Studies on the Influence of Bacteria on the Oxidation-Reduction Potential of Milk

Abstract

Changes in the oxidation-reduction potential of milk caused by various milk organisms in pure culture were determined and plotted. Direct microscopic counts of the numbers of organisms were made on milk and correlated with changes in oxidation-reduction potential (Eh curves). Under similar conditions, each of the organisms studied produced changes in oxidation-reduction potential characteristic of that organism. With many of the organisms studied, the end of the rapid drop in oxidation-reduction potential was almost coincident with the beginning of the rapid rise in numbers of bacteria. This point was well within the phase of logarithmic growth of the organism. Some organisms were found to produce a system with a weak quantity factor; others, produced a system with a strong quantity factor. 2 different strains of Strep, lactis showed variation in the quantity factor. Pairs of spp. of milk bacteria were grown together in sterile milk at 37[degree] C. Changes in oxidation-reduction potential were measured and compared with changes in numbers of bacteria of the 2 spp. Escherichia coli, E. communior and Aerobacter aerogenes, when grown with Strep. lactis, all exerted a restraining influence on the rapid drop in Eh values usually caused by pure cultures of Strep. lactis. The larger the proportion of actively growing colon-aerogenes organisms, the greater restraining action. When growing with one of the colon-aerogenes organisms, apparently more Strep. lactis organisms must be present to cause a drop in Eh values to [long dash]0.2 volt than when Strep. lactis organisms are in pure culture. When Clostridium welchii was grown with Strep. lactis in milk, the lactic organism controlled Eh changes during the first part of the growth, but C. welchii caused a final negative value of about [long dash]0.32 volt. Strep. fecalis and Strep. thermophilus each controlled the changes in Eh when grown with Lactobacillus bulgaricus in milk, Bacillus subtilis and B. albolactis did not materially affect the Eh changes when grown with Strep. lactis in milk.