Chain elongation for the production of caproic acid from green waste using a mixed culture: Effect of the concentration of electron donor and acceptor

Caproic acid production through chain elongation (CE) is influenced by the microorganism's ability to tolerate changes in electron donor (ED) and acceptor (EA) concentrations. This study assessed the performance of a mixed CE performing (MC) culture derived from arrested anaerobic digestion (AAD) effluent in defined media with ethanol and various short-chain carboxylic acids (SCCA), as well as AAD effluent with varying ethanol concentrations and SCCA's compositions. Results showed that the MC culture dominated by Clostridium sensu stricto 12 grew fastest on acetic acid, followed by butyric acid and propionic acid, with the highest carbon and electron distribution and efficiency with butyric acid as the EA. Compared to Clostridium kluyveri (Ck), the MC culture demonstrated greater resistance and adaptability, tolerating ethanol concentrations up to 18 g/L and acetic acid concentrations of 7 g/L when grown on AAD effluent. Under these conditions, the maximum caproic acid titer achieved was 12.05 g/L, with a carbon distribution to caproic acid of 77.8 %. High initial propionic acid concentrations reduced caproic acid production and ethanol conversion efficiency, whereas higher butyric acid concentrations slightly enhanced caproic acid production. The MC culture's robustness under varying ED and EA conditions highlights its potential for enhanced caproic acid production, outperforming Ck in titer and efficiency.