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Some Physical Effects of Freezing upon Milk and Cream^*

Research Laboratories, Bureau of Dairy Industry, U. S. Department of Agriculture, Washington, D. C.

ABSTRACT

1. Slow freezing of milk or cream caused a gradual precipitation of the caseinate system and an immediate destruction of the fat emulsion.
   
2. Freezing did not alter the heat stability of skim milk until the product had been held frozen for several months at –18° C. (0.4° F.) or below. Freezing caused an immediate increase in the amount of casein which could be centrifuged from milks heated before freezing. Freezing, therefore, caused a slow and gradual increase in the size of the casein aggregates but the change was not noticeable until the freezing period was well advanced.
   
3. The destruction of the fat emulsion in cream during slow freezing was lessened by adding cane sugar or increasing the solids-not-fat content of the cream before freezing. Homogenization slightly retarded fat separation when low fat creams were frozen. Freezing destroyed the fat clumps formed in cream by homogenization and restored to the cream the heat stability which it possessed before processing.
   
4. Fresh whole milk was pasteurized, condensed to its weight, canned and frozen without any detrimental effects to the body or flavor of the product. This milk when held frozen at a low temperature and reconstituted at any time within a four-week period by the addition of cold water, yielded a product which often could not be distinguished from fresh market milk. Its use where fresh market milk is expensive or not available was suggested.
   
5. A process for the preparation of large quantities of normal undenatured casein and of milk serum was developed. Frozen homogenized cream was thawed at a temperature below the melting point of the fat; clear milk serum was collected from the melting mass and the residual mixture of fat and casein was utilized in the preparation of normal casein or to raise the protein solids of ice cream mix.
   

^* Presented at the 29th Annual Meeting of the American Dairy Science Association, Geneva, N. T., June 28, 1934.