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

The potential for growth of enterotoxigenic Bacillus cereus in reconstitute d dry foods is a concern, especially when they are consumed by infants or t he immunosuppressed. The ability of a four-strain mixture of spores or vege tative cells of psychrotrophic B. cereus to survive in a commercial, dry in fant 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 degrees C) was investigated. The rate of death of vegetative cells in d ry cereal stored for 36 weeks was not affected by a(w) or pH. Death of spor es in cereal stored at 45 degrees C for up to 48 weeks was enhanced at a, 0 .78 but was unaffected by pH; loss of viability at 5, 25, and 35 degrees C was largely unaffected by differences in a,. The effect of temperature (8, 15, 21, and 30 degrees C) on outgrowth of spores of B. cereus inoculated at three levels (0.14, 14, and 133 CFU/g, dry weight basis) into cereal recon stituted with apple juice and commercial pasteurized milk (2% fat) was also studied. Outgrowth of spores did not occur in cereal reconstituted with ap ple 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, respectivel y, at 21 degrees C. Populations in cereal reconstituted with milk and inocu lated with 133 CFU of B. cereus spores per g reached 7.11, 7.72, and 7.40 l og CFU/g within 12, 48, and 72 h when stored at 30, 21, and 15 degrees C, r espectively. The organism grew in cereal reconstituted with milk and held a t 8 degrees C for 72 h; however, enterotoxin was not detected. In reconstit uted cereal inoculated with 133 spores per g, enterotoxin was detected (det ection limit 16 ng/g) after 24, 48, and 72 h at 30, 21, and 15 degrees C, r espectively, when the population of B. cereus reached >7 log CFU/g. It is r ecommended that reconstituted infant foods be either consumed immediately o r held at less than or equal to 8 degrees C and consumed within 48 h after preparation.