Screening of gamma-aminobutyric acid-producing lactic acid bacteria and its application in Monascus-fermented rice production.

Abstract

Gamma-aminobutyric acid (GABA), with an antidepressant effect, and Monacolin K, with a cholesterol-lowering effect, are the main bioactive ingredients in Monascus-fermented rice (MFR). The simultaneous enrichment of both ingredients can effectively enhance the health benefits of MFR. However, the capacity of Monascus spp. to produce GABA is limited. Seventeen lactic acid bacteria (LAB) strains were preliminarily screened for GABA-producing by whole-cells bioconversion of L-glutamate, followed by rescreening through fermentation with the addition of the precursor L-glutamic acid. Subsequently, the bioconversion conditions (temperature, metal ions, and pH) for the conversion of L-monosodium glutamate (MSG) were investigated. Additionally, the GABA-producing LAB was co-inoculated with a monacolin K producing strain Monascus anka 20-2, and the ratio of M. anka 20-2 to LAB in microbial consortia was optimized for MFR production. The strain Lactobacillus plantarum 8014 was screened out for its ability to produce GABA. At an optimal temperature of 33°C and pH 7.5, with the addition of 0.05 g/L ZnSO4, the strain showed an L-glutamate conversion rate of 100%. The ratio optimization of M. anka 20-2 to L. plantarum 8014 in microbial consortia showed that when the dry cell ratio was 2:1, the content of monacolin K and GABA in the MFR simultaneously reached 2.22 mg/g and 29.9 mg/g, respectively. A two-stage fermentation using microbial consortia containing M. anka 20-2 and L. plantarum 8014 was developed for the production of bioactive MFR, in which the active ingredients monacolin K and GABA were simultaneously enriched, with good consumer acceptability due to the aromatic scent produced by lactic acid bacteria.