Changes in Microbial Population and Growth of Bacillus cereus During Storage of Reconstituted Dry Milk

Abstract

Eight brands of retail nonfat and whole dry milk were reconstituted and analyzed for changes in the predominant bacterial population and for the proliferation of Bacillus cereus throughout storage at 30, 20, and 5°C. All brands yielded similar results. Bacillus and Micrococcus predominated in the freshly reconstituted milk. During storage at 30°C, the Bacillus population proliferated initially, but was gradually replaced by enterococci. At the time of spoilage, Bacillus counts had dropped by several orders of magnitude. The proportion of Micrococcus gradually declined. B. cereus counts reached hazardous levels as early as 10 h after reconstitution, which was before spoilage was evident. Similar changes occurred in reconstituted milk stored at 20°C, but the time course for the changes was longer, and the Bacillus counts did not decline as rapidly as they did at 30°C. Again, counts of B. cereus reached hazardous levels before the milk showed signs of spoilage. At 5°C, the milk showed no signs of spoilage for 4 to 5 weeks. Bacillus constituted more then 90% of the bacteria isolated after the first week. Bacillus counts continued to increase slowly, but the relative proportion decreased as the gram-negative rods, especially Enterobacter, proliferated. B. cereus never reached numbers great enough to cause disease. The results revealed that the microbial profile of reconstituted dry milk changed significantly over time and that the temperature of storage determined the eventual microbial composition. The results also showed that B. cereus is an omnipresent health hazard in reconstituted milk that is not properly refrigerated.