544. Factors influencing the lactic acid-producing properties of streptococci used in the manufacture of Cheddar cheese: I. Observations relating inhibitory and stimulatory phenomena

Abstract

1. Many single-strain lactic streptococci used as cheese starters exhibited a difference in susceptibility to an inhibitory substance present in raw and H.T.S.T. pasteurized milk, some being markedly affected whilst others were completely resistant to this inhibitory aubstance.2. The inhibitory effect of raw or partially heated milk on ‘susceptible’ strains of starter streptococci appears to be a widespread phenomenon. It can be said that the inhibitory substance is normally present in all milk supplies throughout the year. It occurs in milk freshly drawn from the udder aseptically.3. Heating the milk to boiling-point for variable periods or autoclaving the milk destroys the inhibitory effect of the milk on ‘susceptible’ cultures.4. The inhibitory substance is closely associated with the fat globules in milk and can exert a marked effect owing to the large surface area exposed.5. Growth-stimulating substances prepared from extracts of fowl liver, separator slime, and yeast were able to overcome the inhibitory effect when added to raw milk. However, the effect of these substances were markedly depressed on the addition of small amounts of freshly separated cream to the cultures.6. It is suggested that the above growth-stimulating substances contain a common growth factor which enters into competition with an inhibitory substance closely associated with the fat globule.7. The inhibitory substance appears to be bacteriostatic in its effect and not bactericidal.8. Owing to the heat lability of the inhibitory substance and its presence in close association with the fat globule, it is suggested that it may be enzymic in nature.