Estimation of Microbial Count in Raw Milk Based on Catalase-activity

Abstract

The microbial count is an index of suitability of raw milk handling and also one of the most essential item to judge milk quality. Sometimes, the high microbial count is responsible for high titratable acidity and/or off-flavor, and inducesheat-unstability or contamination of heat-stable enzymes. Therefore, the high microbial count milk is not available for manufacturing. The measurment of the microbial count is usually carried out by direct counting with a micro scope or by colony counting on an agar plate. But these methods contain complicated procedures and need special techniques. To solve these problems a method by measuring catalase-activity was investigated, since all aerobic microbes have catalase. To separate the catalase-activity of microbes from that of bovine catalase contained in raw milk, the pH-profile of each activity and inhibitory effects of L-ascorbic acid were investigated. To measure the catalase-activity, an increasing rate of dissolved oxygen was measured by mean of an oxygen electrode just after the addition of hydrogen peroxide. The bovine catarase-activity tended to decrease definitely in the pH range higher than 9, but those of bacteria suspensions and raw milk had peak around pH 11. At pH11, there fore, the cat alase-activity was expected to be that of microbes. In experiments using 3 species of bacteria, a high linear relation was obtained between bacterial count and catalase-activity at pH 11, and the lowest bacterial count which can be estimated from the catalase-activity at pH 11, was spposed to be around 103/ml. In experiments using raw milk samples from a tank lorry, catalase-activity at pH 11 was shown to be highly related to the microbial count. A regression for mula to estimate the microbial count from catalaseactivity at pH 11 was obtained and was shown to be available for raw milk samples.