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Investigation of Factors Contributing to the Bacterial Count of Bulk Tank Milk. III. Increase in Count, from Cow to Bulk Tank, and Effects of Refrigerated Storage and Preliminary Incubation¹

Statistical Research Service, Department of Agriculture Research Branch, Ottawa, Canada

H. Jackson

Department of Food Science, University of Alberta, Edmonton, Canada

C. H. McNaughton

Department of Food Science, University of Manitoba, Winnipeg, Canada

A. G. Leggatt

Department of Dairy Science, University of Guelph, Guelph, Canada

G. B. Landerkin

Food Research lnstitute, Department of Agriculture Research Branch, Ottawa, Canada

C. K. Johns

Formerly Department of Agriculture Research Branch, Ottawa, Canada

ABSTRACT

At 48 farms drawn from three regions, milk samples were taken at various stages: from the pails after milking with University-sanitized machines and pails; similarly, but after using farm-washed equipment; and after entry into the tank in both cases. Standard plate counts were made both with and without preliminary incubation (PI), and before and after refrigerated storage. The equipment was inspected and rated for condition; cleaning and sanitizing procedures were noted. In this way the increase of bacteria could be related to the various possible contributory factors, or groups of factors. Particular interest lay in any difference between the counts with and without preliminary incubation as contributions from the different sources.

For milk not subjected to preliminary incubation, the major increase in count came when farm-washed machines and pails of less-than-satisfactory condition were allowed to contribute. Preliminary incubation generally enhanced both the initial count and the increase in count at all stages of sampling, though in a very different way at each of the three centers under study. Possible factors responsible for high counts after preliminary incubation, besides unclean machines and pails, are poor sanitizing procedures, water supplies with high bacterial counts, and unsatisfactory bulk tank conditions. As a basis for control of production conditions, a standard plate count of 50,000 appears to be too high.

¹ Food Research lnstitute Publication no. 86.