The growth of aerobic spore-forming bacilli in sterilized milk

Abstract

SummaryThe sterilization of homogenized milk at 115·5°C for 15 min in bottles having a partial vacuum in the headspace produced conditions inhibitory to the growth from very small numbers of spores ofBacillus subtilis, B. licheniformis, B. cereusandB. breviswhen compared with growth in the same milk sterilized in open bottles.B. circulansdiffered from the other strains tested in showing greater inhibition in milk sterilized in open bottles than in milk sterilized under partial vacuum.The extent of the inhibition became less as the size of the inoculum was increased. It became less also as the temperature of incubation approached the optimum growth temperature of the bacillus, and was influenced by the strain of the bacillus and the source of the milk but not by the degree of heat treatment within the range 107–117·5°C for 15 min. Inhibition was manifested by a prolongation of the lag phase, and in addition with some strains inhibition of spore germination could be demonstrated.Spore formation following vegetative growth occurred more readily in milk sterilized in open than in evacuated bottles.Milks sterilized under partial vacuum frequently failed to show any growth from small inocula in 30 days at 22°C although growth occurred readily in milk sterilized in open bottles.