Bacillus cereus-spores in raw milk. Factors affecting the contamination of milk during the grazing period

Growth of psychrotrophic strains of Bacillus cereus is a limiting factor for the keeping quality of Swedish pasteurised milk. This is the case during summer and autumn and seems to coincide with the grazing period. A group of 8 cows was studied intensively during three periods of 2 weeks during the summer. Factors studied were: the content of spores in milk from individual cows and bulk tank milk, spores in rinse water from the milking equipment, grass, hay, silage, concentrates, air, spores in faeces from individual cows and in soil from various locations. Furthermore the dirtiness of individual cows (udder, teats and teat tips) was graded as well as the dirtiness of leys, holding area and herd road. Meteorological observations were collected. The weather was extremely hot and dry during the first study periods and quite rainy during the last period. A highly significant increase in spores in milk was noted in the third period. The content of spores in fresh grass as well as hay, silage and concentrates rarely exceeded a few hundred spores/g indicating that they were not important sources of spores. A significant increase of spores in faeces was found during the third period, which could be explained by an increased intake of soil during grazing in wet weather. Spores in soil varied between less than 100/g and 140000/g, with a significant positive correlation with the water content of the soil. A high correlation coefficient was found between the average daily contamination score of the teats and the spore content of milk. Furthermore, significantly high negative correlation coefficients were found between the dry matter content of soil, the evapotranspiration and the spore content of milk. These results indicate that the most important source of contamination of milk is by soil via dirty teats and is influenced by weather conditions. Support for this hypothesis was found by comparing RAPD-PCR profiles of isolates of Bacillus cereus from milk and soil. Although the milking equipment did not seem to contribute to the spore content in the milk, we were able to demonstrate low levels of a strain with identical RAPD profile in rinse water during several days in connection with a cleaning problem. The content of spores in the air during milking was much too low to be important as a contamination source.