Ammonium enhances food waste fermentation to high-value optically active L-lactic acid

Fermentation of food waste to L-lactic acid (L-LA), a high value-added platform molecule, is a green approach for resource recovery. However, low yield and optical activity (OA) of L-LA are key limiting factors for such efforts. Here, we report that ammonium addition (300 mg NH4+-N/L) can double the yield of LA and increase OA of L-LA by fivefold during repeated batch fermentation of food waste. This coincided with a three-fold increase in glycolysis activity and an increase in relative abundance of key lactic acid bacteria (LAB) genera and ldhL gene associated with L-LA production. Ammonium addition provided essential nitrogen for LAB growth (47% of 15NH4+-N underwent assimilation versus 15% oxidized to 15NO3--N and 31% to 29N2 and 30N2), and resulted in a stable reducing environment (the oxidation-reduction potential, ORP ranged from -470 to -320 mV) that favors reduction of pyruvate to L-lactate. Specifically, the added ammonium promoted beneficial population and metabolic shifts, including an increase...