Process optimization of GABA enrichment and sensory flavor improvement in Huangjinya green tea through Saccharomyces cerevisiae fermentation

Gamma-aminobutyric acid (GABA) is typically processed through anaerobic fermentation, which often compromises its sensory quality. Therefore, this study implemented a flavor-directed microbial fermentation strategy for GABA-enriched green tea. Comparative evaluation of four microbial strains showed that S. cerevisiae YA200 produced the highest GABA content while imparting a distinct sweet aroma. Both the selection of optimal fermentation parameters (time, concentration, temperature) and the orthogonal test optimization were guided by sensory evaluation and GABA content. Subsequent process optimization increased the GABA yield to 1.75 mg/g while maintaining a pronounced sweet flavor profile after 3 h of fermentation at 25 °C with a 6% inoculum concentration. UPLC revealed a significant increase in threonine (282.05%), alongside notable reductions in lysine (38.48%) and methionine (65.37%), indicating their important role in modulating sweetness perception. HS-SPME-GC-MS detected 382 differentially accumulated volatiles between the control group (CK) and GABA tea (302 downregulated, 80 upregulated), fermentation generated 67 novel volatiles that were absent in CK. The relative odor activity value (rOAV) analysis and molecular docking noted that a significant depletion of 3,4-dehydro-β-ionone and (E)-β-ionone, coupled with a marked accumulation of 2,2,6-trimethylcyclohexanone, which may be responsible for the reduction in fresh aroma and the enhancement of sweet aroma in tea. This study establishes a microbial fermentation strategy to overcome flavor barriers in GABA tea production.