A redox-based strategy to enhance propionic and butyric acid production during anaerobic fermentation.

This study investigated the selective production of volatile fatty acids (VFAs) during anaerobic mixed-culture fermentation. The experiment used chicken manure (CM) as a potential substrate to produce high added-value propionic acid and butyric acid under an alkaline environment. The conversion of CM into selective VFAs depends highly on operational conditions such as pH and redox balance. Therefore, the current experiment is designed to employ amino acid addition and develop a redox balance control method to control the final VFA profile. This study showed that 0.2-5.0 % valine and threonine addition successfully enhanced propionic acid and butyric acid production during alkaline fermentation and hence decreased the proportion of acetic acid from 83 % to approximately 47 %. The oxidation-reduction potential (ORP) and redox cofactor ratio (NADH/NAD) were measured to support the selective VFA production mechanism. The results obtained in this study bring extra value to the valorization of CM within the circular economy concept for selective value-added VFA production.