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The depletion of fossil fuels has prompted an urgent search for alternative chemicals from renewable sources. Current technology in medium chain fatty acids (MCFAs) production though chain elongation (CE) is becoming increasingly sustainable, hence the motivation for this review, which provides the detailed description, insights and analysis of the metabolic pathways, substrates type, inoculum and fermentation process. The main rate-limiting steps of microbial MCFAs production were comprehensively revealed and the corresponding innovative solutions were also critically evaluated. Innovative strategies such as substrate pretreatment, electrochemical regulation, product separation, fermentation parameter optimization, and electroactive additives have shown significant advantages in overcoming the rate-limiting steps. Furthermore, novel regulatory strategies such as quorum sensing and electronic bifurcation are expected to further increase the MCFAs yield. Finally, the techno-economic analysis was carried out, and the future research focuses were also put forward., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Substrate toxicity would limit the upgrading of waste biomass to medium-chain fatty acids (MCFAs). In this work, two fermentation modes of electro-fermentation (EF) and traditional fermentation (TF) with different concentration of liquor fermentation waste (20%, 40%, 60%) were used for MCFAs production as well as mechanism investigation. The highest caproate (4.04 g/L) and butyrate (13.96 g/L) concentrations were obtained by EF at 40% substrate concentration. TF experiments showed that the substrate concentration above 40% severely inhibited ethanol oxidation and products formation. Compared with TF mode, the total substrates consumption and product yields under EF mode were significantly increased by 2.6%-43.5% and 54.0%-83.0%, respectively. Microbial analysis indicated that EF effectively alleviated substrate toxicity and enriched chain elongation bacteria, particularly Clostridium_sensu_stricto 12, thereby promoting ethanol oxidation and products formation. Caproiciproducens tolerated high-concentration substrates to ensure normal lactate metabolism. This study provides a new way to produce MCFAs from high concentration wastewater., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

[Display omitted] • MCFAs obtained from liquor brewing wastewater by Microbial electrosynthesis. • The −1.0 V cathode potential and CO 2 supply increased the MCFAs yield by 32.6 %. • Co-electron donors in wastewater effectively avoided the competing acrylate pathway. • Clostridium sensu Stricto 12 and Eubacterium were key genera for MCFAs production. Chinese liquor brewing wastewater (CLBW) is rich in ethanol and lactate, but the high extraction cost limits the recovery of carbon resources. In this study, microbial electrosynthesis (MES) was used to upgrade CLBW to biofuels such as medium chain fatty acids (MCFAs), and the effect of different cathode potentials and CO 2 supply on the MES production performance was investigated. The results showed that CLBW could be upgraded to the highest caproate concentration and selectivity of 7.98 g/L and 47.10% under −1.0 V/CO 2 condition, which were 32.6% and 31.9% higher than the control group. The further decrease of the cathode potential led to the deterioration of MCFAs production performance. Electron donor metabolic analysis showed that −1.0 V promoted the conversion of electron donors to caproate. The superiority of −1.0 V/CO 2 condition was further confirmed by pH analysis, the weakly basic pH avoided the toxicity of undissociated carboxylic acids. Electrochemical analysis also indicated that stronger electron transfer ability and higher electrochemical activity occurred at −1.0 V/CO 2 condition. The results of microbial diversity analysis showed that Clostridium sensu stricto 12 and Eubacterium would be enriched under −1.0 V/CO 2 condition. In addition, co-electron donors in CLBW could effectively avoid the competing acrylate pathway. This study provides a new pathway for MCFAs production. [ABSTRACT FROM AUTHOR]