Properties and microbial communities of caproic acid production from food waste through lactate-based chain...

Caproic acid (CA) production from food waste (FW) can not only reduce environmental pollution, but also obtain high-value chemicals. However, the low CA yield due to shortage of electron donors (EDs) seriously restricts practical application in one-stage reactor. This study provided a novel two-stage fermentation process (lactic acid fermentation combined with chain elongation) and compared its performance with the traditional one-stage fermentation. It was found that carbohydrate removal was not affected in both reactors, but the two-stage reactor exhibited higher stability, hydrogen recovery rate and chain elongation potential. Prevotella_7 (71.08%) and Bifidobacterium were the dominant genera in one-stage reactor, which mainly converted organics into short-chain fatty acids, and resulted in lower CA yield (0.13 g-COD/g-COD). While due to the supply of EDs from lactic acid fermentation slurry, higher abundance of functional bacteria including Caproiciproducers (23.78%) and Clostridium_sensu_12 (13.24%), as well as associated genes were maintained in the two-stage reactor, which was favorable for conducting chain elongation and promoting CA production (0.23 g-COD/g-COD). Batch tests also verified that with the supply of fermented FW, microorganisms in the two-stage reactor exhibited higher CA fermentation capacity, and more carbon was fluxed towards chain elongation. This study provided useful information for CA production and feasible methods for FW valorization.