Survival, Growth, and Toxin Production of Bacillus cereus During Cooking and Storage of Fresh Rice Noodles

Retail stores maintain fresh rice noodles (FRNs) at room temperature because refrigeration negatively impacts FRNs' texture. The room temperature and high water activity of FRNs help spore-forming Bacillus cereus to grow and produce toxins. In this study, the effect of steam cooking on survival and different storage temperatures on the growth and enterotoxins production of B. cereus in FRNs were investigated. White rice flour was used to make FRNs. Three treatments of FRNs were used in this study; uninoculated, inoculated (with 4.0 log CFU/ml of B. cereus spores), and autoclaved as a negative control. A slurry of rice flour, cornstarch, and water was steam cooked for 4 min at 90°C and incubated for 168 h at 4°C, and for 72 h at 22 and 32°C. Incubated FRNs were tested for pH, B. cereus growth, and enterotoxins production. Steam cooking reduced the total number of B. cereus spores by 0.7 ± 0.3 log CFU/g. Surviving B. cereus spores in inoculated and uninoculated FRNs germinated over 72 h of storage. No B. cereus was detected in negative controls. An interaction was observed across storage temperatures and time (p < 0.05). The B. cereus population in uninoculated FRNs increased by more than 7.0 log CFU/g at 22 and 32°C over 72 h, while inoculated FRNs showed a 5.0 log bacterial increase at these storage temperatures. No growth was observed at 4°C in both inoculated and uninoculated FRNs. The pH of inoculated FRNs was reduced from 6.9 ± 0.1 to 5.7 ± 0.0 at 32°C and to 6.2 ± 0.1 at 22°C, and the pH of uninoculated FRNs was reduced from 7.0 ± 0.1 to 5.8 ± 0.2 at 32°C and to 6.5 ± 0.0 at 22°C, indicative of FRNs spoilage. B. cereus in inoculated FRNs produced enterotoxins after 12 h of storage at 32°C, and over 24 h of storage at 22°C, while no toxin was detected at 4°C. Our findings show that storing FRNs at room temperature for 24 h leads to enterotoxin production, emphasizing the importance of proper FRN storage and their potential risk to consumers. Nevertheless, further research should investigate the effect of other foodborne pathogens on these products.