Sustainable food waste biorefinery: insights into the conversion of endogenous lactate into caproate

Anaerobic digestion (AD) is a common technology for food waste (FW) valorisation, despite methane belongs to the category of low-value commodities. Short-circuiting the AD process to avoid methanogenesis is an efficient method to enhance the recovery of higher-value products such as volatile fatty acids (VFAs). VFAs are short-chain carboxylates widely applied in industrial production (Shi et al., 2021). Under favourable thermodynamic conditions, VFAs could be converted into medium-chain carboxylic acids (MCCAs) during a biological process called chain elongation (CE), by using ethanol or lactic acid as electron donors. Lactic acid, reported as one of the main fermentation products for FW, can be converted to caproate through the reverse ?-oxidation pathway. The long-term conversion of FW to MCCAs without external electron donor supplementation has yet to be fully elucidated. Aim of this study was to evaluate the effectivity of producing caproate from FW liquid extract at high organic loading rates (OLRs) without electron donor external supplementation. Both processes of fermentation and CE, conducted in mesophilic semi-continuous bioreactors (V = 3 L) with weekly feeding cycles consisting in two feeding days per week (Monday and Thursday) and with manual pH control at 6, were deeply investigated also as regards the microbial community successional changes with a high-throughput 16S rRNA gene sequencing analysis.