A study of the causes of periodic abnormalities of a high grade pasteurized milk supply

Abstract

The causes of acid liquefaction in a high grade pasteurized milk were investigated.

1. Liquefying milk produced much less acid and had a much lower plate count than normal milk.

2. <i>Bacillus albolactis</i> was found regularly in the milk. It made up a much larger percentage of the total flora in liquefying than in normal milk.

3. Associated growth experiments showed that <i>Strep. lactis</i> could control the liquefying activities of <i>Bacillus albolactis</i>.

4. Pasteurization experiments showed that milk heated for 20 minutes at temperatures higher than 61°C. would undergo acid liquefaction. This appeared to be due to the increase in the percentage of <i>Bacillus albolactis</i>.

5. Inoculation of pasteurized samples with <i>Strep. lactis</i> prevented acid liquefaction.

6. Comparison with a type species showed that the strains of <i>Strep. lactis</i> repeatedly isolated from the milk studied were of the <i>tardus</i> variety.

7. The liquefaction was probably primarily due to an increased percentage of <i>Bacillus albolactis</i> brought about by pasteurization; the lack of the type of species of <i>Strep. lactis</i> and the absence of lactobacilli might also have been contributing factors.