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A Study of Methods for Determining Numbers of Moulds and Yeasts in Butter

2. The Influence of Temperature and Time of Incubation

Division of Dairy Research, Dept. of Agriculture, Ottawa, Canada

ABSTRACT

The data obtained in this study show that the average mould and yeast counts of all samples and the counts for the majority of individual samples were slightly higher at 25° than at 30°C for both incubation periods of 48 hours and 5 days, although the differences could not be considered of practical significance. A few of the yeast cultures, however, did not grow as well at 30° as at 25°C. as indicated by the number of colonies. At 37°C. the mould and yeast counts in almost all cases were appreciably lower than at 25° and at 30°C., due to the fact that some of the types of moulds and yeasts did not grow at all or made only scant growth at this high temperature.

When a temperature of 25°C. for 5 days was compared to 30°C. for 48 hours, there was a considerable variation in the yeast counts of individual samples in favour of the lower temperature for the longer incubation period. Many of the counts at 30°C. for 48 hours were over 40 per cent lower than at 25°C. for 5 days. The counts at 37°C. for forty-eight hours were very much lower than at the other two combinations of temperature and incubation period.

The length of the incubation period did not affect the mould and yeast counts to any marked degree at any of the temperatures when a good hand lens was used in counting after 48 hours. The results show, however, that in general the counts were higher at an incubation period of 5 days than at 48 hours. If counting at the shorter incubation period had been done with the naked eye the counts would have been appreciably lower than recorded, as many of the yeast colonies were not apparent without the aid of a lens. At 5 days moulds and yeasts were easily counted with the naked eye at all temperatures used.

The size of the colonies and rate of growth of the moulds and yeasts growing at all temperatures was not influenced to any noticeable extent by temperatures of 30° and 37°C. as compared to 25°C. but was due rather to the length of the incubation period. It was evident that even for the short incubation period of 48 hours there was no advantage in using a higher temperature than 25°C. from the standpoint of size of colonies and rapidity of growth.

The data presented indicate that if mould and yeast counts of butter analyzed at different laboratories are to be comparable a uniform temperature and incubation period should be adopted, and 25°C. for 5 days gave the most satisfactory results in this study. If laboratory conditions necessitate a shorter incubation period than 5 days, counting should always be done with the aid of a good hand lens.