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Observations on the Freezing of Milk and Cream

I. The Effect of Fat Concentration upon the Distribution of Constituents in the Frozen and Unfrozen Portions of Partially Frozen Milk and Cream^*

Pennsylvania Agricultural Experiment Station, State College, Pa.

ABSTRACT

When whole milk is partially frozen in an undisturbed condition, the fat concentration in the frozen and unfrozen portions over the entire range of freezing is dependent upon the cream rising phenomenon of the milk and its speed relative to the speed of freezing; also to some extent, naturally, on the shape and size of the container. Milk that exhibits normal creaming ability shows variable concentrations of fat in the frozen and unfrozen portions from the time freezing first becomes evident until the milk is completely frozen.

When the creaming ability of the milk is destroyed (by heating or homogenization), the fat concentration of the unfrozen portion increases progressively with the degree of freezing, while that of the frozen portion is decreased at first, but finally approaches the fat percentage of the original milk as the degree of freezing approaches 100 per cent.

Increasing fat concentrations in milk or cream retard the diffusion or concentration of the milk constituents (both colloidal and soluble) into the unfrozen portion of the freezing products, and when the fat concentration reaches 25 per cent, such diffusion is practically prevented and the cream freezes homogeneously. The diffusion is possibly inhibited because of the increased viscosity with increasing fat content and also because of the more effective sealing of the interstices between the developing ice crystals by the increased amount of fat in the form of solidified globules.

Homogenization acts similarly to increasing the fat content in restricting the concentration of ingredients in the unfrozen portions of freezing cream provided there is sufficient fat present to give an increase in viscosity.

^* Publication authorized by the Director of the Pennsylvania Agricultural Experiment Station, April, 1935, as Technical Paper No. 687. The data presented herein are a portion of those assembled in a thesis by the senior author and submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy at The Pennsylvania State College.