Widely targeted metabolomic analysis reveals metabolite changes induced by incorporating black tea fermentation techniques in oolong tea processing for quality improvement.

Oolong tea, a semi-fermented tea, has a prominent fruity and floral aroma, resembling green tea's astringency and pungency but lacking black tea's mellowness. Oolong tea incorporating black tea fermentation techniques (OT-IBTFT) not only retained its excellent floral and fruity aroma but also reduced astringency and enriched taste. However, metabolite changes remain unknown. In this study, widely targeted metabolomic analysis showed OT-IBTFT reduced prunin, gallocatechin, methyl gallate, and increased loliolide by changing the flavonoid biosynthesis and biosynthesis of secondary metabolites pathways, thereby reducing the astringency and increasing the mellow taste and richness to improve oolong tea quality. In addition, Wrap-rolling 5 times increased fermentation, improving the color and aroma of oolong tea by increasing theaflavic acid and 2-furoic acid, is more significant than wrap-rolling once. In conclusion, these findings provide a theoretical basis for the improvement of oolong tea processing techniques and flavor and quality control.

Effect of Black Tea Polysaccharides on Alleviating Type 2 Diabetes Mellitus by Regulating PI3K/Akt/GLUT2 Pathway.

The bioactivity of tea polysaccharides (TPs) has been widely reported, but studies to date have focused on green tea. Some human health investigations have implied that black tea may possess potential antidiabetic effects, but less is known about their potential role and related antidiabetic mechanism. The present study was, therefore, conducted to investigate the chemical properties and antidiabetic activity of TPs from black tea. Monosaccharide composition revealed that Alduronic acid (77.8 mol%) considerably predominated in the fraction. TP conformation analysis indicated that three components in TPs were all typical of high-branching structures. Oral administration of TPs could effectively alleviate fasting blood glucose in type 2 diabetes mellitus (T2D) mice, with the values 23.6 ± 1.42, 19.6 ± 2.25, and 16.4 ± 2.07 mmol/L in the 200, 400, and 800 mg/kg·BW groups, respectively. Among these TPs groups, the 800 mg/kg·BW groups significantly decreased by 37.88% when compared with the T2D+water group (p < 0.05). Further studies demonstrated that TP treatment upregulated the expression of p-Akt/p-PI3K (p < 0.001). Additionally, TP treatment significantly promoted glucose transporter protein 2 (GLUT2) translocation in the liver (p < 0.001). These findings suggest that TPs from black tea protect against T2D by activating PI3K/Akt/GLUT2 signaling and might serve as a novel therapeutic candidate for T2D.

Mechanisms and quality variations of non-volatile and volatile metabolites in black tea from various ages of tea trees: Insights from metabolomics analysis.

The sensory quality of black tea (BT) influenced by various factors, among which tree age is particularly significant. People prefer BT produced by fresh leaves from old tea trees, yet the correlation between tree age and tea quality has not been thoroughly investigated. In this study, we analyzed the quality of BT from young trees (H-JYH) and old trees (H-OJYH) using e-tongue technology and sensory evaluation. Our findings revealed that H-OJYH had stronger sweetness and sourness, richer flavor, and diminished bitter-astringency compared to H-JYH. 1231 non-volatile metabolites and 504 volatile metabolites were discovered by ultra-performance liquid chromatography (UPLC) and gas chromatography-mass spectrometry (GC-MS). L-tartaric acid and trans-citridic acid were found to contribute to increase acidity, and 7,8-dihydroxy-6-methoxycoumarin and d-fructose 6-phosphate were associated with enhanced sweetness in H-OJYH. Additionally, lower levels of octyl gallate and vanillic acid in H-OJYH contributed to the diminished bitter-astringency. ß-ionone, 2-phenylethanol and phenylacetaldehyde merged as characteristic compounds of older tree BT with stronger floral and sweet aroma. Our study serves as a guideline to explore the relationship between tree age and tea quality.

Microbial Fermentation Enhances the Effect of Black Tea on Hyperlipidemia by Mediating Bile Acid Metabolism and Remodeling Intestinal Microbes.

Black tea (BT), the most consumed tea worldwide, can alleviate hyperlipidemia which is a serious threat to human health. However, the quality of summer BT is poor. It was improved by microbial fermentation in a previous study, but whether it affects hypolipidemic activity is unknown. Therefore, we compared the hypolipidemic activity of BT and microbially fermented black tea (EFT). The results demonstrated that BT inhibited weight gain and improved lipid and total bile acid (TBA) levels, and microbial fermentation reinforced this activity. Mechanistically, both BT and EFT mediate bile acid circulation to relieve hyperlipidemia. In addition, BT and EFT improve dyslipidemia by modifying the gut microbiota. Specifically, the increase in Lactobacillus johnsonii by BT, and the increase in Mucispirillum and Colidextribacter by EFT may also be potential causes for alleviation of hyperlipidemia. In summary, we demonstrated that microbial fermentation strengthened the hypolipidemic activity of BT and increased the added value of BT.

Study on flavor quality formation in green and yellow tea processing by means of UPLC-MS approach.

Yellow tea (YT) has an additional process of yellowing before or after rolling than green tea (GT), making YT sweeter. We analyzed the variations of composition and taste throughout the withering, fixing and rolling steps using UPLC-MS/MS and sensory evaluation, and investigated the influence of various yellowing times on flavor profile of YT. 532 non-volatile metabolites were identified. Withering and fixing were the important processes to form the taste quality of GT. Withering, fixing and yellowing were important processes to form flavor profile of YT. Withering mainly regulated bitterness and astringency, and fixing mainly regulated bitterness, astringency and sweetness of YT and GT. Yellowing mainly regulated sweetness of YT. Trans-4-hydroxy-L-proline and glutathione reduced form as the key characteristic components of YT, increased significantly during yellowing mainly through Arginine and proline metabolism and ABC transporters. The paper offers a systematic insight into intrinsic mechanisms of flavor formation in YT and GT.

Analysis of aroma quality changes of large-leaf black tea in different storage years based on HS-SPME and GC-MS.

The reasons for the change in volatile metabolites and aroma of black tea during storage remain unclear. Therefore, we used HS-SPME and GC-MS methods to analyze the aroma compounds of new tea (2021) versus aged tea groups (2015, 2017, and 2019). A total of 109 volatile components were identified. During storage, 36 metabolites mainly with floral and fruity aromas decreased significantly, while 18 volatile components with spicy, sour, and woody aromas increased significantly. Linalool and beta-ionone mainly contributed to sweet and floral aromas of freshly-processed and aged black tea, respectively. Isovaleric acid and hexanoic acid mainly caused sour odor of aged black tea. The monoterpene biosynthesis and secondary metabolic biosynthesis pathways might be key metabolic pathways leading to changes in the relative content of metabolites during storage of black tea. Our study provides theoretical support for fully understanding the changes in the aroma quality of black tea during storage.

Effects of pile-fermentation on the aroma quality of dark tea from a single large-leaf tea variety by GC × GC-QTOFMS and electronic nose.

Aroma is one of the significant quality factors of dark tea (DT). However, for a single large-leaf tea variety, there are few studies analyzing the effect of pile-fermentation on the aroma quality of DT. The GC × GC-QTOFMS, electronic nose (E-nose) and GC-olfactometry (GC-O) techniques were employed to analysis the difference of tea products before and after pile-fermentation. A total of 149 volatile metabolites (VMs) were identified, with 92 VMs exhibiting differential characteristics. Among these, 31 VMs with OAV > 1.0 were found to be correlated with E-nose results (|r| > 0.8). Additionally, GC-O analysis validated seven major differential metabolites. Notably, naphthalene, 2-methylnaphthalene, and dibenzofuran were found to enhance the woody aroma, while (Z)-4-heptenal, 2-nonenal and 1-hexanol were associated with an increase in mushroom, fatty and sweet odors, respectively. Moreover, 1-octen-3-ol was linked to reducing pungent fishy smell. These findings could provide a certain theoretical basis for understanding the influence of pile-fermentation on the aroma quality of dark tea.

The formation mechanism of aroma quality of green and yellow teas based on GC-MS/MS metabolomics.

Aroma is a crucial determinant of tea quality. While some studies have examined the aroma of yellow tea, there are no reports of the difference and formation mechanism of aroma quality between yellow and green teas from the same tea tree variety. This study employed gas chromatography-mass spectrometry to investigate the difference and formation mechanism of the aroma of yellow and green tea at the omics level, based on sensory evaluation. The sensory evaluation revealed that green tea has a distinct faint scent and bean aroma, while yellow tea, which was yellowed for 48 h, has a noticeable corn aroma and sweet fragrance. A total of 79 volatile metabolites were detected in the processing of yellow and green tea, covering 11 subclasses and 27 were differential volatile metabolites. Benzoic acid, 2-(methylamino-), methyl ester, terpinen-4-ol ethanone, 1-(1H-pyrrol-2-yl-), 3-penten-2-one, 4-methyl- and benzaldehyde were characteristic components of the difference in aroma quality between green and yellow teas. Eleven volatile metabolites significantly contributed to the aroma quality of green and yellow teas, especially acetic acid, 2-phenylethyl ester, with rose and fruity aromas. KEGG enrichment analysis showed that the arginine and proline metabolism might be the key mechanism of aroma formation during green and yellow teas' processing. These finding provide a theoretical basis way for the aroma formation of green and yellow teas.

Metabolome and Microbiome Analysis to Study the Flavor of Summer Black Tea Improved by Stuck Fermentation.

Tea is the most popular and widely consumed beverage worldwide, especially black tea. Summer tea has a bitter and astringent taste and low aroma compared to spring tea due to the higher content of polyphenols and lower content of amino acids. Microbial fermentation is routinely used to improve the flavor of various foods. This study analyzed the relationship between the quality of black tea, metabolic characteristics, and microbial communities after microbial stuck fermentation in summer black tea. Stuck fermentation decreased the bitterness, astringency sourness, and freshness, and increased the sweetness, mellowness, and smoothness of summer black tea. The aroma also changed from sweet and floral to fungal, with a significant improvement in overall quality. Metabolomics analysis revealed significant changes in 551 non-volatile and 345 volatile metabolites after fermentation. The contents of compounds with bitter and astringent taste were decreased. Sweet flavor saccharides and aromatic lipids, and acetophenone and isophorone that impart fungal aroma showed a marked increase. These changes are the result of microbial activities, especially the secretion of extracellular enzymes. Aspergillus, Pullululanibacillus, and Bacillus contribute to the reduction of bitterness and astringency in summer black teas after stuck fermentation, and Paenibacillus and Basidiomycota_gen_Incertae_sedis contribute positively to sweetness. In addition, Aspergillus was associated with the formation of fungal aroma. In summary, our research will provide a suitable method for the improvement of tea quality and utilization of summer tea, as well as provide a reference for innovation and improvement in the food industry.

Metabolomics reveals the effects of different storage times on the acidity quality and metabolites of large-leaf black tea.

Most aged tea has superior sensory qualities and good health benefits. The content of organic acids determines of the quality and biological effects of aged tea, but there are no reports of the effect of storage on the composition and relative proportion of acidic compounds in black tea. This study analyzed and compared the sourness and metabolite profile of black tea produced in 2015, 2017, 2019 and 2021 using pH determination and UPLC-MS/MS. In total, 28 acidic substances were detected, with 17 organic acids predominating. The pH of black tea decreased significantly during storage from pH 4.64 to pH 4.25 with significantly increased in l-ascorbic acid, salicylic acid, benzoic acid and 4-hydroxybenzoic acid. The metabolic pathways ascorbate biosynthesis, salicylate degradation, toluene degradation, etc. were mainly enriched. These findings provide a theoretical basis to regulate the acidity of aged black tea.