Purpose: This study investigated the occurrence of spore formers in processed milk samples collected from household refrigerators and the effect of pasteurization temperatures on Bacillus spores activation.
Methods: Twenty-four samples of ultra-high temperature (UHT) processed milk and pasteurized milk were collected from 24 households and analyzed for the presence of sporeformers. For the spore activation study, raw milk samples were collected from a local dairy farm and subjected to four different pasteurization regimes (65°C for 30 mins, 72°C for 15 s, 80°C for 30 mins, and 100°C for 2 s). Thereafter, pasteurized milk samples were stored for seven days at 5°C and 10°C and analyzed daily.
Results: The average number of aerobic sporeformers in UHT milk and pasteurized milk were 5.77 CFU/ml and 5.88 CFU/ml, respectively. DNA extraction and sequencing further revealed that the mixed culture contained Bacillus cereus, Bacillus thuringiensis, and Bacillus toyonensis for both pasteurized and UHT milk samples. The average number of aerobic sporeformers in raw milk for samples stored at 5°C ranged between 4.67 to 6.00 CFU/ml and 4.84 to 6.00 CFU/ml at 10°C. There was no significant increase in the number of colonies as storage days increased; however, significant decreases were observed (P<0.05) with increases in pasteurization temperature.
Significance: This work showed that household milk may constitute a public health risk. Also, pasteurization regimes that have been studied may not effectively inactivate all aerobic sporeformers in milk because of Bacillus spores activation.