THE DEVELOPMENT OF BACTERIAL POPULATIONS IN MILK

Abstract

The dominant bacteria in different classes of milk have been identified by taking samples of the colonies on quantitatively inoculated plates. Many of the bacteria that occur commonly in milk find it a relatively unfavorable medium. In the temperature range 10°–22 °C. the organisms that multiply actively are the Streptococcus lactis group, a species of Leuconostoc, coagulase-negative staphylococci, and Gram-negative rods (chiefly Alcaligenes viscosus and fluorescent and non-fluorescent pseudomonads). Plate counts of organisms that resist pasteurization for 30 minutes at 63 °C. are unsuitable for demonstrating the multiplication of Leuconostoc or Alcaligenes tolerans, which show variable and frequently low rates of survival in the heat treatment. In laboratory-pasteurized milk incubated without recontamination, the surviving organisms, unlike the populations of raw milk, do not show distinctly the effects of a differential selection. The only thermoduric organism that was found to produce rapid deterioration of milk is Bacillus cereus (including B. mycoides).