The detection of penicillin in buttermilk

When 59 buttermilk samples were tested for the presence of antibiotics or chemostherapeutic substances by a method using the spores of Bacillus stearothermophilus var. chalidolactis all samples gave a positive reaction corresponding to about 0.01 IU penicillin per ml. When the buttermilk samples were diluted 1:1 (v/v) by a phosphate buffer, pH 6.8, only 2 were positive which were confirmed to contain penicillin. The mean pH of the unbuffered buttermilks was found to be 4.53 +/- 0.11 and when buffered 6.46 +/- 0.05. An in vitro addition of benzylpenicillin to buttermilk, to a concentration of 0.01 IU per ml, resulted in a significantly higher concentration of penicillin in the casein fraction than in the buttermilk or in the whey (Table I).