This review aims to provide a holistic synopsis of the current state of the art knowledge on caproic acid production via anaerobic fermentation from the perspectives of microbes and metabolic pathway. The pure bacterial isolations with ability of caproic acid synthesis were summarized for metabolic identification and syntrophic potentials. Compared with single bacterium, mixed microflora with caproic acid production capacity is more conducive to caproic acid recovery and stability of the fermentation system. The anaerobic fermentation influencing factors, such as pH, temperature, electron donor and acceptor, hydrogen partial pressure, and competition between microbes were discussed. It is found that these factors essentially affect the performance of microbes causing system fluctuation thus decreased caproic acid production. Facing with the immature caproic acid separation technology, were compared from the prospect of application and electrodialysis was proposed as the mainstream technology in future. Simultaneously, it is believed that in-situ separation strategy can improve the yield and purity of caproic acid. Targeting on a large scale caproic acid production through anaerobic fermentation, it is also necessary to strengthen the research on the synergy and competition between microbes in the process of anaerobic fermentation, and to establish a readiness caproic acid separation technology. This work is expected to contribute to the goal of carbon summit and carbon neutrality. 

Caproic acid production from anaerobic fermentation of organic waste - Pathways and microbial perspective

Product spectrum analysis and microbial insights of medium-chain fatty acids production from waste biomass during liquor fermentation process: Effects of substrate concentrations and fermentation modes.

Caproic acid (CA) production from food waste (FW) is a promising way for waste recycling, while the fermentation processes need further exploration. In this study, FW acidogenic fermentation under different pH (uncontrolled, 4, 5, 6) using indigenous microbiota was investigated. Result showed that substrate hydrolysis, carbohydrate degradation and acidogenesis increased with the increase of pH. Although various microbial communities were observed in FW, lactic acid bacteria (Lactobacillus and Limosilactobacillus) were enriched at pH lower than 6, resulting in lactic acid accumulation. CA (88.24 mM) was produced at pH 6 accounting for 31.23% of the total product carbon. The enriched lactic acid bacteria were directionally replaced by chain elongators (Caproicibacter, Clostridium_sensu_stricto, unclassified_Ruminococcaceae) at pH 6, and carbohydrates in FW were firstly transformed into lactic acid, then to butyrate and CA through lactate-based chain elongation processes. This work provided a novel CA fermentation pathway and further enriched the FW valorization. 

Caproic acid production from food waste using indigenous microbiota: Performance and mechanisms.

Abstract Hexanoate is a valuable chemical that can be produced by microorganisms that convert short-chain- to medium-chain carboxylic acids through a process called chain elongation. These microorganisms usually produce mixtures of butyrate and hexanoate from ethanol and acetate, but direct conversion of ethanol to hexanoate is theoretically possible. Steering microbial communities to ethanol-only elongation to hexanoate circumvents the need for acetate addition and simplifies product separation. The biological feasibility of ethanol elongation to hexanoate was validated in batch bioreactor experiments with a Clostridium kluyveri -dominated enrichment culture incubated with ethanol, acetate and butyrate in different ratios. Frequent liquid sampling combined with high-resolution off-gas measurements allowed to monitor metabolic behavior. In experiments with an initial ethanol-to-acetate ratio of 6:1, acetate depletion occurred after ± 35 h of fermentation, which triggered a metabolic shift to direct conversion of ethanol to hexanoate despite the availability of butyrate (± 40 mCmol L −1 ). When only ethanol and no external electron acceptor was supplied, stable ethanol to hexanoate conversion could be maintained until 60–90 mCmol L −1 of hexanoate was produced. After this, transient production of either acetate and butyrate or butyrate and hexanoate was observed, requiring a putative reversal of the Rnf complex. This was not observed before acetate depletion or in presence of low concentrations (40–60 mCmol L −1 ) of butyrate, suggesting a stabilizing or regulatory role of butyrate or butyrate-related catabolic intermediates. This study sheds light on previously unknown versatility of chain elongating microbes and provides new avenues for optimizing (waste) bioconversion for hexanoate production. 

Physiological and stoichiometric characterization of ethanol-based chain elongation in the absence of short-chain carboxylic acids

Enhanced ethanol-driven carboxylate chain elongation by Pt@C in simulated sequencing batch reactors: Process and mechanism.

High-rate conversion of lactic acid-rich streams to caproic acid in a fermentative granular system.

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High-rate conversion of lactic acid-rich streams to caproic acid in a fermentative granular system.