Influence of the C/N ratio on lactic acid production from food waste under high substrate concentration

Converting food waste (FW) into a high-value-added chemical product, lactic acid (LA), through anaerobic fermentation provides a sustainable route for waste management and biorefining. The C/N ratio has been widely studied because it critically influences LA fermentation. However, most existing studies focus on low substrate concentrations, which inevitably lead to low LA levels, making downstream separation and purification economically unfavorable. This study investigated the effects of different C/N ratios (10, 15, 20, and 25) on LA production in an open fermentation system operated at a high substrate concentration (162.7 g TCOD/L, 35 °C, 150 rpm, uncontrolled pH). Results showed that lowering the C/N ratio from 25 to 10 resulted in a 1.4-fold increase in LA production, reaching a peak concentration of 25.0 g/L, and promoted a dominant lactic acid bacteria (LAB) community (from 82.8% to 95.5%). Mechanistic analysis indicates that, under conditions of high substrate concentration, a low C/N ratio promotes an increase in LA yield; this is attributed to the fact that an abundant nitrogen source accelerates the rate of substrate hydrolysis and increases the relative abundance of key metabolic genes. Further statistical analysis revealed that proteins and potassium promoted LA production, whereas lipids inhibited it. This work elucidated the effects and underlying mechanisms of the C/N ratio on LA production from FW under high substrate concentration, thereby providing further insights into the high-value utilization of FW.