Some Physical Effects of Freezing upon Milk and Cream

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Abstract
Many observations on the physical effects of freezing upon pure sols have been reported and an interesting review of some of these studies is given by Jones and Gortner (3). An investigation of the effects o~ freez- ing upon milk and cream is complicated by the heterogeneous character of the material. The constituents of milk with which this study is chiefly concerned are the casein and the fat. A hydrophilic sol which has been frozen will repeptize upon thawing whereas a hydrophobic sol similarly treated will precipitate when the frozen mass is melted. The casein of milk, being weakly hydrated retains its normal degree of dispersion under the usual conditions of freezing and thawing. The fact, however, has been noticed that when milk is held in the frozen state for considerable periods of time the casein gradually becomes insoluble (1, 2, 5). Evidently its hydrophilic properties are altered during storage in a frozen condition. The fat is present in milk as an emulsion and is surrounded by adsorbed protein. If milk or cream has been frozen slowly free fat separates or oils off during thawing, especially when the thawing is conducted at high tem- peratures. If the product is frozen rapidly enough destruction of the fat emulsion can largely be prevented. It is to be noted that while a destruction of the colloidal character of the caseinate system during freezing involves a considerable storage period, the effects harmful to the fat emulsion have occurred by the time the material is completely frozen. A change in the distribution of the constituents when milk is partially frozen has been reported (6, 7, 11, 14). The results show a concentration of constituents in the unfrozen liquid which is to be expected since the solid to separate is pure ice. A condensing process utilizing this principle has