Sorbitol-enhanced butyric acid production in mixed-culture anaerobic fermentation of potato peel waste.

The production of bio-based butyric acid mainly relies on NADH as a reducing agent. Enhancing NADH availability is a promising strategy to increase butyric acid yields in mixed-culture fermentation (MCF) systems. Compared with glucose, sorbitol degradation generates one additional mole of NADH per mole. Accordingly, this study investigated the effect of sorbitol addition on butyric acid production in MCF using potato peel waste as feedstock. NADH levels and microbial community dynamics were further analyzed. The results revealed that sorbitol supplementation significantly increased butyric acid production and selectivity, driven by higher NADH levels and enhanced cooperative microbial interactions. Maximum butyric acid production (14.78 g chemical oxygen demand/L) and selectivity (83.49%) were achieved with 5 g/L sorbitol. Higher sorbitol levels increased ethanol production and reduced acetic acid concentration. Gene abundance analysis indicated that butyric acid biosynthesis was mainly catalyzed by phosphate butyryltransferase (EC:2.3.1.19) and butyrate kinase (EC:2.7.2.7) via the buk pathway. Sorbitol addition significantly enhanced the efficiency of the buk pathway. Additionally, key functional microorganisms involved in butyric fermentation-unclassified_Enterobacteriaceae, Lactobacillus, Clostridium_sensu_stricto, and unclassified_Bifidobacteriaceae-were enriched. These results indicate that sorbitol supplementation can promote the biological synthesis of butyric acid.