Enhancing butyrate production from food waste fermentation: an overlooked role of fungal yeast.

The recovery of valuable butyrate from organic waste through microbial fermentation represents a significant advancement in sustainable development. An efficient microbiome is a prerequisite for efficient butyrate production. This study conducts a systematic investigation into the efficacy of four representative inocula, e.g., yeast-rich distiller yeast (DY), bacteria-dominated waste activated sludge (WAS) and anaerobic sludge (AS), and cellulose-degrading fungi-rich forest soil (FS), during food waste fermentation. The fermentation results indicated that the use of DY led to a peak in butyrate production on the fourth day, achieving a concentration of 17.54 ± 0.29 g COD/L, which is 2.88, 13.29, and 2.71 times higher than that of inoculating WAS, AS, and FS, respectively. Fermentation pathway analysis indicated that DY enhanced substrate degradation while effectively in situ generating and utilizing electron donors, particularly ethanol. Subsequently, combined sequencing technologies revealed that the relative abundance of butyrate-producing bacteria (Clostridium) in the DY reactor was 15.58 %. Furthermore, the relative abundance of ethanol-producing bacteria (Kosakonia) was 17.33 %, and ethanol-producing yeast was significantly enriched at 92.27 %. Functional gene analysis revealed that butyrate synthesis in the DY reactor primarily relied on the reverse β-oxidation pathway. Further external yeast supplement experiments suggested that the yeast significantly promotes butyrate production through yeast-bacteria synergy. The neglected iso-butyrate biosynthesis and enhancement strategies were proposed and deeply discussed. This study provides new insights into yeast-bacteria interactions, offering a promising strategy for high-value valorization of food waste and advancing the circular bioeconomy.