Comprehensive insights into the impacts of capsaicin on food waste dark fermentation for hydrogen production: Characteristics of fermentation products, microbial community and metabolic pathways

Dark fermentation can simultaneously achieve food waste treatment and hydrogen production. Capsaicin, a natural antibacterial substance, is prone to accumulate in food waste due to the capsicum use. However, the impacts of capsaicin on food waste dark fermentation have not yet been studied. To fill this knowledge gap, in this study the response of fermentation products, microbial community, and metabolic pathways to capsaicin was explored by setting different dosage of capsaicin during dark fermentation and using metagenomic sequencing. Results demonstrated that cumulative hydrogen yield at a capsaicin dosage of 40 mg/g volatile solids (VS) decreased by 29.6 % compared to that of control. The macro-reasons for the reduction in hydrogen yield was the decrease in degradation efficiency of VS and conversion efficiency of soluble organic matters. Taxonomic annotations analysis indicated that capsaicin reduced hydrogen producing bacteria (Clostridium, Bacteroides, and Paraclostridium) and enriched lactate producing bacteria (Bifidobacterium). Bifidobacterium competed with hydrogen producing bacteria. KEGG annotations analysis showed that capsaicin inhibited hydrogen production pathway, including NADH pathway, formate-decomposition pathway, and pyruvate pathway, while enhanced the competitive reaction of hydrogen production, such as lactate production pathway, which were the micro-reasons for the reduction in hydrogen yield. Besides, capsaicin decreased microbial activity by inhabiting ABC transporter, biofilm formation, ribosome activity, and inducing apoptosis. This study revealed the underlying mechanisms that capsaicin inhibited hydrogen production, raising concerns for the elimination of capsaicin during dark fermentation of food waste.