Effect of pH regulation mode on byproduct ethanol generated from the lactic acid fermentation of Sophora flavescens residues

Abstract

Sophora flavescens residues (SFRs) were used to efficiently produce lactic acid (LA) through simultaneous saccharification and fermentation (SSF). When the pH value was adjusted to 8.5 with NaOH every 12 h, the maximum LA concentration and yield were 51.1 g/L and 0.85 g/g in the non-sterile fermentation system, respectively, and the byproduct ethanol concentration was lower than that of the group with adjusted pH of 5.5. The relative abundance of Lactobacillus was always over 99.6% during LA accumulation, indicating that bacterial community analysis could not explain the difference of byproduct ethanol concentration under different pH values. Further investigation revealed that the cell concentration of ethanol-producing yeast with adjusted pH of 8.5 every 12 h was lower than that with adjusted pH of 5.5. The experiment also showed that the byproduct ethanol was mainly produced from the metabolic activity of ethanol-producing microorganisms (such as ethanol-producing yeast) rather than from the heterofermentation of Lactobacillus. This study suggests that LA production by SSF can be achieved using pre-treated SFRs, and properly adjusting the pH value of fermentation can inhibit the growth of some microorganisms and increase the production of LA.