Inhibition of the Psychrotrophic Bacteria of Raw Milk by Addition of Lactic Acid Bacteria

Two commercial cultures of lactic acid bacteria (LAB) were added to raw milk in order to inhibit the development of psychrotrophic bacteria. The effects of inoculation level, pH development, media for culture preparation, and milk heat treatment on the inhibitory activity of LAB were studied. Inoculation levels of 2.5 × 10⁶/mL and less of LAB did not significantly reduce psychrotrophic growth. Maximum inoculation level of LAB studied was 25 × 10⁶/mL and this reduced psychrotrophic development by a factor of 10; however, milk pH was reduced to 6.54 over 48 h of incubation at 7°C with this inoculation level. This acidification was not responsible for the inhibitory activity of LAB on psychrotrophs. The LAB were more effective when psychrotroph populations had high multiplication rates. Initial populations of Pseudomonas putida in pasteurized milk did not influence the activity of the LAB. Pseudomonas putida grew faster in sterile milk than in pasteurized milk, but average inhibition rates of LAB were similar in both media.