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A double-blind experiment with 34 healthy human volunteers, aged between 20 and 60 years, was conducted to obtain information about the allowable concentration of B. cereus in pasteurized milk. During a period of 3 weeks the subjects were exposed to B. cereus naturally present in pasteurized milk following storage for 3 to 14 days at 7.5°C. Of 259 milk exposures, gastrointestinal complaints were observed in 18 cases. According to total numbers of B. cereus ingested per exposure the complaints per number of exposures were distributed as follows: < 106: 5 in 132; 106 to 107: 2 in 32; 107 to 108 : 2 in 26; and > 108: 9 in 69. Symptoms, however, were not typical of those caused by B. cereus. There was a weak significance (P ≤ 0.1) for symptoms when > 108 B. cereus cells were ingested. Milk with 106 to 107 B. cereus cells per ml showed a very low diarrheal enterotoxin titer. Pure cultures of B. cereus strains isolated from samples with high B. cereus concentrations also showed a low production of enterotoxin. It can be concluded that for healthy adults the probability of become diseased from cold-stored pasteurized milk is small. From the results no evidence is obtained that B. cereus concentrations less than 105/ml will cause intoxication.

In the dairy industry microbiological contamination may arise from equipment used for handling or processing. In this study it was demonstrated that Bacillus cereus spores could adhere stainless steel, germinate and/or multiply in a tube heat exchanger. The levels of B cereus detect in the tubes, after 24 h at 20°C, varied from less than 1 to 2600 cm‐2. Adhering cells and/or spores were more resistant to cleaning with K500 and sodium hydroxide, determined by laboratory suspension tests and surface tests on stainless steel. This was confirmed in a field trial with a tube heat exchanger after cleaning B cereus could still be isolated from all tubes determined by swab samples of individual tubes. The identity of the isolates was checked by polymerase chain reaction/randomly amplified polymorphic DNA typing to confirm that the organisms found on the surfaces were the same as the bacteria in the ingoing milk.

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