Inhibition of Psychrotrophic Bacteria by Lactic Cultures in Milk Stored at Low Temperature

The effect of lactic cultures on the growth of psychrotrophic bacteria in milk stored at 3.5 and 7 C was investigated. Psychrotrophs were those in raw milk, or those isolated from raw milk and added to sterile milk as pure cultures. Of the lactic cultures investigated, certain commercial multi-strain cultures (lactic acidproducing streptococci plus Leuconostoc cremoris) were the most effective in restricting psychrotrophic growth. Lactic culture was added to milk at .5% in all experiments. Inhibition of psychrotrophs was due to a reduction in growth rate because reduction in the initial psychrotroph population usually was not observed. The degree of inhibition varied with the lactic culture, the initial population of psychrotrophs, the psychrotroph culture, storage temperature and time. Inhibition due to lactic culture was decreased by addition of catalase, suggesting that hydrogen peroxide was the inhibitor. Small amounts of hydrogen peroxide were detected in sterile milk inoculated with an inhibitory culture and stored at 3.5 or 7 C. Hydrogen peroxide decreased in the presence of psychrotrophs, and hydrogen peroxide was not in raw milk.