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Frozen Homogenized Milk. V. Effect of Age Before Freezing on the Keeping Quality of Frozen Homogenized Milk
FROZEN HOMOGENIZED MILK. V. E F F E C T OF AGE B E F O R E F R E E Z I N G ON T H E K E E P I N G QUALITY OF FROZEN HOMOGENIZED MILK C. J. BABCOCK,1 CAPT. J O S E P H N. S T A B I L E , SNC., USA, E R N E S T W I N D H A M , L A R U E B. EVANS, A•I) COL. RAYMOND RANDALL, VC, USA. Veterinary Division, Army Medical Department Research and Graduate School, Army Medical Center, Washington, D. C.
Earlier studies (2, 6, 7, 9, 10) have shown that freezing and storage temperatures affect the physical character of homogenized milk. Previous studies (3, 4, 8) also have shown that when homogenized milk was frozen, the solid components tended to concentrate in the lower portion of the sample during the freezing process; apparently there was no further movement of these solids after the milk was frozen. A more recent study (5) has shown that frozen homogenized milk of good quality can be stored at usual storage temperatures after thawing without deterioration for longer periods of time than usually are necessary before use. The literature does not contain information regarding the effect of the age of homogenized milk before freezing on its keeping quality after freezing. The present study was undertaken to determine the effect of age before freezing on the keeping quality of frozen homogenized milk. PROCEDURE
Homogenized milk samples with a fat content of 3.8 per cent packaged in one-half pint paper containers by a commercial dairy in Washington, D. C., were used. The milk had been pasteurized at 155 ° F. for 30 minutes. Fifty-six samples were taken directly from the filler and divided into seven groups of eight samples each. One sample was examined immediately for flavor, bacterial count, coliform organisms, titratable acidity, pH, and sediment and the remaining seven samples of this group were placed in a freezer held at -17.5 ° C. 2 The remaining samples were held at 1.67 ° C. Eight samples, each representing a separate group, were removed from the 1.67 ° C. storage after 12, 24, 48, 72, 96 and 120 hours. Each time, one of the eight samples of the respective group was used for laboratory examination and the remaining seven were placed in a freezer at - 17.5 ° C. The samples were removed from the -17.5 ° C. storage and thawed for laboratory examination after having been held in the frozen state for 5, 28, 38, 47, 56, 76, and 86 days. The determinations for bacterial content, Received for publication April 30, 1948. 1 Formerly Lieutenant Colonel, SnC., AUS. Present address: Dairy Branch, Production and Marketing Administration, U. S. Department of Agriculture. 2 The thermostat controlling the temperature of this freezer had sufficient lag to cause a temperature variation of about eight degrees. 811
TABLE
Effectofagebeforefreezing Age before freezing (Hr.)
1
on t he ~ e e p i n g q u a l i t y o f f r o z e n h o m o g e n i z e d
Not frozen
mil~
No. of days frozen 5
28
38
47
56
76
86
V . S1. ox. Normal Normal Normal Normal V . S1. o x i dized S1. o x i dized
S1. ox.a Normal S1. ox.a Normal Normal Normal
S1. s t a l e - - o x . S1. s t a l e - - o x . S1. s t a l e - - o x . Stale--ox. Stale---ox. Stale---ox.
Normal
Stale--ox.
•........... •........... ............ •........... ............ ............ ............
4900 + 2800 26002300 2500 1100 1200 -
Flavor 0 12 24 48 72 96
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal .........
120
Normal
Normal
Normal
Normal
Normal
Bacterial 0 12 24 48 72 96 120
3600 + 3400 1500900 700 1100 1100 +
1000 1000 1300 900 600 600 700
+ + -
countb
2100 +
1400 -
1600 -
1500 -
1600 -
1600 +
12002500 1800 + 1300 500 -
13001100 + 1700 1200 + 1100 -
1800 1100 1000 1400 800
-
2200 2600 1600900 ............ + 1500 1600 +
©
T A B L E 1 (continued)
Effect o f age before freezing on the keeping quality of froze~ homogenized milk Sediment
(mZ.)
(ml.)
(ml.)
(m~.)
(m~.)
(m~.)
(~z.)
(m~.)
0 12 24 48
0.02 0.03 0.03 0.02
2.20
0.02 0.02
0.60 0.25 0.30 0.50 0.60 ......... 0.65
1.10
96 120
0.40 0.60 0.40 0.20 0.90 0.50 0.40
1.20 1.30
0.02
0.25 0.10 0.10 0.55 0.30 0.40 0.20
1.20 0.70
72
0.02 0.02 0.02 0.02 0.03 0.02 0.02
0.60 1.10 ] .30 0.60
2.70 2.60 1.60 2.00
4.50 3.50 3.80 2.50 4.50 2.20 3.00
Acidity
0 ]2 24 48 72 96 120
Titratable 0.115 0.125 0.120 0.120 0.120 0.120 0.120
pH 6.67 6.79 6.64 6.60 6.63 6.65 6.64
Titratable 0.125 0.120 0.120 0.115 0.110 0.115 0.115
pH 6.62 6.63 6.6l 6.53
6.59 6.65 6.62
Titratable 0.115 0.115 0.120 0.115 0.110 0.115 0.115
pH 6.62 6.58 6.60 6.72 6.72 6.586.62
Titratable 0.110 0.110 0.110 0.115 0.115 0.115 0.110
pH 6.66 6.62 6.63 6.62 6.61 6.60 6.61
TitrapH table 0.115 6.68 0.115 6.62 0.115 6.64 0.110 6.67 0.110 6.64 ................... 0.115 6.63
a ox. -- oxidised b + and - denote presence or absence of coliform o r g a n i s m s in 1-ml. samples.
Titratable 0.115 0.110 0.115 0.110 0.110 0.110 0.110
pH 6.53 6.52 6.51 6.52 6.53 6.53 6.60
Titratable 0.]30 0.130 0.125 0.125 0.130 0.130 0.130
o
pH
Titratable
pH
6.56 6.55 6.61 6.59 6.63 6.62 6.62
0.130 0.130 0.125 0.125 0.130 0.130 0.130
6.56 6.55
6.61 6.59 6.63 6.62 6.42
o 0
814
C. J . BABCOCK E T AL.
coliform organisms, titratable acidity, and p H were made in accordance with the methods outlined in S t a n d a r d Methods for the Examination of Dairy Products (1). The plates were incubated at 37 ° C. for 48 hours. Flavor determinations were made by a panel of three men, all of whom were experienced in milk judging. The sediment was determined by the method used by the authors in their earlier studies (2, 3). RESL~LTS
The effect of age before freezing on the keeping quality of frozen homogenized milk is shown in table 1. The milk was of good flavor throughout the 120 hours that it was held at 1.67 ° C. before freezing. The flavor of the milk remained good when it was held in the frozen state for 47 days, regardless of its age before freezing. Some of the samples that were thawed after they had been frozen for 56 days and for 76 days had developed a slight oxidized flavor. However, there was no correlation between the age of the samples before freezing and the development of this flavor. When the samples were thawed after holding in the frozen state for 86 days, they had a stale and oxidized flavor. There was an insignificant tendency for these flavors to be more pronounced as the age of the samples before freezing was increased. Table 1 also shows that there was no significant change in the bacterial content of the samples either in the 120 hours that they were held at 1.67 ° C. or in the 89 days that they were held in the frozen state at - 1 7 . 5 ° C. There apparently was a slight coliform contamination of the milk that was used in the preparation of the samples. With the exception of those samples which were held at 1.67 ° C. for 24 hours before freezing, at least one sample in each age group was positive for coliform organisms in ]-ml. portions either before or after freezing. Two of the samples before freezing and two each of those held in the frozen state for 5, 28, 47 and 56 days gave positive coliform tests. Of these samples held in the frozen state for 47 and for 86 days, one each gave a positive test. There was no correlation between the positive coliform tests in the samples before and after freezing. Table 1 shows f u r t h e r that the quantity of sediment remained low and constant in the milk samples before they were frozen. It did not increase when the samples were held in the frozen state for 5 days. When held for 38 days, however, the quantity of sediment had materially increased and it continued gradually to increase as the storage time lengthened. Significant separation, as shown by the sediment readings, had occurred when the samples were thawed after they had been held for 47 days in the frozen state, and considerable sediment ~vas present in all the samples when they were thawed after having been frozen for 89 days. There was, how-
AGE AND KEEPING QUALITY OF HOMOGENIZED MILK
815
ever, no correlation between the degree of separation and the age of the sample prior to freezing. The acidity as shown by titration and by pH determinations did not vary significantly either before the samples were frozen or while they were held in the frozen state. The titratable acidity ranged from 0.110 to 0.130 per cent and the pI-t values from 6.51 to 6.79. These variations were well within the limits of experimental error. The titratable acidity values were lower than usually encountered because of the dilution technic employed. CONCLUSION
It is recognized that homogenized milk should be frozen as soon as possible after processing, but if the milk is of good quality it may be kept as long as 120 hours at 1.67 ° C. before freezing without adversely affecting the keeping quality of the frozen product. The authors wish to express their appreciation to Edith Giltner and Elmina Dickson for their assistance with the analytical determinations. REFERENCES (1) AMERICAN PUBLIC HEALTH ASSOCIATION. Standard Methods for the Examination of Dairy Products. 8th edn. New York, N . Y . 1941. (2) BABCOCK,C. 5., ROERIQ, R. N., STABILE, J. •., DUNLAP, W. A., AND RANDALL,R. Frozen and Homogenized Milk. I. Effect of Freezing and Storage Temperatures on the Physical Characteristics of Homogenized Milk. J. Dairy Sci. 29: 699-706. 1946. (3) BABCOCK,C..I., ROERIG, R. ~.~ STABILE, J. ~.~ DUNLAP, W. A., AND RANDALL, R. Frozen Homogenized Milk. ][I. Effect of Freezing and Storage Temperatures on the Chemical and Bacteriological Properties of Homogenized Milk. J. Dairy Sci., 30: 49-54. 1947. (4) BABCOCK,C. J., STABILE, J. •., R_kNDALL,R.. AND WINDHA~f, E. S. Frozen Homogenized Milk. I l l . Stability of Milk Solids Distribution in Frozen Homogenized Milk. J. Dairy Sci., 30: 733-736. 1947. (5) BABCOCK,C. 5., STABILE, J. •., RANDALL,R., AND WINDIIA~, E . S . Frozen Homogenized Milk. IV. Keeping Quality of Frozen Homogenized Milk after Thawing. J. Dairy Sci., 31: 805-810. 1948. (6) CIVTL, ft. Einfluss des Gefrierens auf Einige Bestandteile und Eigenschaften der Milch. Milchw. Forsch.. 12: 409-432. 1931. (7) ROADHOUSE, C. L., AN]) HENDERSON, J. L. How Quick Freezing Affects Keeping Quality of Milk. Food Inds., 12: 54-55. 1940. (8) THURSTON, L. M., BROWN, W. C., AND DUST~IAN, R . B . O~dized Flavor in Milk. J. Dairy Sci., 19: 671-681. 1936: (9) TROUT, G . M . The Freezing and Thawing of Milk Homogenized at Various Pressures. J. Dairy Sci., 24: 277-287. 1941. (10) WEBB, B. H., AND HALL, S. A. Some Physical Effects of Freezing upon Milk and Cream. J. Dairy Sci., 18: 275-287. 1935.
Earlier studies (2, 6, 7, 9, 10) have shown that freezing and storage temperatures affect the physical character of homogenized milk. Previous studies (3, 4, 8) also have shown that when homogenized milk was frozen, the solid components tended to concentrate in the lower portion of the sample during the freezing process; apparently there was no further movement of these solids after the milk was frozen. A more recent study (5) has shown that frozen homogenized milk of good quality can be stored at usual storage temperatures after thawing without deterioration for longer periods of time than usually are necessary before use. The literature does not contain information regarding the effect of the age of homogenized milk before freezing on its keeping quality after freezing. The present study was undertaken to determine the effect of age before freezing on the keeping quality of frozen homogenized milk. PROCEDURE
Homogenized milk samples with a fat content of 3.8 per cent packaged in one-half pint paper containers by a commercial dairy in Washington, D. C., were used. The milk had been pasteurized at 155 ° F. for 30 minutes. Fifty-six samples were taken directly from the filler and divided into seven groups of eight samples each. One sample was examined immediately for flavor, bacterial count, coliform organisms, titratable acidity, pH, and sediment and the remaining seven samples of this group were placed in a freezer held at -17.5 ° C. 2 The remaining samples were held at 1.67 ° C. Eight samples, each representing a separate group, were removed from the 1.67 ° C. storage after 12, 24, 48, 72, 96 and 120 hours. Each time, one of the eight samples of the respective group was used for laboratory examination and the remaining seven were placed in a freezer at - 17.5 ° C. The samples were removed from the -17.5 ° C. storage and thawed for laboratory examination after having been held in the frozen state for 5, 28, 38, 47, 56, 76, and 86 days. The determinations for bacterial content, Received for publication April 30, 1948. 1 Formerly Lieutenant Colonel, SnC., AUS. Present address: Dairy Branch, Production and Marketing Administration, U. S. Department of Agriculture. 2 The thermostat controlling the temperature of this freezer had sufficient lag to cause a temperature variation of about eight degrees. 811
TABLE
Effectofagebeforefreezing Age before freezing (Hr.)
1
on t he ~ e e p i n g q u a l i t y o f f r o z e n h o m o g e n i z e d
Not frozen
mil~
No. of days frozen 5
28
38
47
56
76
86
V . S1. ox. Normal Normal Normal Normal V . S1. o x i dized S1. o x i dized
S1. ox.a Normal S1. ox.a Normal Normal Normal
S1. s t a l e - - o x . S1. s t a l e - - o x . S1. s t a l e - - o x . Stale--ox. Stale---ox. Stale---ox.
Normal
Stale--ox.
•........... •........... ............ •........... ............ ............ ............
4900 + 2800 26002300 2500 1100 1200 -
Flavor 0 12 24 48 72 96
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal Normal
Normal Normal Normal Normal Normal .........
120
Normal
Normal
Normal
Normal
Normal
Bacterial 0 12 24 48 72 96 120
3600 + 3400 1500900 700 1100 1100 +
1000 1000 1300 900 600 600 700
+ + -
countb
2100 +
1400 -
1600 -
1500 -
1600 -
1600 +
12002500 1800 + 1300 500 -
13001100 + 1700 1200 + 1100 -
1800 1100 1000 1400 800
-
2200 2600 1600900 ............ + 1500 1600 +
©
T A B L E 1 (continued)
Effect o f age before freezing on the keeping quality of froze~ homogenized milk Sediment
(mZ.)
(ml.)
(ml.)
(m~.)
(m~.)
(m~.)
(~z.)
(m~.)
0 12 24 48
0.02 0.03 0.03 0.02
2.20
0.02 0.02
0.60 0.25 0.30 0.50 0.60 ......... 0.65
1.10
96 120
0.40 0.60 0.40 0.20 0.90 0.50 0.40
1.20 1.30
0.02
0.25 0.10 0.10 0.55 0.30 0.40 0.20
1.20 0.70
72
0.02 0.02 0.02 0.02 0.03 0.02 0.02
0.60 1.10 ] .30 0.60
2.70 2.60 1.60 2.00
4.50 3.50 3.80 2.50 4.50 2.20 3.00
Acidity
0 ]2 24 48 72 96 120
Titratable 0.115 0.125 0.120 0.120 0.120 0.120 0.120
pH 6.67 6.79 6.64 6.60 6.63 6.65 6.64
Titratable 0.125 0.120 0.120 0.115 0.110 0.115 0.115
pH 6.62 6.63 6.6l 6.53
6.59 6.65 6.62
Titratable 0.115 0.115 0.120 0.115 0.110 0.115 0.115
pH 6.62 6.58 6.60 6.72 6.72 6.586.62
Titratable 0.110 0.110 0.110 0.115 0.115 0.115 0.110
pH 6.66 6.62 6.63 6.62 6.61 6.60 6.61
TitrapH table 0.115 6.68 0.115 6.62 0.115 6.64 0.110 6.67 0.110 6.64 ................... 0.115 6.63
a ox. -- oxidised b + and - denote presence or absence of coliform o r g a n i s m s in 1-ml. samples.
Titratable 0.115 0.110 0.115 0.110 0.110 0.110 0.110
pH 6.53 6.52 6.51 6.52 6.53 6.53 6.60
Titratable 0.]30 0.130 0.125 0.125 0.130 0.130 0.130
o
pH
Titratable
pH
6.56 6.55 6.61 6.59 6.63 6.62 6.62
0.130 0.130 0.125 0.125 0.130 0.130 0.130
6.56 6.55
6.61 6.59 6.63 6.62 6.42
o 0
814
C. J . BABCOCK E T AL.
coliform organisms, titratable acidity, and p H were made in accordance with the methods outlined in S t a n d a r d Methods for the Examination of Dairy Products (1). The plates were incubated at 37 ° C. for 48 hours. Flavor determinations were made by a panel of three men, all of whom were experienced in milk judging. The sediment was determined by the method used by the authors in their earlier studies (2, 3). RESL~LTS
The effect of age before freezing on the keeping quality of frozen homogenized milk is shown in table 1. The milk was of good flavor throughout the 120 hours that it was held at 1.67 ° C. before freezing. The flavor of the milk remained good when it was held in the frozen state for 47 days, regardless of its age before freezing. Some of the samples that were thawed after they had been frozen for 56 days and for 76 days had developed a slight oxidized flavor. However, there was no correlation between the age of the samples before freezing and the development of this flavor. When the samples were thawed after holding in the frozen state for 86 days, they had a stale and oxidized flavor. There was an insignificant tendency for these flavors to be more pronounced as the age of the samples before freezing was increased. Table 1 also shows that there was no significant change in the bacterial content of the samples either in the 120 hours that they were held at 1.67 ° C. or in the 89 days that they were held in the frozen state at - 1 7 . 5 ° C. There apparently was a slight coliform contamination of the milk that was used in the preparation of the samples. With the exception of those samples which were held at 1.67 ° C. for 24 hours before freezing, at least one sample in each age group was positive for coliform organisms in ]-ml. portions either before or after freezing. Two of the samples before freezing and two each of those held in the frozen state for 5, 28, 47 and 56 days gave positive coliform tests. Of these samples held in the frozen state for 47 and for 86 days, one each gave a positive test. There was no correlation between the positive coliform tests in the samples before and after freezing. Table 1 shows f u r t h e r that the quantity of sediment remained low and constant in the milk samples before they were frozen. It did not increase when the samples were held in the frozen state for 5 days. When held for 38 days, however, the quantity of sediment had materially increased and it continued gradually to increase as the storage time lengthened. Significant separation, as shown by the sediment readings, had occurred when the samples were thawed after they had been held for 47 days in the frozen state, and considerable sediment ~vas present in all the samples when they were thawed after having been frozen for 89 days. There was, how-
AGE AND KEEPING QUALITY OF HOMOGENIZED MILK
815
ever, no correlation between the degree of separation and the age of the sample prior to freezing. The acidity as shown by titration and by pH determinations did not vary significantly either before the samples were frozen or while they were held in the frozen state. The titratable acidity ranged from 0.110 to 0.130 per cent and the pI-t values from 6.51 to 6.79. These variations were well within the limits of experimental error. The titratable acidity values were lower than usually encountered because of the dilution technic employed. CONCLUSION
It is recognized that homogenized milk should be frozen as soon as possible after processing, but if the milk is of good quality it may be kept as long as 120 hours at 1.67 ° C. before freezing without adversely affecting the keeping quality of the frozen product. The authors wish to express their appreciation to Edith Giltner and Elmina Dickson for their assistance with the analytical determinations. REFERENCES (1) AMERICAN PUBLIC HEALTH ASSOCIATION. Standard Methods for the Examination of Dairy Products. 8th edn. New York, N . Y . 1941. (2) BABCOCK,C. 5., ROERIQ, R. N., STABILE, J. •., DUNLAP, W. A., AND RANDALL,R. Frozen and Homogenized Milk. I. Effect of Freezing and Storage Temperatures on the Physical Characteristics of Homogenized Milk. J. Dairy Sci. 29: 699-706. 1946. (3) BABCOCK,C..I., ROERIG, R. ~.~ STABILE, J. ~.~ DUNLAP, W. A., AND RANDALL, R. Frozen Homogenized Milk. ][I. Effect of Freezing and Storage Temperatures on the Chemical and Bacteriological Properties of Homogenized Milk. J. Dairy Sci., 30: 49-54. 1947. (4) BABCOCK,C. J., STABILE, J. •., R_kNDALL,R.. AND WINDHA~f, E. S. Frozen Homogenized Milk. I l l . Stability of Milk Solids Distribution in Frozen Homogenized Milk. J. Dairy Sci., 30: 733-736. 1947. (5) BABCOCK,C. 5., STABILE, J. •., RANDALL,R., AND WINDIIA~, E . S . Frozen Homogenized Milk. IV. Keeping Quality of Frozen Homogenized Milk after Thawing. J. Dairy Sci., 31: 805-810. 1948. (6) CIVTL, ft. Einfluss des Gefrierens auf Einige Bestandteile und Eigenschaften der Milch. Milchw. Forsch.. 12: 409-432. 1931. (7) ROADHOUSE, C. L., AN]) HENDERSON, J. L. How Quick Freezing Affects Keeping Quality of Milk. Food Inds., 12: 54-55. 1940. (8) THURSTON, L. M., BROWN, W. C., AND DUST~IAN, R . B . O~dized Flavor in Milk. J. Dairy Sci., 19: 671-681. 1936: (9) TROUT, G . M . The Freezing and Thawing of Milk Homogenized at Various Pressures. J. Dairy Sci., 24: 277-287. 1941. (10) WEBB, B. H., AND HALL, S. A. Some Physical Effects of Freezing upon Milk and Cream. J. Dairy Sci., 18: 275-287. 1935.