Effect of heat treatment on bacteriological quality and phosphatase activity of market cream

Abstract

The bacterial count of good quality cream after pasteurization did not vary greatly with increasing intensity of pasteurization until 93°C was used. In the case of poorer quality creams the total count decreased slightly with increasing temperatures of short-time heating. There was no marked relationship between counts before and after pasteurization.
For determining the end-point in keeping quality tests, reduction of methylene blue was found to be unsatisfactory, and reduction of resazurin to the bright pink stage was used. Resazurin reduction time after keeping quality storage showed a general relationship with later deterioration detected by taste and smell.
When the cream was pasteurized at the higher temperatures, the predominant organisms surviving were spore-forming bacteria which produced thickening and bitterness in keeping quality tests.
Storage at low temperatures for up to 3 days did not materially alter the bacterial count, but there was a tendency for keeping quality to be lowered slightly as storage progressed. The temperature of incubation in the keeping quality . test markedly influenced the result. An incubation temperature of 22°C for 24 hr appeared to be too high to suit the purposes of the test. ·
With all intensities of heating, phosphatase tests were always negative immediately after pasteurization, but some samples, especially those treated at lower temperatures, showed reactivation on storage, which was apparently due to bacterial development. With these samples phosphatase activity increased as the time and temperature of storage increased.