Bacillus cereus sensu lato is a challenge in solid-state fermented brewers' spent grain

Brewers' spent grain (BSG), as the main by-products from the beer industry, has gained interest in circular bioeconomy which promotes the use of BSG in food applications. Fungal Solid-State Fermentation (SSF) with Rhizopus sp. to produce Tempeh-like products is emerging for its numerous advantages. In this study, we investigated the potential microbiological safety issues regarding the use of BSG for SSF with R. oryzae. Our results showed that spore-forming bacteria were dominant in BSG. Bacillus cereus sensu lato was identified as a potentially significant microbiological hazard in BSG, with a prevalence of 100% and concentrations of 1.8–4.2 log₁₀ CFU/g (n = 15). Characterization of 149 B. cereus s. l. isolates revealed the diversity of B. cereus s. l. in BSG, with panC group II as the dominant group. The results of SSF, performed in both microclimate-controlled bioreactor and lab-simulated condition, showed that B. cereus s. l. could grow to over the critical level at 5 log₁₀ CFU/g. Therefore, it is urgent to investigate more control measures to prevent the growth and toxin production of B. cereus s. l. in BSG during SSF for food applications. Furthermore, our results revealed that the SSF of Tempeh-like products should not be too long, due to an increased risk of B. cereus s. l. growth and spoilage. Storage of fermented BSG and other Tempeh-like products at chilled temperatures (preferably at ≤4 °C) is recommended to prevent the growth of B. cereus s. l.