The Germicidal Effect of Lactic Acid in Milk

Abstract

Reference to the germicidal effect of sour milk or buttermilk on pathogenic bacteria is occasionally found in the literature. It has been intimated that lactic-acid-producing bacteria are a protection against infectiousness of contaminated milk, inasmuch as the acid produced by these bacteria is said to destroy pathogenic bacteria. There is however little positive evidence to show whether this assumption is true or not. Earlier work by Barthel,1 Bassenge,2 and Behla3 is contradictory. An investigation by Northrup4 as to the fate of typhoid bacilli in milk and in lactose broth, acidified by cultivation of lactic acid bacteria and filtered through a Chamberland filter, seems to show that acid produced by different lactic acid organisms varies in degree of germicidial action. While one strain of Bacterium lactis acidi destroyed typhoid bacilli when 0.33 percent acid (as lactic acid) was present, other strains, including the bacillus bulgaricus, had the same effect only when nearly twice as much acid was present. Krumwiede and Noble5 determined the longevity of typhoid bacilli in sour cream. The authors make the following deductions from their work: