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Assaying Sterile Concentrated Milk for Native Proteolytic Enzymes¹

Department of Food Science, University of Illinois, Urbana

ABSTRACT

Deproteinized filtrates of hemoglobin that have been acted upon by a protease have an optical density peak at 400 mµ due to hemin-containing split products. This technique was used to assay milk in borate buffer, pH 8.6, for native proteolytic enzymes.

Proteolysis in raw milk was evident by increases in the optical density of filtrates after adding heated enzyme-free hemoglobin and incubating. Preheating at 94 C for 4 sec greatly retarded enzyme activity in milk and in its concentrate. Concentrated milk sterilized at 150 C, its autoclaved control, and autoclaved hemoglobin revealed the same linear optical density curves at 280 and 400 mµ, indicating no measurable proteolytic activity during storage for 30 days. Optical densities of filtrates from three concentrated milks were essentially the same at 400 mµ after storage for 14 days; whereas, at 280 mµ they were significantly different due to nonenzymatic proteolysis. Incubating sterile concentrated milk at 65 and 80 C for 63 hr increased nonprotein N which was heat-induced, but the milk was unaffected at 20 to 65 C for 37 hr. Native milk proteases were of no significance in the gelation of concentrated milk sterilized by the high-temperature short-time method.

¹ This research was supported by U. S. Public Health Grant EF-141.

² Research Associate.