Bacillus cereus spores can survive in dried milk products and may proliferate and produce toxins upon rehydration and storage at ambient temperatures. This study compared the growth and enterotoxin production of B. cereus in rehydrated commercially available infant milk formula (IMF), whole milk powder (WMP), non-fat dried milk (NFDM), goat milk powder (GMP), oat milk powder (OMP), and soymilk powder (SMP) at 4°, 22°, 37°C during a storage of 24 hours. The milk powders used in this study were mist inoculated with approximately 1.75 log CFU/ml B. cereus spores and dried back to their original aw of 0.20 to 0.22. Inoculated and uninoculated milk powders were individually hydrated according to the manufacturer’s instructions and stored at 4°, 22°, 37°C for 24 hours. Samples of rehydrated powders were taken at 0 h, 4 h, 8 h, 12 h, and 24 h of storage and tested for B. cereus growth, enterotoxins, and pH. The highest growth in B. cereus cells was observed at 37°C, followed by 22°C, with no growth observed at 4°C. Enterotoxins were first detected in all rehydrated inoculated powders except in OMP after 8 hours of storage at 37°C, and toxin levels increased from mild to heavy with longer storage time. No significant effect of milk powder type on B. cereus growth and toxin production was observed under all storage conditions. At refrigeration temperatures (4°C), no significant growth in bacteria was observed, but as storage temperatures increased to 22°C and 37°C, the bacterial population and enterotoxin levels increased along with an increase in storage time.
Sathish Yerrapati (Thu,) studied this question.