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Effects of Preliminary Incubation on the Bacterial Flora of Bulk-Tank Milks Produced in Three Different Canadian Milk Sheds¹

Formerly Department of Agriculture Research Branch, Ottawa, Canada
Food Research Institute, Department of Agriculture Research Branch, Ottawa, Canada

ABSTRACT

Regional differences in response of producer raw milk samples to preliminary incubation at 13 C for 18 hr appeared to be correlated with the percentages of gram-negative rods present. Edmonton milks, which showed the least response, had appreciably fewer of these organisms than those from Winnipeg, which responded the most. Those from Guelph were intermediate in both respects.

Marked variations were encountered between results from different farms in each milk shed, and even between different samples from the same farm. As the initial Standard Plate Count increased so did the proportion of gram-negative rods. After preliminary incubation, milk showing >8-fold increase in count contained much higher percentages of these rods than those showing <3-fold increase.

Although equipment condition and percentage of gram-negative rods frequently showed a negative correlation, the rapid growth of these rods suggests that they are a better indicator of contamination than are the streptococci.

¹ Food Research Institute Publication No. 132.