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Effect of Freezing and Length of Storage on Milk Properties1
Effect of Freezing and Length of Storage on Milk Properties S, J. WEESE, D. F. BUTCHER and R. O. THOMAS
Department of Animal Industry and Veterinary Science West Virginia University, Morgantown 26506 Abstract
One hundred six composite samples of milk were collected for 10 weeks from evening and morning milkings of individual cows. Samples were divided into three portions and analyzed fresh and after storage at --26 C, for 7 and 90 days. Concentrations of milk fat, solids-not-fat, total solids, chloride and protein, and the freezing point were determined. Differences between fresh samples and those that had been frozen were significant except for lactose and chloride. Introduction
I t is not always practical to analyze milk the same day samples are taken. This experiment was designed to study what effect freezing and thawing had on commonly used measurements of composition of individual cow samples. Experimental Procedures
One pint samples made up of the evening's and the following morning's milk from 16 cows for first collection and 18 cows for the five subsequent collections were taken for 10 weeks. All samples were divided into three equal portions. One portion was analyzed fresh (one day); the second portion was frozen, stored seven days and analyzed after thawing, and the third portion was frozen and stored for 90 days before being thawed and analyzed. All determinations were made by the same person. The Babcock test was used for determining milk fat, the Golding bead test (3) for solidsnot-fat and total solids, the Mojonnier test for total solids, and the Fiske cryoscope for freezing point. Lactose was determined by a modified method of Folin and W u (4) for blood sugar. Orange G dye-binding (1) readings of optical density were used instead of being converted to protein. The same Orange G solution was used for all determinations. Received for publication December 20, 1968. 1 Published with the approval of the Director of the West Virginia University Agricultural Experiment Staiton as Scientific Paper no. 1050.
Paired comparison t-tests were used to analyze differences between determinations on fresh and frozen samples, l~or each constituent, three comparisons were computed for all samples. 1) Determinations on sample when fresh minus determinations on sample after being frozen for seven days. 2) Determinations on sample when fresh minus determinations on sample after being frozen for 90 days. 3) Determinations on sample after being frozen for seven days minus determinations on sample after being frozen for 90 days. The paired comparison t-test was computed by dividing the mean difference by the standard error of the mean difference. These means are not independent because Difference c is equal to Difference b minus Difference a. Results and Discussion
Table 1 gives mean differences for the nine constituents measured on all samples. All comparisons for per cent milk fat and for dye-binding readings were signi~cantly different. Fresh samples had the highest per cent milk fat, followed by those that were frozen and stored 90 days and then those stored seven days. The increase in mean per cent milk fat determination from 7 to 90 days may be due to warming the milk which was frozen for 90 days, whereas milk frozen seven days was not warmed. Although the mean per cent milk fat for milk frozen 90 days was closer to that for fresh milk, the correlations (Table 2) indicate that the accuracy of evaluating milk fat content by the Babcock method is considerably less when milk had been frozen 90 days, and warmed to 34 C, before analyzing, than when it had been frozen seven days and analyzed immediately after thawing. Unfortunately, the effects of length of storage and warming cannot be separated in these data. The Orange G dye-binding readings were highest for samples frozen 90 days, with samples frozen seven days ranking second and fresh samples were the lowest. As the dyebinding reading increased, per cent protein decreased, agreeing with the micro-Kjeldahl method except that micro-Kjeldahl values for fresh milk and milk frozen seven days were essentially the same.
1724
FREEZING ~iILK
1725
TABLE 1. Means, standard deviations, and differences due to freezing and thawing and length of storage for various properties of milk.
Constituents
Mean
SD
7 Days
90 Days
Frozen 7 days --frozen 90 days
Milk fat (%) Solids-not-fat (%) Total solids (%) Chloride (% × 103) Freezing point (C) Micro-Kjeldahl (%N) Lactose ( % ) ~ojonnier, total solids (%) Dye-binding
3.98 8.51 12.47 95.5 --0.539 3.09 4.76 12.46
0.572 0.311 0.751 18.11 0.007 0.325 0.503 0.820
0.2189 ~ 0.0211 0.2106 ~° 0.3255 0.004 0.0046 0.0533 0.1855 ~
0.0858 * 0.1524 ~ 0.2081 ~* --0.6066 0.004 0.0498 ~ -- 0.0809 0.200 ~
--0.1330 ~ 0.1312 *~ --0.0024 0.9321 0.004 0.0452 ** --0.1342 ~ 0.0155
559.2
69.6
Fresh
Freshfrozen
--4.047 '~~
--22.97 ~
--18.92 **
Significantly different from zero at .05 level. ~ Significantly different from zero at .01 level. Determinations for per cent solids-not-fat and for freezing point revealed the same trend, with fresh samples being essentially the same as samples frozen seven days; whereas those frozen 90 days were significantly lower than fresh samples and those frozen seven days. Length of storage appears to cause a decrease in the mean value of these determinations, whereas the effect of freezing itself seems to TABLE 2. Correlations between measurements on the same sample of milk after different lengths of storage.
Frozen 7 days
Frozen 90 days
Frozen 7 days versus frozen 90 days
0.895
0.736
0.729
0.815
0.656
0.666
0.878 0.973
0.759 0.945
0.741 0.953
0.868
0.799
0.880
0.970 0.746
0.960 0.460
0.975 0.445
0.864
0.885
0.841
0.954
0.850
0.897
Fresh versus : Constituents Fat (%) Solids-notfat (%) Total solids ( % ) Chloride (%) Freezing point MicroKjeldahl Lactose Mojonnier solids Dyebinding
have little effect, the relative accuracy (Table 2) of these determinations also decreased with length of time stored. Length of time samples were stored had no influence on either measure of per cent total solids, but fresh samples were significantly higher in total solids than frozen samples by both Mojonnier and Golding bead methods. The correlation coefficients (Table 2) indicated that there may have been a decrease in accuracy of the Go]ding bead method of estimating total solids as the length of storage increased, however, this decline in accuracy did not occur with the l~ojonnier method. A significant increase in detectable lactose concentration occurred in milk from the seventh to ninetieth day of frozen storage, while the level of lactose in fresh milk was not significantly different from that of frozen milk. Chloride was not influenced by freezing and thawing or by length of storage; and the relative accuracy of detection was high after freezing for 7 and 90 days. Conclusions
Our data indicate that milk fat, total solids and dye-binding estimates are significantly affected by the freezing and thawing process, whereas milk fat, Golding bead, freezing point, micro-Kjeldahl, lactose and dye-binding estimates are affected by length of frozen storage. These data indicate that accurate determination of chloride content could be made on frozen milk. Since correlation coefficients are large, micro-Kjeldahl values could be used to acJ. DAIRY SCIENCE V0r,. 52, ~O. 11
1726
W E E S E E T AL.
curately r a n k animals for protein content if all milk was held in frozen storage the same length of time; however, this would not be an accurate method to determine protein content due to the significant differences between the means. One should be careful in determining milk fat, total solids, dye-binding, freezing point and lactose on frozen milk, since the results may be quite different f r o m those obtained on fresh milk. Some of these differences are undoubtedly due to coagulation when frozen milk is thawed and some may be due to degradation o f constituents.
J. DAIRY SCIENCE VOL. 52, NO. 11
References
(1) Ashworth, U. S., R. Seals, a~d R. E. Erb. 1960. An improved procedure for the determination of milk proteins by dye-binding. J. Dairy Sci., 43: 614. (2) Association of Official Agricultxlral Chemists. 1965. Official Methods of Analysis. 18th ed., Washington, D.C. (3) Golding, N. S. 1964. Procedure for the Golding plastic bead test for solids-not-fat in milk. Washington State Univ. Inst. Agr. Sci. Ext. Cir., 340. (4) Oser, B. L. 1965. Hawk's Physiological Chemistry. 14th ed., McGraw-Hill Book Co., New York.
Department of Animal Industry and Veterinary Science West Virginia University, Morgantown 26506 Abstract
One hundred six composite samples of milk were collected for 10 weeks from evening and morning milkings of individual cows. Samples were divided into three portions and analyzed fresh and after storage at --26 C, for 7 and 90 days. Concentrations of milk fat, solids-not-fat, total solids, chloride and protein, and the freezing point were determined. Differences between fresh samples and those that had been frozen were significant except for lactose and chloride. Introduction
I t is not always practical to analyze milk the same day samples are taken. This experiment was designed to study what effect freezing and thawing had on commonly used measurements of composition of individual cow samples. Experimental Procedures
One pint samples made up of the evening's and the following morning's milk from 16 cows for first collection and 18 cows for the five subsequent collections were taken for 10 weeks. All samples were divided into three equal portions. One portion was analyzed fresh (one day); the second portion was frozen, stored seven days and analyzed after thawing, and the third portion was frozen and stored for 90 days before being thawed and analyzed. All determinations were made by the same person. The Babcock test was used for determining milk fat, the Golding bead test (3) for solidsnot-fat and total solids, the Mojonnier test for total solids, and the Fiske cryoscope for freezing point. Lactose was determined by a modified method of Folin and W u (4) for blood sugar. Orange G dye-binding (1) readings of optical density were used instead of being converted to protein. The same Orange G solution was used for all determinations. Received for publication December 20, 1968. 1 Published with the approval of the Director of the West Virginia University Agricultural Experiment Staiton as Scientific Paper no. 1050.
Paired comparison t-tests were used to analyze differences between determinations on fresh and frozen samples, l~or each constituent, three comparisons were computed for all samples. 1) Determinations on sample when fresh minus determinations on sample after being frozen for seven days. 2) Determinations on sample when fresh minus determinations on sample after being frozen for 90 days. 3) Determinations on sample after being frozen for seven days minus determinations on sample after being frozen for 90 days. The paired comparison t-test was computed by dividing the mean difference by the standard error of the mean difference. These means are not independent because Difference c is equal to Difference b minus Difference a. Results and Discussion
Table 1 gives mean differences for the nine constituents measured on all samples. All comparisons for per cent milk fat and for dye-binding readings were signi~cantly different. Fresh samples had the highest per cent milk fat, followed by those that were frozen and stored 90 days and then those stored seven days. The increase in mean per cent milk fat determination from 7 to 90 days may be due to warming the milk which was frozen for 90 days, whereas milk frozen seven days was not warmed. Although the mean per cent milk fat for milk frozen 90 days was closer to that for fresh milk, the correlations (Table 2) indicate that the accuracy of evaluating milk fat content by the Babcock method is considerably less when milk had been frozen 90 days, and warmed to 34 C, before analyzing, than when it had been frozen seven days and analyzed immediately after thawing. Unfortunately, the effects of length of storage and warming cannot be separated in these data. The Orange G dye-binding readings were highest for samples frozen 90 days, with samples frozen seven days ranking second and fresh samples were the lowest. As the dyebinding reading increased, per cent protein decreased, agreeing with the micro-Kjeldahl method except that micro-Kjeldahl values for fresh milk and milk frozen seven days were essentially the same.
1724
FREEZING ~iILK
1725
TABLE 1. Means, standard deviations, and differences due to freezing and thawing and length of storage for various properties of milk.
Constituents
Mean
SD
7 Days
90 Days
Frozen 7 days --frozen 90 days
Milk fat (%) Solids-not-fat (%) Total solids (%) Chloride (% × 103) Freezing point (C) Micro-Kjeldahl (%N) Lactose ( % ) ~ojonnier, total solids (%) Dye-binding
3.98 8.51 12.47 95.5 --0.539 3.09 4.76 12.46
0.572 0.311 0.751 18.11 0.007 0.325 0.503 0.820
0.2189 ~ 0.0211 0.2106 ~° 0.3255 0.004 0.0046 0.0533 0.1855 ~
0.0858 * 0.1524 ~ 0.2081 ~* --0.6066 0.004 0.0498 ~ -- 0.0809 0.200 ~
--0.1330 ~ 0.1312 *~ --0.0024 0.9321 0.004 0.0452 ** --0.1342 ~ 0.0155
559.2
69.6
Fresh
Freshfrozen
--4.047 '~~
--22.97 ~
--18.92 **
Significantly different from zero at .05 level. ~ Significantly different from zero at .01 level. Determinations for per cent solids-not-fat and for freezing point revealed the same trend, with fresh samples being essentially the same as samples frozen seven days; whereas those frozen 90 days were significantly lower than fresh samples and those frozen seven days. Length of storage appears to cause a decrease in the mean value of these determinations, whereas the effect of freezing itself seems to TABLE 2. Correlations between measurements on the same sample of milk after different lengths of storage.
Frozen 7 days
Frozen 90 days
Frozen 7 days versus frozen 90 days
0.895
0.736
0.729
0.815
0.656
0.666
0.878 0.973
0.759 0.945
0.741 0.953
0.868
0.799
0.880
0.970 0.746
0.960 0.460
0.975 0.445
0.864
0.885
0.841
0.954
0.850
0.897
Fresh versus : Constituents Fat (%) Solids-notfat (%) Total solids ( % ) Chloride (%) Freezing point MicroKjeldahl Lactose Mojonnier solids Dyebinding
have little effect, the relative accuracy (Table 2) of these determinations also decreased with length of time stored. Length of time samples were stored had no influence on either measure of per cent total solids, but fresh samples were significantly higher in total solids than frozen samples by both Mojonnier and Golding bead methods. The correlation coefficients (Table 2) indicated that there may have been a decrease in accuracy of the Go]ding bead method of estimating total solids as the length of storage increased, however, this decline in accuracy did not occur with the l~ojonnier method. A significant increase in detectable lactose concentration occurred in milk from the seventh to ninetieth day of frozen storage, while the level of lactose in fresh milk was not significantly different from that of frozen milk. Chloride was not influenced by freezing and thawing or by length of storage; and the relative accuracy of detection was high after freezing for 7 and 90 days. Conclusions
Our data indicate that milk fat, total solids and dye-binding estimates are significantly affected by the freezing and thawing process, whereas milk fat, Golding bead, freezing point, micro-Kjeldahl, lactose and dye-binding estimates are affected by length of frozen storage. These data indicate that accurate determination of chloride content could be made on frozen milk. Since correlation coefficients are large, micro-Kjeldahl values could be used to acJ. DAIRY SCIENCE V0r,. 52, ~O. 11
1726
W E E S E E T AL.
curately r a n k animals for protein content if all milk was held in frozen storage the same length of time; however, this would not be an accurate method to determine protein content due to the significant differences between the means. One should be careful in determining milk fat, total solids, dye-binding, freezing point and lactose on frozen milk, since the results may be quite different f r o m those obtained on fresh milk. Some of these differences are undoubtedly due to coagulation when frozen milk is thawed and some may be due to degradation o f constituents.
J. DAIRY SCIENCE VOL. 52, NO. 11
References
(1) Ashworth, U. S., R. Seals, a~d R. E. Erb. 1960. An improved procedure for the determination of milk proteins by dye-binding. J. Dairy Sci., 43: 614. (2) Association of Official Agricultxlral Chemists. 1965. Official Methods of Analysis. 18th ed., Washington, D.C. (3) Golding, N. S. 1964. Procedure for the Golding plastic bead test for solids-not-fat in milk. Washington State Univ. Inst. Agr. Sci. Ext. Cir., 340. (4) Oser, B. L. 1965. Hawk's Physiological Chemistry. 14th ed., McGraw-Hill Book Co., New York.