Inhibitory effects of garlic, cinnamon, and rosemary on viability, heat resistance, and biofilm formation of Bacillus cereus spores in the broth of a fermented soybean paste stew, Cheonggukjang jjigae.

Foods prepared through heating, including broths, have the potential and risk of survival of Bacillus cereus, which has the ability to form spores and biofilms. This study evaluated the efficacy of various natural products (particularly spices) in mitigating B. cereus contamination in Cheonggukjang jjigae (CJ) broth. The following characteristics of B. cereus were examined: viability of vegetative cells (including other pathogenic bacteria) and planktonic spores, heat resistance of planktonic spores and spores in intact biofilms, and biofilm formation and persistence. In an antimicrobial test to evaluate the inhibitory effects of spice and cruciferous vegetable extracts on B. cereus CH3 vegetative cells, cinnamon, garlic, and rosemary extracts were selected as they have shown significant inhibitory effects, with inhibition zones of 20-29 mm in diameter at the highest concentration tested (160 mg/mL, unless otherwise stated). These spice extracts also exhibited antimicrobial activity against other foodborne pathogens, including Staphylococcus aureus, Listeria monocytogenes, Salmonella Typhimurium, and Escherichia coli O157:H7. Garlic extract showed the greatest inhibitory effect on the viability and heat resistance of planktonic spores of B. cereus CH3, and cinnamon and rosemary extracts exhibited similar effects. Garlic extract reduced B. cereus CH3 spore counts in phosphate buffer solution (PBS) and CJ broth by 20.22 % and 14.08 %, respectively, compared to control (treated with the same ethanol amount instead of the extract), and effectively weakened spore heat resistance, reducing the D-values of planktonic spores of B. cereus CH3 in PBS and CJ broth by 32.89 % and 23.08 %, respectively, compared to control. As for the characteristics related to biofilm, garlic extract showed the highest inhibitory effect on biofilm formation and persistence and heat resistance of spores in intact biofilms, followed by rosemary and cinnamon extracts. All three spice extracts completely inhibited biofilm formation even at the lowest concentration (20 mg/mL) at the early stage of biofilm formation. They completely eradicated biofilm persistence formed in brain heart infusion (BHI) and CJ broth at the highest concentration. A high garlic extract concentration (80 mg/mL) also reduced the D-values of spores in biofilms formed in BHI and CJ broth by 16.34 % and 9.00 %, respectively, compared to control. Taken together, garlic extract was most effective in mitigating B. cereus contamination in a concentration-dependent manner in in vitro-menstrua and CJ broth. This study may provide one of the promising strategies to reduce the risk of B. cereus in soybean stews such as CJ.