Bacillus cereus Spores in Raw Milk: Factors Affecting the Contamination of Milk During the Grazing Period

Psychrotrophic Bacillus cereus is a limiting factor for the shelf-life of pasteurized milk, particularly during the grazing season. Potential sources of contamination and factors that might affect the spore content of milk were studied in detail for a group of eight cows during three 2-wk study periods from June to September over 2 yr. The spore content of milk was strongly associated with the degree of contamination of the teats with soil. High water content of soil, low evaporation of water and dirty access alloys were the most important factors correlating with high spore concentrations. The spore content of soil varied from < 50 to 380,000/g, depending on time and sampling site. The milking equipment did not contribute significantly to the contamination. The spore contents in air during milking (< 100 cfu/m3) and in feed (silage, hay, fresh grass, and concentrates) were too low to be of importance for contamination. The spore content in dung was also low. Further support that soil was the major contamination source was found by comparison of genetic fingerprints by random amplified polymorphic DNA polymerase chain reaction of isolates of B. cereus from soil and milk and by teat cleansing experiments, which resulted in reduced contamination levels in milk.