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Proteolysis of High Quality Raw Milk During Storage at Various Temperatures

Department of Animal and Dairy Science

J. F. Frank

Department of Food Science and Technology, University of Georgia, Athens 30602

ABSTRACT

Proteolysis in raw milk resulting from the action of native milk protease was studied. Low bacteria count milk was preserved using thimerosal (sodium ethylmercurithiosalicylate) and incubated at 4, 11, 21, and 37°C. Treatments included the addition of urokinase (a plasminogen activaror), trypsin inhibitor, and porcine plasmin. Proteolysis was estimated by measuring increases in noncasein N and proteose-peptone N. Proteose-peptone N analysis was superior to noncasein N analysis in the differentiation of various levels of plasmin activity. During 4 d of incubation at 4°C, slight increases in proteose-peptone N were observed in milk treated with urokinase and 3 mg/L plasmin. Trypsin inhibitor had no effect on proteose-peptone N levels at 4°C. At 11°C, proteolysis was apparent in untreated milk after 1 d of incubation (proteose-peptone N increased by .15 mg/ml). Proteolysis at 11°C was stimulated by the addition of urokinase and 1.5 mg/L plasmin and was inhibited by trypsin inhibitor. Similar but more pronounced results were obtained at 21 and 37°C. Results indicate that storage of raw milk at temperatures above 4°C, even when bacterial proteases are absent. Stimulation of proteolysis by urokinase and inhibition by trypsin inhibitor indicate that plasmin activity is responsible for much of the observed increase in proteose-peptone N levels.