Assaying Sterile Concentrated Milk for Native Proteolytic Enzymes

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Abstract
Depro-teinized filtrates of hemoglobin acted upon by a protease have an optical density peak at 400 m[mu] due to hemin-containing split products. Pro-teolysis in raw milk was evident by increases in the optical density of filtrates after adding heated enzyme-free hemoglobin and incubating. Preheating at 94[degree] C for 4 sec. greatly retarded enzyme activity in milk and its concentrate. Concentrated milk sterilized at 150[degree] C, its autoclaved control and autoclaved hemoglobin revealed the same linear optical density curves at 280 and 400 m[mu], indicating no measurable proteolytic activity during storage for 30 days. Optical densities of filtrates from 3 concentrated milks were essentially the same at 400 m[mu]. after storage for 14 days; they were significantly different at 280 m[mu]. due to nonenzymatic proteolysis. Incubating sterile concentrated milk at 65[degree]and 80[degree] C for 63 hr. increased nonprotein-N which was heat-induced, but the milk was unaffected at 20[degree]-65[degree]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.