Optimization of fermentation for γ-aminobutyric acid (GABA) production by yeast Kluyveromyces marxianus C21 in okara (soybean residue)

γ-Aminobutyric acid (GABA) is a non-protein amino acid with a variety of physiological functions. Recently, yeast Kluyveromyces marxianus strains involved in the catabolism and anabolism of GABA can be used as a microbial platform for GABA production. Okara, rich in nutrients, can be used as a low-cost fermentation substrate for the production of functional materials. This study first proved the advantages of the okara medium to produce GABA by K. marxianus C21 when L-glutamate (L-Glu) or monosodium glutamate (MSG) is the substrate. The highest production of GABA was obtained with 4.31 g/L at optimization condition of culture temperature 35 °C, fermentation time 60 h, and initial pH 4.0. Furthermore, adding peptone significantly increased the GABA production while glucose and vitamin B6 had no positive impact on GABA production. This research provided a powerful new strategy of GABA production by K. marxianus C21 fermentation and is expected to be widely utilized in the functional foods industry to increase GABA content for consumers as a daily supplement as suggested.