THERMODURIC ORGANISMS IN MILK. PART II. PRELIMINARY STUDIES ON TESTING TECHNIQUES

Abstract

Summary:To define a suitable technique for estimating thermoduric bacteria in raw milk supplies, experiments were made to determine whether any difference could be detected between colony counts after laboratory pasteurization at 145 or 150° F. (62.8 or 65.5°) and whether colony counts are affected by temperature and period of storage of samples before laboratory pasteurization. Temperatures above 63.5±0.5° occasionally gave underestimates of the thermoduric population. Within limits time and temperature of storage generally had no effect on numbers of thermoduric bacteria and samples to be examined for thermoduric content can be aged at temperatures between 4 and 20° and examined at any time up to 24–26 hr.It is recommended that the pasteurizing bath shall be capable, under working load, of heating the samples to 63.5° within 5 min. To ensure this with certain types of bath it may be necessary to circulate the water, or to avoid overloading.A limited study of types of thermoduric organisms predominating after different incubation periods and temperatures of storage revealed that the type initially predominant was generally present in greatest numbers after the milk had been aged for 24 hr. Occasionally Gram‐positive sporing rods overgrew other types and some micrococci were not favoured by storage at 20°.