Effects of Anaerobic Treatment at Different Withering Degrees on the Chemical Composition and Sensory Quality of γ-Aminobutyric Acid-Enriched White Tea

To investigate the effects of anaerobic treatment at different withering degrees on the components and taste quality of γ-aminobutyric acid (GABA)-enriched white tea, tea samples subjected to anaerobic treatment after withering to water contents of 60%, 50%, and 40% were analyzed using sensory evaluation according to the national standard procedure and using metabolomics based on ultra-high performance liquid chromatography-quadrupole orbitrap mass spectrometry (UPLC-Q-Exactive/MS). The results showed that the infusion of tea processed with anaerobic treatment at a fresh leaf water content of 60% exhibited a bright apricot-yellow color and a rich sweet-fruity aroma. A total of 151 compounds were identified across all samples, including 19 amino acids, 11 catechins, 15 dimeric catechins, 16 alkaloids, 32 flavonoid glycosides, 8 N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs), 13 phenolic acids, 9 organic acids, 2 aroma glycosides, 16 lipids, and 10 other compounds. The partial least squares-discriminant analysis (PLS-DA) and one-way analysis of variance (ANOVA) results indicated significant differences in the chemical composition of white tea processed at different withering degrees. In total, 125 compounds exhibited significant inter-group differences (P < 0.05). The highest GABA content was found in tea subjected to anaerobic treatment at a fresh leaf water content of 60%. The levels and proportions of amino acids in GABA-enriched white tea fluctuated across different withering degrees. Notably, the contents of catechins, proanthocyanidin dimers, and most flavonoid glycosides significantly decreased after anaerobic treatment, while the levels of theasinensins and theaflavin-3-gallate significantly increased. Additionally, metabolic pathway enrichment analysis revealed that anaerobic treatment at varying withering degrees significantly affected flavonoid and flavonol biosynthesis, as well as alanine, aspartate, and glutamate metabolism in GABA-enriched white tea. This study provides a theoretical basis for the production of GABA-enriched white tea.