Bacilli in Milk. I. Spore Germination and Growth

Abstract

The influence of the heat treatment of the spore and of the milk upon germination, outgrowth, and subsequent vegetative cell growth of Bacillus cereus 7 spores was determined in raw (unheated), pasteurized (62.8 C-30 min), high-heated (100 C-10 min), autoclaved (121 C-10 min) skimmilks. Pollowing 2 hr incubation at 35 C, the per cent germination for the unheated spores was 27.3, 89.7, 65.2, and 15.6, and for the heat-shocked (80 C-12 min) spores it was 67.8, 99.5, 94.9, and 83.5 in raw, pasteurized, high-heated, and autoclaved skimmilks, respectively. The time required for the spores to germinate, outgrow, and initiate active cell multiplication ranged from 124 min for the heat-shocked spores in pasteurized milk to 355 min for the unheated spores in raw milk. When vegetative cells in exponential growth were inoculated into the various milk systems, the rate of growth for the first 30 min in raw milk was approximately one-half that for pasteurized or high-heated milk. However, upon continued incubation no differences in generation time or number of generations were noted between the raw and heated milks. Heat treatment of the spore stimulated germination and subsequent outgrowth; whereas, heat treatment of the milk affected the initial rate of active cell multiplication. © 1968, American Dairy Science Association. All rights reserved.