Bactericidal mechanisms of radio frequency energy combined with UV light against Bacillus cereus spores based on low-moisture food substitutes

Abstract

Bacillus cereus spores represent a considerable challenge in food processing due to their high resistance to harsh environments. Radio frequency (RF) heating combined with ultraviolet (UV) treatment was proposed for inactivating thermostable spores in low-moisture (LM) foods. This study explored the bactericidal mechanisms of RF coupling with UV treatment sequentially to inactivate B. cereus spores based on LM materials. Results showed that the inactivation effect of B. cereus spores in silica gel particles was similar with that in cereal grains, thus these particles served as substitute for LM foods. Cellular level analysis indicated that the apparent morphology, internal structure and inner membrane of B. cereus spores on silica gel particles were seriously damaged after RF heating alone, which led to the leakage of intracellular pyridine-2,6-dicarboxylic acid (DPA), nucleic acid and protein, and the reduction of growth and reproduction rate. However, damage on the cellular level of B. cereus spores was limited under UV irradiation at 1.00 mW/cm² for 10 min. Importantly, compared with the individual treatment, combined treatment resulted in the most severe damage to cell morphologies and the highest leakage level of contents. The non-targeted proteomics analysis indicated that numbers of differently expressed proteins increased from 193 (RF treatment) or 178 (UV treatment) to 270 after combined treatment, thus enhancing interference of multiple intracellular pathways and leading to additive sporicidal effects against B. cereus spores. These results provided an in-depth understanding at cellular and proteomics levels to reveal the bactericidal mechanism of combined RF and UV treatments on B. cereus spores in LM foods.