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

ABSTRACT

BACKGROUND: 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. METHODS: 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. RESULTS: 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 21, the content of monacolin K and GABA in the MFR simultaneously reached 2.22 mg/g and 29.9 mg/g, respectively. CONCLUSIONS: 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.