Advances in Electricity-Steering Organic Waste Bio-Valorization for Medium Chain Carboxylic Acids Production

Medium chain carboxylic acids (MCCAs, e.g., caproic acid, caprylic acid, etc.) with 6-12 carbon atoms are valuable platform chemicals produced from organic waste via microbial chain elongation metabolism named as reversed beta-oxidation and fatty acid-biosynthesis cyclical pathway. Recently, many articles reported that electricity could not only serve as the external electron donor and provide the reduction equivalent required for chain elongation but also regulate the microbiome structure and metabolic behaviors to promote MCCAs formation. Electricity-steering MCCAs bioproduction has become an appealing technique to valorize low-value organic waste, paving an alternative pathway for net-zero carbon emission energy systems and sustainable socio-economic development. However, the MCCAs' bioproduction from organic waste steered by electric field has not been comprehensively reviewed. From a systematical analysis of publicly available literature, we first covered the basic working principle, fermentation architecture, functional microflora, and metabolic pathway of MCCAs production driven by electricity. The strategies of substrate modulation, applied voltage/current regulation, electrode optimization, and microbial cooperation and stimulation for boosting electricity-driven MCCAs bioproduction are then scrutinized and extensively discussed. Ultimately, the pressing knowledge gaps and the potential path forward are proposed to provide pointers for consistently higher MCCAs yield and the transition from laboratory to market.