Survival and growth of psychrotrophic Bacillus cereus in dry and reconstituted infant rice cereal

Abstract

The potential for growth of enterotoxigenic Bacillus cereus in reconstituted dry foods is a concern, especially when they are consumed by infants or the immunosuppressed. The ability of a four-strain mixture of spores or vegetative cells of psychrotrophic B. cereus to survive in a commercial, dry infant rice cereal as affected by water activity (a(w); 0.27 to 0.28, 0.52 to 0.55, and 0.75 to 0.78), pH (5.6 and 6.7), and temperature (5, 25, 35, and 45°C) was investigated. The rate of death of vegetative cells in dry cereal stored for 36 weeks was not affected by a(w) or pH. Death of spores in cereal stored at 45°C for up to 48 weeks was enhanced at a(w) 0.78 but was unaffected by pH; loss of viability at 5, 25, and 35°C was largely unaffected by differences in a(w). The effect of temperature (8, 15, 21, and 30°C) on outgrowth of spores of B. cereus inoculated at three levels (0.14, 14, and 133 CFU/g, dry weight basis) into cereal reconstituted with apple juice and commercial pasteurized milk (2% fat) was also studied. Outgrowth of spores did not occur in cereal reconstituted with apple juice. Cereal reconstituted with milk and inoculated with 0.14, 14, and 133 spores per g contained >3 log CFU/g within 24, 9, and 6 h, respectively, at 21°C. Populations in cereal reconstituted with milk and inoculated with 133 CFU or B. cereus spores per g reached 7.11, 7.72, and 7.40 log CFU/g within 12, 48, and 72 h when stored at 30, 21, and 15°C, respectively. The organism grew in cereal reconstituted with milk and held at 8°C for 72 h; however, enterotoxin was not detected. In reconstituted cereal moculated with 133 spores per g, enterotoxin was detected (detection limit 16 ng/g) after 24, 48, and 72 h at 30, 21, and 15°C, respectively, when the population of B. cereus reached >7 log CFU/g. It is recommended that reconstituted infant foods be either consumed immediately or held at ≤8°C and consumed within 48 h after preparation.