Background Dark tea is a post-fermented tea with unique organoleptic characteristics. A unique microbial fermentation procedure is involved in dark tea processing that is considered a major factor to manage the quality of dark tea. Dark tea is gaining increasing attention recently because it contains various bioactive compounds with diverse biological functions. Scope and approach This review summarizes the microbial fermentation process, main bioactive compounds, and important biological functions or health benefits of dark tea, emphasizing its protective effects against different diseases and relevant molecular mechanisms, and proposes the perspectives in research and application of dark tea. Key findings and conclusions Functional core microorganism-mediated bioconversion is crucial for the formation of dark tea's main quality components during the fermentation period. Dark tea contains a variety of bioactive compounds, including alkaloids, free amino acids, peptides, polyphenols, pigments, polysaccharides, and volatile compounds. Consumption of dark teas has been linked to multiple health benefits, such as antioxidant, anti-obesity, anti-diabetic, anti-cancer, cardiovascular protective, gastrointestinal protective, hepatoprotective, neuroprotective, and photoprotective activities. Future studies focusing on the health mechanisms of dark teas and the potential applications of dark tea in the food, pharmaceutical, and medical industries are guaranteed.

State-of-the-art review of dark tea: From chemistry to health benefits

Qingzhuan tea (QZT) is a unique type of dark tea exclusively produced in Hubei Province of China. In the current study, liquid chromatography-mass spectrometry (LC-MS) coupled with multivariate analysis was applied to characterize the chemical composition of QZT and investigate the effect of QZT processing on its metabolic profile and sensory quality. The contents of polyphenols and flavonoids decreased significantly while the polysaccharides content remained stable, while the theabrownin content inversely increased during QZT processing. LC-MS-based metabolomics analyses revealed that the tea sample after microbial fermentation (MFT) was dramatically different from the sample before microbial fermentation (UFT), while MFT was very similar to QZT. A total of 102 compounds were identified as critical metabolites responsible for metabolic changes caused by QZT processing, with the contents of catechins and flavonoids significantly decreased, and some novel phenolic acids and catechin derivatives were formed. The sensory quality of QZT was mainly formed during microbial fermentation, which greatly reduced the astringency and bitterness of raw tea leaves and produced its characteristic woody and stale aroma as well as mellow taste. These results suggested that microbial fermentation is the critical process in changing the metabolic profile of raw tea leaves and forming the sensory quality of QZT.

Distinct Changes of Metabolic Profile and Sensory Quality during Qingzhuan Tea Processing Revealed by LC-MS-Based Metabolomics

Fu brick tea (FBT) is a microbial-fermented tea, which is produced by the solid-state fermentation of tea leaves. Previous studies have proved that FBT aqueous extracts could attenuate obesity and gut microbiota dysbiosis. However, the bioactive components in FBT that contribute to these activities remain unclear. In this study, we aimed to investigate the effects of FBT polyphenols (FBTPs) on obesity, gut microbiota, and gut microbiota-related intestinal oxidative stress and barrier function and to further investigate whether the antiobesity effect of FBTPs was dependent on the alteration of gut microbiota. The results showed that FBTP supplementation effectively attenuated obesity in high-fat diet (HFD)-fed rats. FBTP supplementation improved the intestinal oxidative stress and intestinal barrier function, including intestinal inflammation and the integrity of the intestinal barrier. Furthermore, FBTP intervention significantly attenuated HFD-induced gut microbiota dysbiosis, characterized by increased phylogenetic diversity and decreased Firmicutes/Bacteroidetes ratio. Certain core microbes, including Akkermansia muciniphila, Alloprevotella, Bacteroides, and Faecalibaculum, were also found to be improved by FBTPs. Moreover, the antiobesity effect of FBTPs was gut microbiota-dependent, as demonstrated by a fecal microbiota transplantation experiment. Collectively, we concluded that FBTPs reduced obesity by modulating the gut microbiota and gut microbiota-related intestinal oxidative stress and barrier function. Therefore, FBTPs may be used as prebiotic agents to treat obesity and gut microbiota dysbiosis in obese individuals.

Polyphenols from Fu Brick Tea Reduce Obesity via Modulation of Gut Microbiota and Gut Microbiota-Related Intestinal Oxidative Stress and Barrier Function.

Molecular mechanisms underlying health benefits of tea compounds

Fubrick tea aqueous extract (FTEs) has been reported to improve lipid metabolism and gut microbiota communities in mice and humans. However, it is still unclear how FTEs prevents obesity through gut microbiota, and whether some other regulatory mechanisms are involved in the process. Here, we found that FTEs supplementation effectively alleviated the body weight gain, visceral fat accumulation, dyslipidemia, and impaired glucose tolerance induced by a high-fat diet (HFD), and fecal microbiota transplantation (FMT) from FTEs-treated mice showed similar protective effects as FTEs supplementation in mice fed with a HFD. The results confirmed that gut microbiota played key roles in attenuating HFD-induced fat deposition and metabolic disorder. In particular, FTEs reversed HFD-induced gut microbiota dysbiosis via increasing the relative abundances of Bacteroides, Adlercreutzia, Alistipes, Parabacteroides, and norank_f_Lachnospiraceae, and reducing that of Staphylococcus. Interestingly, FTEs could still alleviate HFD-induced lipid accumulation in mice treated with antibiotics, which had increased relative abundances of Bacteroidetes, Bacteroides, and Bacteroides_uniformis sp. In addition, supplementation with FTEs also modified the serum metabolome, especially the "caffeine metabolism" pathway. Furthermore, FTEs supplementation increased the concentrations of caffeine, theophylline, and theobromine in serum, which were positively correlated with an abundance of norank_f_Lachnospiraceae. Overall, FTEs exerts beneficial effects against obesity induced by HFD, and the underlying mechanism is partially related to the reprogramming of intestinal microbiota, while the metabolism of caffeine in FTEs also played an important role in the process. This study provides a theoretical basis for the further study of the anti-obesity effects of FTEs and the consideration of gut microbiota as a potential target for the treatment of obesity induced by a HFD.

Fubrick tea attenuates high-fat diet induced fat deposition and metabolic disorder by regulating gut microbiota and caffeine metabolism

An increasing amount of evidence suggests that the metabolic improvement of high-fat diet (HFD)-induced obese mice by Fuzhuan brick tea (FBT) is associated with gut microbiota. However, the causalities between FBT and gut microbiota have not yet been elucidated and the underlying mechanisms of action remain unclear. To impart direct evidence for the essential role of gut microbiota in the attenuation of obesity by FBT, the effects of FBT on healthy mice and microbiota-depleted mice that were treated with antibiotics were compared in an HFD-induced obesity mouse model. The results showed that FBT dramatically ameliorated obesity, serum lipid parameters, blood glucose homeostasis, hepatic steatosis, adipocyte hypertrophy, and tissue inflammation. However, the microbiota-depleted mice with single bacterium (Escherichia-Shigella) after antibiotic treatment were resistant to FBT-induced antiobesity and metabolic improvement. The beneficial effects of FBT resulted from its shift on gut microbiota composition and structure in mice. HFD-induced increase in the phyla Firmicutes/Bacteroidetes (F/B) ratio was remarkably restored by FBT. Furthermore, FBT-induced increase in abundances of beneficial bacteria Clostridiaceae, Bacteroidales, and Lachnospiraceae and decreases in harmful Ruminococcaceae, Peptococcaceae, Peptostreptococcaceae, and Erysipelotrichaceae were causal antecedents for FBT to reduce obesity and improve metabolic disorders.

Fuzhuan Brick Tea Attenuates High-Fat Diet-Induced Obesity and Associated Metabolic Disorders by Shaping Gut Microbiota

Hunan black tea is well-known for its floral-honey aroma, but the volatile components responsible for the fragrance have not been elucidated yet. In this study, the volatile compounds in Hunan black tea were identified and quantified by the headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS). The results showed that 88 compounds were extracted and determined in Hunan black tea, including the dominant components Geraniol, phenethyl alcohol, phenylacetaldehyde, linalool, nonanal and other 5 aromatic compounds. Furthermore, the aroma-active compounds were identified by odor activity value (OAV). It was found that 24 aroma compounds, including geraniol with an OAV≥1 were regarded as the primary active aromatic compounds in Hunan black tea. Finally, partial least squares (PLS) regression analysis was employed and results revealed that Nonanal, trans-nerolidol, benzyl alcohol, and phenylethanol exhibit a positive correlation with the intensity of floral and sweet honey aromas. Overall, this study identified the volatile compounds responsible for the dominant floral-honey aroma in Hunan black tea. 

Identification of volatile and odor-active compounds in Hunan black tea by SPME/GC-MS and multivariate analysis

Characterization of key aroma compounds and core functional microorganisms in different aroma types of Liupao tea.

Comprehensive analysis of carotenoids constituents in purple-coloured leaves and carotenoid-derived aroma differences after processing into green, black, and white tea

https://doi.org/10.1016/j.fochx.2022.100270

Nontargeted and targeted metabolomics analysis provides novel insight into nonvolatile metabolites in Jianghua Kucha tea germplasm (Camellia sinensis var. Assamica cv. Jianghua)

Wenliang Wu

Papers

Fuzhuan Brick Tea Attenuates High-Fat Diet-Induced Obesity and Associated Metabolic Disorders by Shaping Gut Microbiota

Identification of volatile and odor-active compounds in Hunan black tea by SPME/GC-MS and multivariate analysis

Characterization of key aroma compounds and core functional microorganisms in different aroma types of Liupao tea.

Comprehensive analysis of carotenoids constituents in purple-coloured leaves and carotenoid-derived aroma differences after processing into green, black, and white tea

Nontargeted and targeted metabolomics analysis provides novel insight into nonvolatile metabolites in Jianghua Kucha tea germplasm (Camellia sinensis var. Assamica cv. Jianghua)