Inhibitory Effect of Rancid Milk on Certain Bacteria

Summary Milk which had undergone lipolysis to the extent that a reduction of about 10 dynes per centimeter in the surface tension occurred was found to have a definite inhibitory effect toward S. lactis , S. zymogenes and L. casei . The inhibition of S. bovis and E. coli , though detectable, was definitely less than that of the above organisms. L. bulgaricus and Ps. aeruginosa were not measurably affected. Use of control samples in connection with the rancid milk demonstrated that the inhibition displayed by the rancid milk was not caused by either the slight reduction in pH or the reduced surface tension noted in such milk. Sterile active lipase showed no direct effect on the growth of S. lactis other than through its lipolytic action on the fat in the milk. The data indicated that inhibition was caused by some component of rancid milk and that the extent of inhibition of S. lactis was in direct proportion to the degree of lipolysis occurring in the milk.