A study on γ-Aminobutyric acid (GABA) enrichment and its metabolic mechanism in germinating mung beans under the stress of ultrasound combined with slightly acidic electrolyzed water

Ultrasound and slightly acidic electrolyzed water (SAEW) treatments have been widely applied to enrich γ-Aminobutyric Acid (GABA) during the germination of grains and legumes; however, the effects and underlying mechanisms of their combined treatment on GABA enrichment remain unclear. After 72 h of germination, this study investigated the impacts of different treatment methods—distilled water treatment (GM), ultrasound treatment (UGM), SAEW treatment (SGM), and combined treatment (USGM)—on key enzyme activities, protein structural characteristics, and the GABA metabolic pathway in germinated mung beans. The results indicated that the combined treatment of ultrasound and SAEW promoted GABA enrichment in germinated mung beans, with the GABA content reaching 6.25 ± 0.025 mg/g. Additionally, it significantly enhanced the activities of glutamate decarboxylase (GAD), diamine oxidase (DAO), and polyamine oxidase (PAO) to 0.359 ± 0.008 U/g, 0.554 ± 0.021 U/g, and 1.265 ± 0.036 U/g, respectively ( p < 0.05). Fourier-transform infrared spectroscopy and intrinsic fluorescence spectroscopy analyses revealed that the combined treatment significantly increased the content of α-helix and β-turn structures in the protein secondary structure ( p < 0.05), while reducing the proportions of β-sheet and random coil structures, and enhanced protein surface hydrophobicity. Untargeted metabolomics analysis further demonstrated that the combined treatment significantly upregulated the levels of amino acid-related compounds, with a strong positive correlation observed between Glu and GABA content. Moreover, it primarily facilitated GABA synthesis by modulating pathways such as arginine-proline metabolism, arginine biosynthesis, alanine, aspartate, and glutamic acid metabolism, and butanoate metabolism. The findings of this study provide theoretical support for elucidating the molecular mechanisms by which ultrasound and SAEW promote GABA enrichment in mung beans and offer new insights for developing functional mung bean foods rich in GABA.