A survey on occurrence of thermophilic bacilli in commercial milk powders in China

Twenty-two milk powders including infant formula milk powders and whole milk powders collected from different manufacturers in China were examined for the presence of contaminated thermophilic bacilli. The thermophilic isolates were grouped by whole cell protein profiles, and representative strains of each group were further identified by analyzing partial 16S rDNA. 80.5% of all 801 isolates were identified as Bacillus licheniformis, Anoxybacillus flavithermus and Geobacillus stearothermophilus. The most dominant strain was B. licheniformis L1, which accounted for 27.8% of all isolates and appeared in eighteen of twenty-two milk powders. Furthermore, the inactivation effect of conventional heat treatments that were widely used in industry for milk powder was tested on ten representative strains. The three pasteurization processes did not show inactivation effect on most strains, but in some cases even activated thermophilic spores. The UHT process efficiently inactivated both thermophilic vegetative cells and spores.