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The Freezing and Thawing of Milk Homogenized at Various Pressures^*

Department of Dairy Husbandry, Michigan Agricultural Experiment Station

ABSTRACT

The analyses of various portions of milk homogenized at different pressures and frozen partially or wholly have been made.

A slightly greater percentage of homogenized than unhomogenized milk froze in a given period at 0° F.

When creaming was inhibited by heating or by homogenization the unfrozen portion was relatively richer in fat and solids-not-fat than the frozen portion. When creaming occurred, as in the unhomogenized milk, the frozen portion was higher in fat but lower in solids-not-fat than the unfrozen portion.

The titratable acidity of the unfrozen portion was higher than the frozen portion, regardless of creaming. No evidence of creaming was noted in frozen homogenized milk but rather a pronounced settling of the fat which was influenced by the rate of thawing.

Frozen homogenized milk upon thawing exhibited no flakiness which was commonly observed when unhomogenized milk was frozen and thawed, but did exhibit a watery appearance at the surface layers which was more pronounced when the frozen milk was thawed slowly.

Marked settling of the fat and solids-not-fat of milk was noted when homogenized milk was frozen and then thawed. The rate of thawing had a pronounced influence upon the extent of settling of the fat and solids-not-fat; the lower 15 per cent of creaming cylinders of slowly thawed frozen homogenized milk contained as high as 7.7 per cent fat and 24.60 per cent total solids as contrasted to 2.0 and 5.50 per cent, respectively, of the upper 15 per cent layer. The titratable acidity of the upper and lower 15 per cent portions ranged from 0.04 to 0.35 per cent. Similar trends were observed upon freezing and thawing of skimmed milk. When milk was heated to inhibit creaming was frozen and thawed, the fat tended to rise to form a cream layer, a contrast to the movement of fat in the thawed frozen homogenized milk.

The drainage of solids from homogenized milk was slightly faster after the first 100 ml. than that of the unhomogenized milk.

^* Journal Article No. 479 (n.s.) Mich. Agr. Exp. Sta.