Consumption by Healthy Adults of Pasteurized Milk with a High Concentration of Bacillus cereus : A Double-Blind Study

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.