Proteolysis of High Quality Raw Milk During Storage at Various Temperatures

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 37C. Treatments included the addition of urokinase (a plasminogen activator), 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 4C, 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 4C. At llC, proteolysis was apparent in untreated milk after 1 d of incubation (proteose-peptone N increased by .15 mg/ml). Proteolysis at llC 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 37C. Results indicate that storage of raw milk at temperatures above 4C, 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.