Purpose: This work studied the ability of B.cereus spores to germinate and grow, in order to determine a safe cooling rates for cooked rice, beans, pasta, rice/chicken (4:1), rice/chicken/vegetables (3:1:1), rice/beef (4:1), and rice/beef/vegetables (3:1:1).
Methods: Samples were inoculated with a cocktail of four strains of heat-shocked (80°C/10 min) B. cereus spores (NCTC 11143, 935A/74, Brad 1, and Mac 1) to obtain a final spore concentration of approximately two log CFU/g. Thereafter, samples were cooled through the temperature range of 54.5°C to 7.2°C in 6, 9, 12, 15, 18, and 21 h. At the end of the cooling period, samples were removed and plated on mannitol egg yolk polymyxin agar. The plates were incubated at 30°C for 24 h.
Results: While minimal growth was observed in samples from the six hour cooling period, greater than one log CFU/g growth from B. cereus spores was observed in all products during nine hour cooling. The only exception was beans in which, when the time to achieve 7.2°C was extended to 12 h, B. cereus spores germinated and grew from an inoculum of 2.2 log to 4.0 log CFU/g. A public health concern was noted when the extent of growth from spores, in all products, was up to three log CFU/g during cooling to 7.2°C in 15 h.
Significance: The study results suggested safe cooling rates for cooling cooked rice, beans, pasta, rice/chicken, rice/chicken/vegetables, rice/beef and rice/beef/vegetables to guard against B. cereus foodborne disease outbreaks.