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Rancidity Studies on Mixtures of Raw and Pasteurized Homogenized Milk^*

Department of Dairying, Michigan State College, East Lansing, Michigan

ABSTRACT

A rancid flavor and an increase in acidity were found to develop readily on storage when raw milk was mixed with homogenized pasteurized milk. The results secured confirm earlier work (2, 3), that lipolytic activity is not confined solely to homogenized raw milk but to homogenized pasteurized milk as well provided active lipase is present. The maximum increase in acidity occurred when the ratio of raw milk to homogenized pasteurized milk was approximately one to one. As the percentage of raw milk in the homogenized pasteurized milk increased above 50 per cent, the increase in titratable acidity was found to be correspondingly less. When only a small percentage of the sample was homogenized pasteurized milk, very small increases in acidity occurred. These increases in titratable acidity were closely associated with the development of a rancid flavor.

The fact that the greatest increases in acidity in mixtures involving pasteurized milk occurred when the milk was approximately 50 per cent unhomogenized raw and 50 per cent homogenized pasteurized indicates that the amount of increased surface or increased surface activation caused by homogenization and the amount of lipase added by the raw milk are of approximately equal importance in the development of rancidity in homogenized milk. It would appear, therefore, that increases in acidity and the development of rancidity in homogenized raw milk are dependent upon the factors concerned with the increased surface and not upon an activation of lipase by homogenization.

Further evidence of the equal importance of the fat surfaces and the amount of lipase present is shown by the fact that when homogenized raw milk was added to homogenized pasteurized milk the rate of increase in acidity was only slightly greater than when unhomogenized raw milk was mixed with homogenized pasteurized milk. If lipase is activated by homogenization it would seem that these increases should have been considerably faster than those noted. The greater increase which did occur in the homogenized raw and homogenized pasteurized milk mixtures might be explained by the fact that all of the fat had been subjected to homogenization so that there was more fat surface exposed upon which the lipase could act than in the raw milk and homogenized pasteurized milk mixtures in which only a portion of the fat had been subjected to homogenization. In the latter case the amount of lipase added by the raw milk seemed to be the limiting factor in the development of rancidity. The lipase added to the homogenized paseurized milk in the form of unhomogenized raw milk appeared to be just as effective in causing rancidity as was the lipase added by the homogenized raw milk.

From these studies it would seem that homogenized pasteurized milk contaminated with raw milk is as susceptible to lipolysis as homogenized raw milk. In addition, these results indicate the possibility if controlling the extent of the development of rancidity through the use of proper mixtures of homogenized pasteurized and raw milk.

^* Journal Article 515 (n. s.) from the Michigan Agricultural Experiment Station.