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

摘要

This study evaluated the ability of spore-forming Bacillus spp. to resist milk pasteurization conditions from 72 to 150℃. 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 dH_2O, 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℃, 0.39 after 90 min at 78℃, 8.10 after 60 min at 100℃, 7.74 after 2 min at 130℃, and 7.43 after 0.5 min at 150℃. 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℃, 0.21 after 60 min at 78℃, 7.62 after 60 min at 100℃, 7.37 after 2 min at 130℃, and 7.53 after 0.5 min at 150℃. No significant differences (P < 0.05) in thermal resistance were observed for comparisons of spores heated in dH_2O 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 dH_2O, synthetic media, or skim milk.