. 25 Rowland, S. J. 1938. The determination of the nitrogen distribution in milk. J. Dairy Res. 9:42. 26 Selitzer, R. 1976. The dairy industry in America. Magazines for Industry, Inc., New York, NY. 27 Speck, M. L., ed. 1976. Compendium of methods for the microbiological examination of foods. Am. Publ. Health Assoc., Inc., Washington, DC. 28 Weckbach, L. S., and B. E. Langiois. 1977. Effect of heat treatments on survival and growth of a psychrotroph and on nitrogen fractions in milk. J. Food Prot. 40:857. 29 Yates, A. R., and J. A. Elliott. 1977. The influence of psychrotrophs on the protein content of whey. Can. Inst. Food Sci. Technol. J. 10:269.

Journal of Dairy Science Vol. 63, No. 11, 1980