Effect of Heat Treatment of Milk on Activation of Bacillus Spores

The quality and shelf life of fluid milk products are dependent on the amount and type of microorganisms present following pasteurization. This study evaluated the effects of different pasteurization processes on the microbial populations in fluid milk. The objective was to determine whether certain pasteurization processes lead to an increase in the amount of bacteria present in pasteurized milk by activating Bacillus spores. Samples of raw milk were collected on the day of arrival at the dairy plant. The samples were pasteurized at 63°C for 30 min (low temperature, long time), 72°C for 15 s (high temperature, short time), 76°C for 15 s, and 82°C for 30 min. The pasteurized samples were then stored at 6 and 10°C for 14 days. The samples were analyzed for standard plate count and Bacillus count immediately after pasteurization and after 14 days of storage. Pasteurization of milk at 72 and 76°C significantly (P < 0.05) increased the amount of Bacillus spore activation over that of 63°C. There was no detection of Bacillus in initial samples pasteurized at 82°C for 30 min, but Bacillus was present in samples after storage for 14 days, indicating that injury and recovery time preceded growth. The majority of isolates were characterized as Bacillus mycoides and not Bacillus cereus, suggesting that this organism might be more a cause of sweet curdling of fluid milk than previously reported.