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Observations on Milk Protease¹

Department of Dairy Technology The Ohio State University and the Ohio Agricultural Experiment Station Columbus

ABSTRACT

Proteolytic activity in raw milk has been reported by several investigators (1–4, 8, 9). However, in some instances, it is difficult to evaluate the results of previous workers because of possible interference of bacterial protease (1–3, 9). The work of Storrs and Hull (8) appears to be a substantial contribution to the belief that protease is a natural constituent of milk. These authors found that tyrosine-tryptophane increases occurred in three milks incubated at 100° F. for 6 hr. Microbial action would not account for the changes noted, because the milks had low bacterial counts (less than 10,000 microorganisms/milliliter) and contained a mixture of bacitracin and polymyxin to inhibit bacterial growth. However, the limited data of Storrs and Hull, and the fact that their colorimetric method (which uses Folin's Phenol reagent) is lacking in specificity (7), suggested the need for this further investigation.

In one experiment, six samples of fresh milk, obtained aseptically from mastitis-free individual cows, were examined for protease activity immediately after milking, as follows: (a) equal volumes of milk and 0.2 M borax buffer, pH 8.5, were combined and incubated 6–8 hr. at 38° C., (b) the proteins were precipitated with 10% trichloracetic acid, (c) the filtrates thus obtained were extracted four or five times with ethyl ether, concentrated by freeze drying, resuspended in 10% isopropanol, and examined by paper chromatography techniques according to the method of McParren (6).

¹ Technical Paper 13: 60, The Department of Dairy Technology, The Ohio State University and the Ohio Agricultural Experiment Station. Ohio Agricultural Experiment Station Journal Article 50: 60. Supported in part by a University grant-in-aid and by Hatch Funds.