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The Effect of Using Tryptone-Glucose-Skimmilk Agar and 32° C. Incubation on the Bacterial Colony Count of Ice Cream^*

New York State Agricultural Experiment Station, Geneva, New York

ABSTRACT

A comparison between 37° C. standard nutrient agar counts and 32° C. tryptone-glucose-skimmilk agar counts on 112 samples of ice cream representing 12 manufacturers was made on a logarithmic basis.

Tryptone agar counts were higher than standard agar counts in 103 instances (92.0 per cent). The theoretical average percentage increase in the case of individual manufacturers varied from minus 29 to plus 1,221 per cent, but with the exception of these extremes, increases ranged between 27 and 258 per cent.

The fact that the proportion of the number of colonies developing on tryptone agar at 32° C. varied so greatly from standard nutrient agar at 37° C. indicates that the latter is a poor instrument for use in quality measurements and shows that poor quality may masquerade as good quality ice cream, where the present standard method of making agar plate counts is followed.

Wide distribution of the dots in Fig. 1 shows clearly that the number of colonies on standard agar plates incubated at 37° C. does not represent a constant proportion of the total number capable of development.

Based on the entire 112 samples, the average percentage increase in count of 162 per cent in favor of tryptone agar and 32° C. incubation is highly significant in consideration of the fact that it is based on logarithms.

^* Approved by the Director of the New York State Agricultural Experiment Station for publication as Journal Paper No. 213, August 12, 1937.

¹ Associate in Research.

² Student Assistant.