Microbial contamination of tofu in Korea and growth characteristics of Bacillus cereus isolates in Tofu

Highlight
- Microbial contamination of tofu in Korea was monitored from May to August in 2014.
- Dominant spoilage bacteria were isolated from the tofu and confirmed as Bacillus cereus.
- Shelf life of tofu was proposed by estimated growth characteristic of Bacillus cereus isolates from tofu.

This study investigated the microbial contamination of commercial tofu from local supermarkets in Seoul, Korea. Growth modeling of Bacillus cereus isolated from spoiled tofu was used to determine the appropriate and temperature for safe storage of tofu. During the monitoring of 100 commercial tofu products, pathogenic bacteria (B. cereus, Staphylococcus aureus, Listeria monocytogenes, Salmonella spp., and enterohemorrhagic Escherichia coli O157:H7 [EHEC]) were not detected. Aerobic bacteria were isolated from 32 of 100 tofu samples because of different sterilization and packaging methods. To isolate the dominant microorganism involved in tofu putrefaction, tofu was intentionally spoiled by storage at 30 °C for 24 h. After spoilage, the most abundant colonies were harvested and confirmed as B. cereus by analyzing their 16 S ribosomal RNA (rRNA) sequences and fatty acid compositions. To investigate growth properties on tofu, isolated B. cereus was cultured and inoculated on tofu. The growth of B. cereus at different storage temperatures (15 °C, 20 °C, 25 °C, and 30 °C) was analyzed. As a result, the appropriate storage time of tofu stored at 5 °C, 10 °C, and 15 °C was determined to be 9.99, 4.17, and 2.08 days, respectively.