Enhanced fermentation for lactic acid production from food waste via hydrothermal pretreatment: performance evaluation and metagenomic analysis.

Mixed-culture lactic acid fermentation of food waste (FW) commonly requests low pH conditions. However, this can pose the challenge of insufficient substrate hydrolysis. To address this, we investigated hydrothermal pretreatment coupled with centrifugal supernatant recovery as a technical strategy to enhance the FW hydrolysis and lactic acid production. The study characterized the supernatant substrate after the pretreatment at different temperatures (80-140 °C), focusing on organic solubilization, supernatant-solid separation for organic recovery, grease removal, and subsequently evaluated lactic acid production in 8-day batch fermentation. The results showed that hydrothermal pretreatment significantly boosted FW hydrolysis, improved organic recovery in the supernatant, and enhanced lactic acid yields. Optimal lactic acid production (12.4 g/L) representing a 68 % increase over the control without hydrothermal pretreatment, was achieved at 120 °C, despite the maximum hydrolysis occurring at 140 °C. Metagenomic sequencing further revealed that the pretreated substrate fostered the development of a lactic acid bacteria-dominated microbial community at low pH, notably enriched with Lactobacillus amylolyticus and Lactobacillus delbrueckii, along with functional genes associated with lactic acid production. Crucially, the absence of genes related to propionate and butyrate pathways in the dominant bacteria would explain the reduced byproduct spectrum and enhanced fermentation stability. These findings indicate that hydrothermal pretreatment improves both the quantity and quality of FW supernatant substrate, promoting a lactic acid bacteria-dominated community that drives favorable metabolic pathways under low pH conditions for more efficient and stable lactic acid production.