Penicillin present in cheese milk affected the performance of various ‘single strain’ starter cultures in the cheese vat.

A concentration of 0·07 penicillin unit/ml, in the milk was sufficient to cause a complete failure or ‘dead’ vat with the most sensitive strain of Str. cremoris. Two strains ceased to produce acid in the later stages of the manufacturing process when the milk contained 0·10 unit/ml. With three other strains the 0·10 unit/ml, level delayed acid development; the experimental vats required 20–40 min. longer than the controls to reach ‘salting’ acidity.

Flash pasteurization temperatures usually employed in cheese-making (150–160° F.) did not appreciably alter the activity of penicillin in the cheese milk.

The quality of the cheese produced was influenced by the rate and extent of acid production by the starter cultures in the vat during manufacture.

Experience in the field indicates that where a large number of cows are treated simultaneously for mastitis, there may be sufficient penicillin in the cheese milk to affect the starter, if the milk from quarters undergoing treatment is included in the bulk milk.