Evaluating the Impact of Cooling Techniques on Bacillus cereus Populations in Brown Rice

Introduction:  In institutional settings, large quantities of food may be cooked, cooled, and stored for later service. Improper, or slow, cooling has been identified by the United States Food and Drug Administration as a contributing factor in foodborne illness outbreaks. Therefore, validating cooling methods that are feasible and effective at preventing pathogen growth is critical for public health.

Purpose:  This study was designed to test the efficacy of cooling technique combinations on controlling Bacillus cereus growth within brown rice.

Methods:  Brown rice was prepared according to product label instructions and then cooled to 135 to 140°F before inoculation with B. cereus (10⁴ CFU/g of spores). All pans were stored in a commercial walk-in freezer (-20°C) or placed in ice water baths stored inside a commercial walk-in refrigerator (4°C), either uncovered or covered with one or two layers of aluminum foil. Samples were obtained at 0, 4, 8, 12, and 24 hours, plated onto Mannitol Egg Yolk with Polymyxin B agar, and incubated for 24 to 48 hours to enumerate B. cereus populations.

Results:  Treatment*time (P=0.0026) and product depth*time (P=0.0268) were statistically significant for B. cereus populations within the brown rice product during cooling. Bacillus cereus populations decreased by 0.37 log CFU/g between zero and 24 hours when stored in the freezer, whereas populations decreased by 0.09 log CFU/g between aero and 24 hours when stored in the refrigerator. Bacillus cereus populations decreased in both two- and three-inch product depths between zero and 24 hours by 0.21 log CFU/g and 0.25 log CFU/g, respectively.

Significance: The slight decrease in B. cereus populations observed over the 24-hour cooling period combined with no significant difference (P>0.05) in B. cereus population observed for the cover (two layers, one layer, uncovered) variable indicate that all cooling techniques were effective at controlling B. cereus population outgrowth in prepared rice.