pH stabilization of lactic acid fermentation via the glutamate decarboxylation reaction: Simultaneous production of lactic acid and γ-aminobutyric acid

Neutralization of H+ enhances the substrate conversion rate and affords high product concentrations during lactic acid fermentation. Glutamate decarboxylase (GAD) consumes H+ and catalyzes the α-decarboxylation of glutamate to generate neutral γ-aminobutyric acid (GABA). In this study, the glutamate decarboxylation reaction, mediated by immobilized GAD, was applied to consume the H+ ions produced during lactic acid fermentation by Rhizopus oryzae As3.819. The glutamate decarboxylation reaction could neutralize the H+ produced during fermentation, thereby stabilizing the pH during lactic acid production, while lactic acid fermentation provided H+ for GABA production. By coupling the two biochemical processes (pH 4.7-5.0), a lactic acid concentration of 80.2 g/L with a yield of 67% from glucose and a GABA concentration of 44.8 g/L with 96.7% conversion from glutamate were obtained. These results indicated that the two biochemical processes could be coupled in a single reactor, thereby enabling the production of lactic acid and GABA simultaneously and economically.