Hexanoic Acid Production from Chinese Cabbage Waste Driven by In Situ Lactic Acid Pre-Fermentation: Effect of pH

Chain elongation (CE) with lactic acid as electron donor is an important way for the biosynthesis of hexanoic acid, and pH is crucial for the carbon flow direction in the CE process. In this study, a high concentration of lactic acid was achieved through the pre-fermentation of Chinese cabbage wastes (CCW), and the effects of pH on the metabolic flow of lactic acid and hexanoic acid yield during CE reaction were investigated. The coexistence of Lactobacillus, Caproiciproducens, and Acinetobacter at pH 5.5 facilitated the CE process driven by lactic acid, obtaining the highest hexanoic acid yield of 5.01 ± 0.01 g COD/L. At pH 6.0, the high abundance of propionic acid-producing bacteria, such as Blautia and Succiniclasticum, converted lactic acid to propionic acid via the acrylate pathway, resulting in low selectivity of hexanoic acid. Without pH control, acetic acid was the main product due to the low pH at the initial stage of fermentation.