An Assessment of Pasteurization Treatment of Water, Media, and Milk with Respect to Bacillus Spores†

This study evaluated the ability of spore-forming Bacillus spp. to resist milk pasteurization conditions from 72 to 150°C. Spores from the avirulent surrogate Sterne strain of Bacillus anthracis, as well as a representative strain of a common milk contaminant that is also a pathogen, Bacillus cereus ATCC 9818, were heated at test temperatures for up to 90 min in dH2O, brain heart infusion broth, or skim milk. In skim milk, characteristic log reductions (log CFU per milliliter) for B. anthracis spores were 0.45 after 90 min at 72°C, 0.39 after 90 min at 78°C, 8.10 after 60 min at 100°C, 7.74 after 2 min at 130°C, and 7.43 after 0.5 min at 150°C. Likewise, log reductions (log CFU per milliliter) for viable spores of B. cereus ATCC 9818 in skim milk were 0.39 after 90 min at 72°C, 0.21 after 60 min at 78°C, 7.62 after 60 min at 100°C, 7.37 after 2 min at 130°C, and 7.53 after 0.5 min at 150°C. No significant differences (P < 0.05) in thermal resistance were observed for comparisons of spores heated in dH2O or brain heart infusion broth compared with results observed in skim milk for either strain tested. However, spores from both strains were highly resistant (P < 0.05) to the pasteurization temperatures tested. As such, pasteurization alone would not ensure complete inactivation of these spore-forming pathogens in dH2O, synthetic media, or skim milk.