Lactic acid produced by Streptococcus thermophilus activated glutamate decarboxylase (GadA) in Lactobacillus brevis NPS-QW 145 to improve γ-amino butyric acid production during soymilk fermentation

Abstract

γ-Amino butyric acid (GABA)-producing Lb. brevis 145 solely or as a co-culture with Streptococcus thermophilus 1275 was used to ferment 4% soy protein isolate (SPI) medium supplemented with glucose, fructose, galactose, lactose, sucrose or maltose to understand the mechanism of GABA production in the presence of mono-sodium glutamate (MSG). GABA production, remanent MSG, sugar utilization by the two organisms, lactic acid production at different fermentation time points (0 h, 7 h, 24 h, 48 h) were determined. Moreover, pH in mono-culture and co-culture groups was monitored during 48 h of fermentation. Further, HCl, lactic acid, or acetic acid was added separately to adjust the pH of the medium to 4.5 to determine whether pH was a crucial factor for GABA synthesis. S. thermophilus 1275 and Lb. brevis 145 showed different preference to various sugars in SPI medium. The co-culture exhibited enhanced sugar utilization, lactic acid production, improved GABA production and a decrease in MSG level during soymilk fermentation. Importantly, there was a weak correlation between GABA production and sugar utilization, rather it was the acidic environment created by lactic acid production by S. thermophilus from carbohydrate catabolism that influenced gadA activity in Lb. brevis 145 in order to improve GABA production.