Survival and Growth of Foodborne Pathogens during Cooking and Storage of Oriental-Style Rice Cakes

Abstract

Fresh cooked rice cakes for retail sale are typically held at room temperature because refrigeration dramatically reduces their quality. Room temperature, high water activity, and a pH of >4.6 provided an environment conducive to pathogen growth. To date, no studies have been published regarding survival and growth of foodborne pathogens in fresh cooked rice cakes. This study was undertaken to investigate the effect of steam cooking on foodborne pathogens and their subsequent growth in five varieties of rice cakes made from flours of regular rice, sweet rice, white rice, tapioca, and mung bean. Bacillus cereus spores were detected in white rice, tapioca, and mung bean samples. The rice cake flours were inoculated with non–spore-forming foodborne pathogens (Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, Listeria monocytogenes, and Staphylococcus aureus) or spore-forming bacteria (Bacillus cereus) and steam cooked (100°C) for 30 min. Steam cooking significantly reduced (>6 log CFU/g) non–spore-forming foodborne pathogens in all samples and inactivated spores of B. cereus by 1 to 2 log CFU/g. Although spores of B. cereus survived steam cooking and germinated during 3 days of storage at room temperature, populations in most rice cakes remained below 106 CFU/g, which is the threshold for producing toxin. Rice cakes made from mung bean flour supported growth and germination of B. cereus spores above that critical level. In mung bean rice cakes, enterotoxin production was detected by the second day, when B cereus cell populations reached about 6.9 log CFU/g. The toxin concentration increased with storage time. However, our results suggest that rapid growth of total mesophilic microorganisms by more than 7 to 8 log CFU/ml during the first day of storage produced off flavors and spoilage before B. cereus was able to grow enough to produce toxins. Therefore, steam-cooked rice cakes made from a variety of flours including mung bean flour are safe for sale for up to 1 day after storage at room temperature and are free of B. cereus toxins.