Tea polyphenol (TPs) oxidation caused by polyphenol oxidase (PPO) in manufacturing is responsible for the sensory characteristics and health function of fermented tea, therefore, this subject is rich in scientific and commercial interests. In this work, an in vitro catalysis of TPs in liquid nitrogen grinding of sun-dried green tea leaves by PPO was developed, and the changes in metabolites were analyzed by metabolomics. A total of 441 metabolites were identified in the catalyzed tea powder and control check samples, which were classified into 11 classes, including flavonoids (125 metabolites), phenolic acids (67 metabolites), and lipids (55 metabolites). The relative levels of 28 metabolites after catalysis were decreased significantly (variable importance in projection (VIP) > 1.0, p < 0.05, and fold change (FC) < 0.5)), while the relative levels of 45 metabolites, including theaflavin, theaflavin-3′-gallate, theaflavin-3-gallate, and theaflavin 3,3′-digallate were increased significantly (VIP > 1.0, p < 0.05, and FC > 2). The increase in theaflavins was associated with the polymerization of catechins catalyzed by PPO. This work provided an in vitro method for the study of the catalysis of enzymes in tea leaves.

Sichuan Congou black tea (SCGBT), one of China’s top three high-aroma black teas, enjoys widespread consumer popularity. However, research into optimal processing varieties remains insufficient. This study assessed the quality of SCGBT produced from eight tea varieties: ‘Fudingdabai’ (FDDB), ‘Zhongcha 302’ (ZC302), ‘Wuniuzao’ (WNZ), ‘Chuancha 2’ (CC2), ‘Fuxuan 9’ (FX9), ‘Mingshan 131’ (MS131), ‘Zhongcha 108’ (ZC108), and ‘Huangjinya’ (HJY). Sensory evaluation, quantitative chemical analysis, and metabolomics techniques were employed. Results indicated that HJY, ZC302, and MS131 exhibited strong sweetness, umami, and mellowness, while CC2, FX9, and ZC108 were characterized by intense bitterness and astringency, attributed to high tea polyphenol levels and low free amino acid concentrations. The quality of WNZ and FDDB was relatively average. Using non-targeted metabolomics, 4,476 metabolite ion features were detected, and 75 significantly differential metabolites were identified (p < 0.05 and VIP > 1.0). Correlation analysis revealed that metabolites such as valine, L-glutamic acid, asparagine, L-serine, kaempferol-3-O-sophoroside-7-O-glucoside, quercetin-7-O-glucoside, L-tyrosine, and L-norleucine significantly influenced the taste of the tea infusion (|r| > 0.8, p < 0.05). Elevated levels of amino acids (L-glutamic acid and asparagine) and phenolic acids (gallic acid) contributed to the umami of HJY, ZC302, and MS131. Astringency was primarily determined by variations in flavonol/flavone and flavonol/flavone glycosides, as well as ester catechins. Additionally, the content of sweet-tasting amino acids and non-ester catechins was found to fluctuate. In summary, ZC302, MS131, and HJY are more suitable for processing into higher-grade SCGBT. These findings provide a theoretical and practical foundation for selecting and breeding high-quality SCGBT varieties and ensuring consistent product quality.

Huangjinya has recently seen widespread adoption in key tea-producing areas of China, celebrated for its unique varietal traits. Its leaves are also used to produce black tea with distinctive sensory characteristics. The fragrance-enhancing (EF) process is essential in crafting Huangjinya black tea (HJYBT) and is significant in flavor development. However, the impact of EF on non-volatile metabolites (NVMs), volatile metabolites (VMs), and their interactions remains poorly understood. This study aims to investigate how EF temperatures (60 °C, 70 °C, 80 °C, 90 °C, and 110 °C) influence HJYBT flavor transformation. Quantitative descriptive analysis revealed that EF improved the color, aroma, and appearance of tea leaves. Moreover, after an EF temperature of 80 °C, the HJYBT exhibited lower bitterness and astringency, whereas floral, sweet, and fruity aromas became stronger. However, when EF temperatures exceeded 90 °C, a pronounced burnt aroma developed, with HJYBT at 100 °C exhibiting caramel and roasted notes. Partial least squares discriminant analysis indicated that geraniol and linalool contribute to floral and fruity aromas, while 2-ethyl-6-methyl-pyrazine, furfural, and myrcene are key volatiles for caramel and roast aromas. Heptanal, methyl salicylate, α-citral, 1-hexanol, and (E)-3-hexen-1-ol were found to modify the green and grassy odor. Overall, HJYBT treated at 80 °C EF exhibited the highest umami, sweetness, floral and fruity aromas, and overall taste, exhibiting the least astringency, bitterness, and green and grassy notes. These results provide a significant theoretical basis for enhancing HJYBT quality and selecting the optimal EF method.

Developing an effective method to achieve stability and improve the quality of Tibetan tea has scientific significance. Aspergillus niger K1 isolated and identified from Tibetan tea was inoculated in unsterilized or sterilized tea leaves to develop the bioaugmented fermentation (BF) and normal fermentation (NF) processes of Tibetan tea. The results showed that BF resulted in infusions with a deeper color, a stronger aroma, and a thicker taste compared to NF. The dominant bacterium in BF was Staphylococcus (23.76%), while the dominant fungus was Blastobotrys adeninivorans (50.95%). Moreover, 859 metabolites were identified, and the level of 90 differentially changed metabolites (DCMs) in BF increased significantly (VIP > 1, p < 0.05, FC > 2) compared to those in NF, while the level of 37 DCMs in BF decreased significantly (VIP > 1, p < 0.05, FC < 0.5). Correlation analysis demonstrated that A. niger significantly positively correlated with theabrownins, caffeine, and glutamylisoleucine (p < 0.05, |r| > 0.8). B. adeninivorans showed significant negative correlations with 1-(beta-D-ribofuranosyl)-1,4-dihydronicotinamide and 2-hydroxyacetaminophen sulfate (p < 0.05, |r| > 0.8). Consequently, the inoculation of A. niger for BF has the potential to alter the metabolites in tea through a synergistic interaction with other microorganisms, ultimately improving the sensory quality of Tibetan tea.