Unlocking Chain-Elongating Microbiomes for Highly Selective Food Waste Conversion into Caproic Acid Using a Single-Stage Electro-Fermentation Technique

Food waste holds significant untapped potential to drive the production of sustainable bioproducts that displace fossil fuels and bolster electrification. This study presents a novel single-stage electro-fermentation (EF) reactor designed to convert complex food waste compounds into valuable medium-chain carboxylates (MCCs) such as caproic acid. The reactor incorporates particle-electrode architectures and a diverse microbiome capable of hydrolysis, acidogenesis, and chain elongation in a single unit process, eliminating the need for multistage reactors. EF performance was evaluated under voltages ranging from −0.8 to +0.8 V vs Ag/AgCl. Under −0.8 V, a particle-fluidized (PF) reactor achieved the highest caproic acid selectivity (68.25%) of any EF study using complex substrates. Chemical analyses suggest that hydrolysis rate and H 2 pressures likely influenced carbon flux toward MCCs. Biofilm imaging and genomic analyses revealed the enrichment of chain-elongating taxa such as Clostridium (∼24%, relative abundance) under reducing conditions in PF reactors, indicating that extracellular electron transfer mechanisms vary across voltage and reactor configuration. These results demonstrate that particle-electrodes, voltage, and chain-elongating microbiomes can be leveraged to direct metabolic flux toward MCC production. Altogether, this work provides a new single-stage strategy for converting waste streams into high-value bioproducts, offering a roadmap for future EF reactor design and operation.