Fate of bacillus cereus and naturally occurring microbiota on milled rice as affected by temperature and relative humidity

We studied the survival and growth patterns of Bacillus cereus, mesophilic aerobic bacteria (MAB), and molds and yeasts (MY) on rough and milled brown and white Korean rice stored at 12 and 21°C and 43, 68, and 85% relative humidity (RH) for up to 24wk. The initial populations of MAB present on rough rice, brown rice, and white rice were 7.7, 5.7, and 3.3logCFU/g, respectively, and remained constant or decreased (P≤0.05) by 0.7-1.8logCFU/g during storage. The initial populations of B. cereus on the three types of laboratory-inoculated rice were 3.1-3.8logCFU/g and remained constant (P>0.05) during storage, regardless of degree of milling, storage temperature, and RH. The initial populations of MY on rough rice, brown rice, and white rice were 6.2, 4.2, and 2.1logCFU/g, respectively. At 12°C and 85% RH, the MY increased significantly (P≤0.05) only on brown rice; however, at 21°C and 85% RH, MY increased (P≤0.05) on all types of rice during storage. These observations will be useful when assessing conditions affecting survival of B. cereus and determining environmental conditions necessary to prevent growth of potentially mycotoxigenic molds on various types of milled rice during storage.