Uncovering the Mechanisms of How Capsaicin Affects Short-Chain Fatty Acid Production during Food Waste Valorization

During food waste valorization to produce short-chain fatty acids (SCFAs), organic substrates are biotransformed step by step, but the effects of food additives that are present at high concentrations on these steps and targeted SCFA production have not been investigated. Therefore, this study aimed to address this knowledge gap by selecting capsaicin, a widely used food additive worldwide, as a representative additive. The results demonstrated a dose-dependent hormesis-like effect of capsaicin on the production of SCFAs. Specifically, the presence of 4 mg of capsaicin/g of volatile solids (VSs) increased SCFAs yield by 12.5%, while higher concentrations (8–64 mg of capsaicin/g of VS) caused a decrement of 22.5–62.9%. Notably, low levels of capsaicin accelerated the solubilization, hydrolysis, and acidification processes with the exception of solubilization and protein hydrolysis, which remained unaffected at higher levels. Additionally, the presence of capsaicin altered the bacterial cell membrane fluidity and led to an increase in reactive oxygen species (ROS). Slight accumulation of ROS stimulated metabolic activities, while excessive accumulation increased the level of microbial apoptosis. The low level of capsaicin enhanced SCFA production by upregulating gene expression associated with hydrolysis and SCFA bioconversion. Conversely, at high capsaicin levels, genes encoding components of the ribosomal subunit involved in polypeptide synthesis, cell migration, and survival regulation were downregulated, resulting in reduced microbial cell activity and SCFA production. This study provides valuable insights into the response mechanisms of complex microbial processes to capsaicin at the genetic level, specifically in SCFA production during anaerobic fermentation of food waste, and has implications in resource utilization of food waste (FW) containing exogenous additives.