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Ammonium Enhances Food Waste Fermentation to High-Value Optically Active l‑Lactic acid

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posted on 2019-12-13, 20:03 authored by Wenjuan Zhang, Xianbao Xu, Pingfeng Yu, Pengxiao Zuo, Ya He, Hong Chen, Yanan Liu, Gang Xue, Xiang Li, Pedro J. J. Alvarez
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 of 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 threefold increase in the glycolysis activity and an increase in the relative abundance of key lactic acid bacteria (LAB) genera and the 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 the reduction of pyruvate to l-lactate. Specifically, the added ammonium promoted beneficial population and metabolic shifts, including an increase in intracellular NADH levels (0.46 ± 0.02 vs 0.26 ± 0.01 mM for unamended controls) that significantly increased the l-LA yield. Overall, this study provides a practical way to enhance l-LA production with high OA during food waste fermentation and highlights ammonium as an overlooked biostimulator for food waste biorefinery.

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