Inhibiting methanogenesis with medium-chain fatty acids: strategy for rapid start-up and stable operation of food...

Converting food waste (FW) into medium-chain fatty acids (MCFAs) via chain elongation (CE) is an economical and eco-friendly approach, but methanogenic competition remains a key challenge limiting CE efficiency. Traditional inhibition methods (e.g., pH regulation, hydraulic/solids retention time control, chemical additives) require strict operation or external inputs, causing non-specific microbial inhibition, high costs and environmental risks. Innovatively, MCFAs can inherently suppress methanogens with obvious advantages. However, their inhibition mechanisms and dependence on concentration and carbon chain length remain unclear. This study investigated the effects of butyric acid (C4), caproic acid (C6), and caprylic acid (C8) at different concentrations on methane production, medium- and short-chain fatty acids accumulation, and microbial dynamics in FW anaerobic fermentation. The results indicated that the inhibitory effect was primarily driven by undissociated fatty acids, with the potency increasing with longer carbon chain lengths. Notably, C8 at a low undissociated concentration (0.05 mM) completely inhibited methanogenesis. Higher concentrations of C4, C6, and C8 effectively sustained hydrolysis and acidogenesis while promoting CE and leading to the accumulation of caproic acid and caprylic acid. Metagenomic analysis showed that a decline in methanogenesis-related functional genes was accompanied by an increase in reverse β-oxidation related functional genes. These findings provide a feasible strategy for rapid start-up and stable operation of FW-based CE systems, and present a sustainable route for FW valorization toward high-value biochemicals.