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Further Studies of the Effect of Pasteurization on the Bacterial Flora of Low Count Milk

Washington Agricultural Experiment Station Pullman, Washington

ABSTRACT

Total and differential bacterial counts were made on 75 samples each of raw and commercially pasteurized milk while fresh and again after 24 and 48 hours of incubation at 70° F.

Pasteurization resulted in a decrease in the percentage of the acid-forming bacteria from 29.4 to 18.8, a decrease in the percentage of the proteolytic group from 7.0 to 5.0 and an increase from 63.6 to 76.2 per cent in the alkali-forming and inert group.

After 24 hours the percentage of acid-forming bacteria in the raw and pasteurized samples was 92.1 and 12.4 respectively. No significant differences existed in the proportion of the proteolytic group in the two kinds of milk. The alkali-forming and inert group constituted 7.35 per cent of the raw milk flora as compared to 86.4 per cent of the flora of the pasteurized samples.

After 24 hours the flavors of the pasteurized samples were more desirable than the flavors of the raw samples.

After 48 hours 98.7 per cent of the raw milk samples showed acid coagulation as compared to 57.3 per cent of the pasteurized samples. Ninety-six per cent of the raw milk flora were acid-forming organisms as compared to 76.1 per cent of the pasteurized milk flora. The proteolytic group constituted less than 1.0 per cent of the flora of either type of milk. The alkali-forming and inert group made up 3.96 per cent of the flora of the raw milk and 23.7 per cent of the flora of the pasteurized milk. Twenty per cent of the pasteurized samples developed a pronounced bitter flavor without the accumulation of more than 0.4 per cent acid.

The results obtained in this study bear out the contention that, in many instances, organisms other than the lactic acid types are of major importance in the spoilage of commercially pasteurized milk derived from low count raw milk.