Search

Microorganisms in dark tea play an indispensable role in its ecosystem management. Despite the challenging conditions inherent to dark tea, a mere gram of its matrix can harbor a diverse array of microbial taxa, including bacteria and fungi. The interplay between dark tea and its microbiome is essential for ensuring quality and safety. Beneficial microbial communities significantly contribute to dark tea's rheological properties and organoleptic characteristics. This paper aims to elaborate on microbiome ecosystems in dark tea, microbial functions on dark tea quality, technology advances in microbiome research, and challenges of revealing microbiota-dark tea substrate interactions. The dark tea industry is now well-positioned to identify how to manipulate and manage the dark tea microbiome to enhance flavor, ensure safety, and improve our understanding of the mechanisms by which these ecosystems will adapt to environmental changes. • The structure, origin, and function of dark tea microbiome were introduced. • Integrated methods for zooming the diversity of dark tea microbiomes were introduced. • The new biotechnologies can be applied to tailor microbes for dark tea flavor. [ABSTRACT FROM AUTHOR]

Microbial fermentation, especially the microbes involved, plays a crucial role in the quality formation of dark tea. Over the last decade, numerous microbes have been isolated from dark tea and in turn, applied to dark tea manufacture through pure-strain, mixed-strain, and enhanced fermentation. This article systematically summarizes the specific metabolic function and quality contribution of tea-derived microbes, with special attention paid to their safety risk. Aspergillus niger converts catechins via hydrolysis, addition, oxidative polymerization, and B-ring fission, contributing greatly to the reddish-brown color and mellow taste of dark tea. Aspergillus sydowii and Penicillium simplicissimum are caffeine-degrading microbes, degrading caffeine mainly into theophylline. However, under adverse conditions, Aspergillus , Penicillium , and Fusarium species potentially produce aflatoxins, ochratoxin A, and citrinin, the mycotoxins occurring in dark tea. The in-depth knowledge of tea-derived microbes is important for improving the quality and safety of dark tea, providing a theoretical basis for its industrial modernization. [Display omitted] • The advantages of various artificial fermentation were comparatively analyzed. • The metabolic functions of tea-derived individual microbes were characterized. • Metabolic pathway of catechins and caffeine by specific microbes were elucidated. • The mycotoxins and associated mycotoxigenic microbes in dark tea were clarified. • The major limitations of current research and future directions were proposed. [ABSTRACT FROM AUTHOR]

Dark tea with a unique aroma is a particular type of tea. Post-fermentation is the key manufacturing process that forms a unique aroma, distinguishing dark tea from other types of tea. Different post-fermentation conditions may be the reason for forming unique aromas in various dark teas. Microbial community and composition during post-fermentation are the main contributors to the formation of characteristic aroma in various dark teas. In this paper, we illustrated the differences in fermentation length, pile temperature and water content of ripened Pu-erh tea, Liupao tea and Tibetan tea during post-fermentation, and only six dominant microorganisms and six key aroma-active compounds were found to be consistent among the three dark teas. Furthermore, we elaborated on the correlations between key aroma-active compounds and core functional microorganisms during post-fermentation. Exploring these interrelationships during post-fermentation is crucial and worthwhile to maintain the stability of the aroma quality of the three dark teas. • Different types of dark teas have unique aromas; • The generation of different aromas is closely linked to key factors during the post-fermentation process; • The correlations between key aroma-active compounds and core functional microorganisms were elaborated; • The dominant microorganisms during post-fermentation may not all be the core functional microorganisms. [ABSTRACT FROM AUTHOR]

[Display omitted] • (1) The aroma compounds of dark teas from five different regions were thoroughly studied. • (2) A total of 1372 volatile compounds were detected in five regions dark teas. • (3) β-Ionone, dihydro-β-ionone, etc. compounds were identified as key aroma compounds. • (4) Eighteen aroma compounds can be used as factors to distinguish dark teas in PLS-DA. • (5) Five regions dark teas can be accurately categorized by Pearson correlation analysis. Dark tea (DT) holds a rich cultural history in China and has gained sizeable consumers due to its unique flavor and potential health benefits. In this study, headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC–MS), relative odor activity value (ROAV), and chemometrics approaches were used to detect and analyze aroma compounds differences among five dark teas from different geographical regions. The results revealed that the five DTs from different geographical regions differed in types, quantities, and relative concentrations of volatile compounds. A total of 1372 volatile compounds of were identified in the 56 DT samples by HS-SPME-GC–MS. Using ROAV and chemometrics approaches, based on ROAV>1 and VIP>1. Eighteen key aroma compounds can be used as potential indicators for DT classification, including dihydroactinidiolide, linalool, 1,2,3-trimethoxybenzene, geranyl acetone, 1,2,4-trimethoxybenzene, cedrol, 3,7-dimethyl-1,5,7-octatrien-3-ol, β -ionone, 4-ethyl-1,2-dimethoxybenzene, methyl salicylate, α -ionone, geraniol, linalool oxide I, linalool oxide II, 6-methyl-5-hepten-2-one, α -terpineol, 1,2,3-trimethoxy-5-methylbenzene, and 1,2-dimethoxybenzene. These compounds provide a certain theoretical basis for distinguishing the differences in five DTs from different geographical regions. This study provides a potential method for identifying the volatile substances in DTs and elucidating the differences in key aroma compounds. Abbreviations: DT, dark tea; FZT, Fuzhuan tea; LPT, Guangxi Liupao tea; QZT, Hubei Qingzhuan tea; TBT, Sichuan Tibetan tea; PET, Yunnan Pu-erh tea; ROAV, Relative odor activity value; OT, Odor threshold; HS-SPME, Headspace solid-phase microextraction; GC–MS, Gas chromatography-mass spectrometry; PCA, Principal components analysis; PLS-DA, Partial least squares-discriminant analysis; HCA, Hierarchical clustering analysis. [ABSTRACT FROM AUTHOR]

Theabrownins (TBs) are heterogeneous mixtures of water-soluble brown tea pigments, and important constituents to evaluate the quality of dark tea. TBs have numerous hydroxyl and carboxyl groups and are formed by the oxidative polymerization of tea polyphenols. Many biological activities attributed to TBs, including antioxidant, anti-obesity, and lipid-regulating, have been demonstrated. This review summarizes the research progress made on the formation mechanism and physicochemical properties of TBs. It also discusses their protective effects against various diseases and associated potential molecular mechanisms. Additionally, it examines the signaling pathways mediating the bioactivities of TBs and highlights the difficulties and challenges of TBs research as well as their research prospects and applications. [Display omitted] • Formation, preparation, and physicochemical characteristics of TBs were summarized. • The biological activity and mechanism of TBs were reviewed. • The challenges and strategies of current TB research are deeply analyzed. [ABSTRACT FROM AUTHOR]

The latest insights into the health benefits of various fermented teas, including dark tea, oolong tea, and black tea, underscore the significant role of polyphenols in promoting wellness. The fermentation process in these teas not only enriches their overall polyphenol content but also enhances level of specific polyphenols, improving their health-promoting effects. Dark tea, such as Pu-erh, undergoes microbial fermentation, resulting in unique polyphenol profiles particularly theabrownins that offer potent antioxidant properties. Oolong tea, which is partially oxidized contains catechins and theasinensins, contributing to its potential to enhance weight management and correct metabolic disorders. Black tea, a fully oxidized tea, has a high levels of theaflavins and thearubigins, polyphenols linked to reduced inflammation, decreased incidences of cancer, and enhanced neuroprotection. However, despite these benefits, the emerging understanding of the health risks has associated fermented teas with the presence of potentially harmful compounds formed during fermentation or processing. In dark tea, prolonged microbial fermentation may lead to the production of mycotoxins, such as aflatoxins, which have been linked to carcinogenic effects. Black tea may accumulate heavy metals from soil and processing, posing risks to human health if consumed in excessive amounts. Similarly, oolong tea, while partially fermented, can still contain heavy metals contributing to impaired health. While these risks are generally low and depend on factors such as brewing conditions and consumption frequency, they highlight the importance of conducting future studies to devise preventive measures to mitigate these risks. [ABSTRACT FROM AUTHOR]

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. 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. 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. • Dark teas are rich in diverse bioactive compounds. • Several core microorganisms are involved in dark tea fermentation. • Dark teas possess plenty of biological functions, especially anti-obesity and cardiovascular protective effects. • It is recommended for the public to consume dark teas to prevent and manage certain chronic diseases. [ABSTRACT FROM AUTHOR]

As the critical process for quality formation of ripened Pu-erh tea (RpPT), pile-fermentation mechanism has been widely studied by using omics approach, chemical analysis, microbial distribution and their inoculated fermentation tests. Despite several reviews about chemical compositions and health benefits in RpPT and other dark teas, more comprehensive study is requisite to summarize the pile-fermentation mechanism of RpPT and application prospects of relevant microbes. This paper aims to elaborate dominant microbes, recent advances in single- and multi-omics approaches, and pathways of characteristic components during the pile-fermentation. Microbial effects are summarized and compared, which would enhance their application potential in RpPT production. At least 93 fungi and 53 bacteria have been isolated from RpPT related to the pile-fermentation. Aspergillus niger is regarded as dominant microorganism in pile-fermentation for its higher detection frequency and relative abundance. Aspergillus , Penicillium, Rasamsoni, Rhizomucor and Bacillus genera promote RpPT quality formation. The low mycotoxins contamination risk in pile-fermentation and inoculated fermentation indicates microbial application potential in RpPT production. Several microbes are confirmed to promote formation of characteristic components including theabrownins, puerins, teadenol A, gallic acid, lovastatin, gamma-aminobutyric acid and several flavonols such as quercetin. Future studies focusing on superiority and application potential of relevant microbes in RpPT production through exogenous inoculation in the pile-fermentation are guaranteed, which could improve sensory quality and health benefits of RpPT, and shorten fermentation cycle. • A. niger dominates in pile-fermentation for quality formation of ripened Pu-erh tea (RpPT). • Low mycotoxins risk is confirmed both in pile-fermentation and inoculated fermentation. • Formation mechanisms of RpPT characteristics in pile-fermentation are clarified. • Several microbes promote formation of characteristic components such as theabrownins. • Microbial exogenous inoculation show application potential in RpPT production. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dark tea (DT) improved the progressive loss of motor control in aged mice. • DT ameliorated age-related skeletal muscle atrophy in skeletal muscle of mice. • DT suppressed oxidative stress and enhanced antioxidant capacity and mitochondrial function in mice. • DT alleviated inflammation in skeletal muscle of aged mice. Dark tea (DT), a tea post-fermented by the Eurotium cristatum fungus, is known to attenuate oxidative damage and chronic inflammation. It also extends the lifespan of Caenorhabditis elegans fed a high-sugar diet. However, its impact on age-related muscle loss in mice is unclear. We divided 65-week-old male C57BL/6 mice into three groups: old control received the AIN-93 M diet, while the 0.15DT and 0.3DT groups had 0.15 % and 0.3 % DT for 9 weeks. As a result, DT improved grip strength, treadmill performance, muscle weight, and size in aged mice. It reduced cellular senescence, muscle atrophy markers, and angiopoietin-like 2 expression, while increasing sirtuin 1 protein levels in aged mice. Additionally, DT lowered malondialdehyde content and pro-inflammatory cytokine levels and enhanced total antioxidant capacity and enzymes, ultimately attenuating mitochondrial dysfunction in skeletal muscle. Therefore, our results suggest DT attenuates age-related muscle atrophy by alleviating oxidative stress, mitochondrial dysfunction, and inflammation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The tea samples were processed from a single large-leaf tea variety. • Naphthalene and 2-methylnaphthalene and dibenzofuran could enhance the woody aroma. • 2-Nonenal and 1-hexanol were associated with increasing the fatty and sweet odors. • (Z)-4-heptenal contributed to the formation of a mushroom smell in dark tea. • 1-Octen-3-ol was linked to reducing pungent fishy smell during the pilling process. 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. [ABSTRACT FROM AUTHOR]

• Widely targeted metabolomics reveals chemical periodic change in whole process. • 568 Differential metabolites (VIP > 1.0, P < 0.05, FC > 1.50 or < 0.67) selected. • Anaerobic fermentation is regarded as critical process for the quality formation. • 7 Amino acids, 4 phenolic acids, 6 flavonoids and their glycosides accumulated. • N -, O -methylation, hydrolyzation, glycosylation, oxidation involved in fermentation. Yunnan pickled tea is produced from fresh tea-leaves through fixation, rolling, anaerobic fermentation and sun-drying. In this study, widely targeted metabolomics using UHPLC-QQQ-MS/MS and HPLC analysis were carried out to elaborate its quality formation during the whole process. Results confirmed the contribution of preliminary treatments and anaerobic fermentation to the quality formation. A total of 568 differential metabolites (VIP > 1.0, P < 0.05, FC > 1.50 or < 0.67) were screened through OPLS-DA. (-)-Epigallocatechin and (-)-epicatechin significantly (P < 0.05) increased from the hydrolyzation of ester catechins, such as (-)-epigallocatechin gallate and (-)-epicatechin gallate in anaerobic fermentation. Additionally, the anaerobic fermentation promoted vast accumulations of seven essential amino acids, four phenolic acids, three flavones and flavone glycosides, pelargonidin and pelargonidin glycosides, flavonoids and flavonoid glycosides (i.e. kaempferol, quercetin, taxifolin, apigenin, myricetin, luteolin and their glycosides) through relevant N -methylation, O -methylation, hydrolyzation, glycosylation and oxidation. [ABSTRACT FROM AUTHOR]

Eurotium cristatum is a dominant fungus in the post-fermentation of Fu brick tea, however, its potential role in affecting the volatile profile and non-volatiles metabolites during the liquid-state fermentation of dark tea is not clear. In this study, dark tea was prepared by liquid-state fermentation with E. cristatum. A total of 51 volatile compounds were identified by HS-SPME-GC-MS, which mainly consisted of ketones, aldehydes, esters, and alcohols. There were 12 volatiles (ROAV >1) contributed to the overall aroma, including linalool, citral, methyl salicylate, etc. The overall flavor of tea infusion had been greatly improved after fermentation. The content of ester-type catechins, total phenols, total flavonoids, theaflavins, thearubigins, soluble sugars, and free amino acids decreased, while the content of theabrownins and soluble proteins increased. Meanwhile, the content of nonester type catechins and gallic acid increased and then decreased. The contents of various volatiles and metabolites change under the action of E. cristatum , coordinating to form a fermented dark tea infusion with mint, flower and fruit aroma and mellow and slightly astringent quality. The antioxidant activities showed a positive correlation with substances such as gallic acid and polyphenols. The results showed that the liquid-state fermentation with E. cristatum can effectively influence the flavor characteristics and antioxidant capacity of dark tea infusion. [Display omitted] • The volatile levels in tea infusion increased distinctly after 10 days of fermentation. • The fermented tea infusion with E. cristatum imparted a mint, floral, and fruity flavor. • Linalool, citral, and methyl salicylate have been identified as key odorants of fermented tea infusion. • Submerged fermentation with E. cristatum drastically influenced the level of non-volatile metabolites. • The change in antioxidant activities are closely related to non-volatile metabolites. [ABSTRACT FROM AUTHOR]

[Display omitted] • A temporal clustering strategy for untargeted metabolomics was established. • Dynamic changes of tea metabolites fermented by mixed and single fungi were explored. • Flavonoids and lipids were the key differences in unfermented and fermented tea. • Aspergillus neoniger was in a dominant status among mixed fermentation of tea. Dark tea fermentation involves various fungi, but studies focusing on the mixed fermentation in tea remain limited. This study investigated the influences of single and mixed fermentation on the dynamical alterations of tea metabolites. The differential metabolites between unfermented and fermented teas were determined using untargeted metabolomics. Dynamical changes in metabolites were explored by temporal clustering analysis. Results indicated that Aspergillus cristatus (AC) at 15 days, Aspergillus neoniger (AN) at 15 days, and mixed fungi (MF) at 15 days had respectively 68, 128 and 135 differential metabolites, compared with unfermentation (UF) at 15 days. Most of metabolites in the AN or MF group showed a down-regulated trend in cluster 1 and 2, whereas most of metabolites in the AC group showed an up-regulated trend in cluster 3 to 6. The three key metabolic pathways mainly composed of flavonoids and lipids included flavone and flavonol biosynthesis, glycerophospholipid metabolism and flavonoid biosynthesis. Based on the dynamical changes and metabolic pathways of the differential metabolites, AN showed a predominant status in MF compared with AC. Together, this study will advance the understanding of dynamic changes in tea fermentation and provide valuable insights into the processing and quality control of dark tea. [ABSTRACT FROM AUTHOR]

Dark teas are rich in secondary metabolites, such as phenolics and flavonoids, which have been suggested to be associated with their health benefits. In this study, the concentrations of tea polyphenols, tea pigments, catechins, flavonoids, alkaloid, and volatile components in 44 dark tea samples, including Pu-erh, Fuzhuan and Liubao teas, were systematically examined. Among the samples tested, Pu-erh tea contained the highest total flavonoid content (5.24 ± 0.05%), followed by Liubao (4.45 ± 0.61%) and Fuzhuan teas (3.33 ± 0.23%). The tea polyphenols levels in the dark teas were approximately 10%, and no statistically significant differences ( p > 0.05) were found among the different types. Hexadecanoic acid was the most abundant aroma component in the dark teas, accounting for 15–20% of the total volatile oils. Moreover, the antioxidant activities of these dark teas were analyzed using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS) assay, ferric reducing antioxidant power (FRAP) assay, and cellular antioxidant activity (CAA) assay (HepG2 cells). The fat metabolism modulation activities (FMMA) of the dark teas were tested using a high-throughput screening method (SMMC-7221 cells). The results indicated that the different dark teas had diverse antioxidant activities, and the variation in the activities was significant. Correlation analysis showed that there was a significant positive correlation between the levels of EGCG and antioxidant activities measured using the ABTS ( r = 0.916) and FRAP ( r = 0.853) assays, and the levels of total flavonoids and theabrownins correlated well with the values determined using the CAA ( r = 0.845 and 0.865, respectively) assay. [ABSTRACT FROM AUTHOR]

Liupao tea is a distinctive Chinese dark tea obtained by indigenous tea fermentation facilitated by the symbiotic association of bacteria and fungi. The composition of fungal community in 4 Liupao tea samples stored for several years under natural microbial fermentation was evaluated by MiSeq sequencing. Taxonomic analysis revealed 3 phyla, 6 families, 8 genera. The genera Eurotium and Aspergillus were dominant fungi in almost all the samples. A total of 85 strains found in 41 other tea samples were species of Eurotium . amstelodami , Eurotium . niveoglaucum , Eurotium . repens , Eurotium . rubrum , Eurotium . tonophilum and Eurotium . cristatum by culture-dependent method. Of these species, E . repens , E . rubrum and E . tonophilum have not been previously associated with Liupao tea. This report is the first to reveal fungal flora composition using Illumina-based sequencing and provide useful information for relevant studies on the isolation of Eurotium species in Liupao tea. The predominant molds are Eurotium species, and the comparison of fungal diversity in dark teas is worth considering. The taxonomic analysis of the microbial community would also aid the further study of functional genes and metabolic pathways of Liupao tea fermentation. [ABSTRACT FROM AUTHOR]

• Volatile compounds during the storage of An tea by SPME and SPE extraction. • One hundred and fifty-seven volatile compounds were detected. • The woody, stale and medicinal aromas are revealed during the storage of An tea. • Key aroma compounds related to stale and woody aromas were screened. The aim of this study was to reveal the molecular basis of aroma changes during storage of An tea (AT). The key volatile compounds in AT were screened using SPME-GC–MS and SPE-GC–MS analytical techniques in combination with odor activity value (OAV) and flavor dilution factor (FD). The results showed that with the increase of storage time the stale and woody aromas were revealed. Esters, acids and hydrocarbons are the main types of volatile compounds in AT, and their content accounts for 52.69 %–61.29 % of the total volatile compounds. The key volatile compounds with stale and woody aromas during AT storage were obtained by OAV value and FD value, namely ketoisophorone (flavor dilution factor, FD = 64), linalool oxide C (FD = 64), 1-octen-3-ol (OAV > 1, FD = 32), 1,2-dimethoxybenzene (FD = 16), naphthalene (OAV > 1, FD = 32), 3,4-dimethoxytoluene (FD = 16), and 1,2,3-trimethoxybenzene (FD = 8). Our research provides a scientific basis and insights for the improvement of quality during the storage of dark tea. [ABSTRACT FROM AUTHOR]

[Display omitted] • A QuEChERS method for aflatoxins detection in tea infusion. • Dark tea matrix showed strong adsorbability to aflatoxin. • Only 2.94% AFB 1 was released from the stored fortified dark tea by tea brewing. In daily tea drinking, little is known on aflatoxin transfers from tea-leaf to infusion during brewing and the actual intake. A verified aflatoxins analytical method on tea infusion is significant to the exposure assessment. Here, an optimal method 3 (M3) was screened and validated on four aflatoxins (B 1 , B 2 , G 1 and G 2) simultaneous determination. Recoveries of AFG 1 and AFB 1 were 87.37 %±1.99 %-102.03 %±8.62 %, and AFG 2 and AFB 2 were 83.99 %±8.65 %-100.14 %±1.69 %. The correlation coefficient on the fortified samples determination was > 0.99 for each aflatoxin. In accuracy and precision validation, aflatoxins recoveries in high and low fortified samples were 85.94 %-103.83 %, and RSDs were 2.20 %-9.15 %. Method applicability test showed that the M3 was qualified for six different dark tea types. Extraction rate of 30-days stored AFB 1 fortified tea-leaf by acetonitrile–water (84:16, v/v) was < 50 %; meanwhile, only 2.94 % spiked AFB 1 was released by ten times tea brewing which indicated that Puerh tea have strong adsorbability to aflatoxin. [ABSTRACT FROM AUTHOR]

• E. cristatum used in pure-culture fermentation of primary dark tea. • Metabolites evolutions of dark tea during SSF were determined by UPLC–QQQ–MS/MS. • 574 differential metabolites were detected during SSF of dark tea by E. cristatum. • Phenolic acids and flavonoids are the two largest classes of differential metabolites. • E. cristatum is favorable to the accumulation of theabrownins during SSF of dark tea. Eurotium cristatum is the predominant fungus and key contributor to the characteristics of post-fermented Fu brick tea (FBT) during manufacturing. In this study, the influence of solid-state fermentation (SSF) with E. cristatum on the chemical profile dynamic changes of dark tea was investigated. Results indicated that total phenolics, flavonoids, theaflavins, thearubigins, and galloyl catechins consistently decreased, degalloyl catechins and gallic acid increased in the initial stage of fermentation and decreased after long-term fermentation, and theabrownins continually increased. UPLC–QQQ–MS/MS-based widely targeted metabolomic analysis revealed that the metabolites of dark tea processed by SSF with E. cristatum were drastically different from the raw material. A total of 574 differential metabolites covering 11 subclasses were detected in the whole SSF of dark tea, and the most drastic changes occurred in the middle stage. Phenolic acids and flavonoids were the two major classes of differential metabolites. A series of reactions such as degradation, glycosylation, deglycosylation, methylation, and oxidative polymerization occurred during SSF. Overall, SSF with E. cristatum greatly influenced the metabolites of dark tea, which provided valuable insights that E. cristatum is critical in forming the chemical constituents of FBT. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hypolipidaemic effect of various dark teas were examined using a zebrafish model. • Microbial fermentation significantly improved the hypolipidaemic effect of CDTEs. • The mechanisms underlying hyperlipidaemia regulation by CDTEs were proposed. In order to investigate the hypolipidaemic and antioxidant effects of various dark teas produced from different post-fermentation using the same raw material, a hyperlipidaemia zebrafish model combined with binding bile salts assay and antioxidant assays were performed in this study. Results showed that the hypolipidaemic effect of dark tea extracts increased significantly (p < 0.05) while the antioxidant ability decreased sharply compared with raw material. Particularly, Liupao tea (50%) and Pu-erh tea (48%) showed promising hypolipidaemic potential; however, the antioxidant capacity of Pu-erh tea decreased (31–49%) most dramatically. Besides, the levels of total polyphenols and catechins decreased sharply, but theabrownin, gallic acid, and caffeine increased significantly after post-fermentation. Moreover, the potential mechanisms of regulating hyperlipidaemia by dark tea extracts were discussed. These results suggest that microbial fermentation significantly affects the bioactivity of dark teas, and provide theoretical basis for processing and improving of dark tea products for hyperlipidaemia therapy. [ABSTRACT FROM AUTHOR]

[Display omitted] • Geographic origin, microbes and fermentation are factors of chemical variations. • Pu-erh tea is rich in theaflavins, thearubigins and theabrownins. • Raw and ripened Pu-erh teas differ in flavor, phenolic and antioxidant profiles. • Anti-hyperglycaemic and anti-hyperlipidemia of Pu-erh tea are clinically proven. • Health risks for chemical and biological contaminants of Pu-erh tea are unclear. Pu-erh tea is post-fermented sun-dried tea leaves of Camellia sinensis (Linn.) var. assamica (Masters) Kitamura plant, native to Yunnan, China. Pu-erh tea is a highly prized commodity with unique aroma and taste and multiple health effects. This review overviews the chemical constituents, tentative bioactive compounds and their mechanisms responsible for bioactivities of Pu-erh tea. The bioactivities include antioxidative, antimutagenic, antimicrobial, laxative and neuroprotective activities, and controlling or preventing hypercholesterolemia, hyperglycemia, obesity, diabetes, osteoporosis and Alzheimer's disease. Limited human trials hardly convince the claimed therapeutic efficiency of Pu-erh tea. Raw and ripened Pu-erh tea possess their respective sensory characteristics, chemical and microbial diversities. Chemical and biological differences between Pu-erh and other tea types could be explained by different extents of their respective processing-induced compound transformations. Undesirable heavy metals, mycotoxins and other biocontaminants detected in Pu-erh tea relate to growing conditions of tea plantations, processing and storage conditions. Chemistry- and nutrition-derived mechanisms for tea pricing are lacking. Decontamination strategies and future studies for quality improvement of Pu-erh tea are recommended. [ABSTRACT FROM AUTHOR]

Dark tea, a unique tea fermented primarily in China, has numerous potential beneficial effects. However, harmful substances present in dark tea have provoked significant concern. To conduct a quantitative benefit-risk assessment of dark tea for Chinese residents and provide guidance on rational consumption, a framework of Benefit-Risk Analysis for Foods (BRAFO) and meta-analysis was applied to construct a disability-adjusted life year (DALY). Based on the BRAFO-tiered approach, a reference scenario (no intake) and an alternative scenario (intake of 3 cups/day) were determined. The overall health impacts of dark tea were simulated by comparing the risks of fluoride and AF with benefits of reduced-risk to coronary heart disease (CHD) and diabetes in different scenarios. Three cups of fermented tea consumed per day decreased risks of CHD and diabetes by 8.16% and 12.77% respectively. After quantitative integration of information, the ultimate net health effect was found to be −1958.827 illustrating that the benefits of drinking three cups of dark tea per day outweigh the risks. However, considering the uncertainties in the process, decision-makers should proceed with caution, consulting additional well-conducted studies and further managing harmful substances in dark tea. • Risks and benefits of dark tea were analyzed quantitatively by applying the BRAFO framework or the first time. • Three cups of fermented tea consumed per day were found to reduce the risk of coronary heart disease and diabetes by 8.16% and 12.77% respectively. • The ultimate net health effect was −1958.827 illustrating that the benefits of drinking three cups of dark tea per day outweigh the risks. [ABSTRACT FROM AUTHOR]

[Display omitted] • LBT could alleviate hyperglycemia and its related metabolic disorders. • LBT altered gut microbial structure and composition of hypoglycemic mice. • LBT ameliorated insulin resistance by activating PI3K/Akt pathway. Diabetes and its related metabolic disorders are worldwide public health issues. Many studies have shown that changes in the structure and composition of the intestinal flora are closely related to the host's physiological and pathological processes. In this study, we aim to explore the effect of Liubao tea (LBT) extract on hyperglycemic mice with metabolic disorders and intestinal flora dysbiosis and to further study its regulatory effect on insulin resistance and its potential regulatory mechanism. Our results show that LBT had a good hypoglycemic effect and could significantly alleviate the metabolic disorder evoked by hyperglycemia. The gut microbial sequencing showed that LBT treatment increased the diversity of intestinal flora, increased the abundance of beneficial bacteria, and reduced the abundance of harmful or conditional pathogenic bacteria, as well as significantly altered 39 of the top 50 OTUs with abundance. Besides, LBT could activate the PI3K-Akt-PPARs-GLUT2 cascade signaling pathway to improve metabolic disorders, thereby alleviating insulin resistance. These results suggest that LBT has excellent potential to become a natural functional food for the prevention of hyperglycemia and insulin resistance. [ABSTRACT FROM AUTHOR]

• Analysis of volatiles changes during pile fermentation of dark tea. • Basic volatiles analysis of Yinghong No. 9 dark tea at an industrial scale. • On day 20 of pile fermentation, fresh, clean and chest-nutty smell disappeared. • On day 40 of pile fermentation, stale, fungus and aged fragrance began to present. • β-Myrcene, acetophenone, and terpinen-4-ol may be the key aged aroma compounds. Each type of tea has a unique volatile profile due to its variety, processing technologies and origin. Using HS-SPME and GC/MS, we analyzed the changes of volatile components in cultivar Yinghong No. 9 during pile-fermentation every 10 days. A total of 94 compounds showed significant differences during a total of 60 days mainly including alkanes, ketones, esters, terpenes, aromatics and heterocyclic compounds. Interestingly, 13 metabolites were progressively reduced during the first 20 days and remained unchanged in subsequent procedures, while 17 metabolites remained unchanged in the early stage and progressively increased during the last 20 days of pile fermentation, indicating that they are characteristic volatile compounds of raw material sun-dried green tea and dark tea, respectively. β-ionone, phenylethyl alcohol, and a-ionone could be the top three contributed aroma compounds in the final dark tea. Our study provides a theoretical basis for process and quality improvement of Yinghong No. 9. [ABSTRACT FROM AUTHOR]

• Differences in major chemical constituents existed in different dark teas. • Dark tea promoted gastrointestinal transit in normal mice. • Dark tea restored antibiotic-induced dysbiosis of intestinal microflora. Processing of dark tea varieties, such as Fu brick tea, Liupao tea, Qianliang tea, and Qing brick tea, includes solid-state fermentation involving microorganisms. In this study, we analyzed the major chemical constituents of dark tea extracts and evaluated their modulatory effect on the gastrointestinal function in normal mice, including the improvement of gastrointestinal transit and intestinal microbial, as well as the attenuation of intestinal microbial dysbiosis and intestinal pathological damage, and the adjustment of immune function in antibiotic-treated mice. Substantial differences in major chemical constituents, including total polyphenols, total organic acids, water extract content, 18 free amino acids, gallic acid, and six tea catechins, were observed among Fu brick tea, Qianliang tea, Qing brick tea, and Liupao tea extracts. Extracts from the four dark tea varieties significantly promoted gastrointestinal transit and colonization of beneficial Bifidobacterium and Lactobacillus , and inhibited the growth of harmful Escherichia coli and Enterococcus in normal mice. In addition, Qianliang tea, Qing brick tea, and Liupao tea extracts significantly accelerated the reversal of the ampicillin sodium-induced pathological damage in the ileum, intestinal bacterial dysbiosis (Bifidobacterium , Lactobacillus , E. coli , and Enterococcus), and low immunity. [ABSTRACT FROM AUTHOR]

Chinese dark tea is widely enjoyed for its multiple health-promoting effects and pleasant taste. However, its production involves fermentation by microbiota in raw tea, some of which are filamentous fungi and thus potential mycotoxin producers. Accordingly, whether mycotoxins pose health risk on dark tea consumption has become a public concern. In this study, a cleaning method of multi-functional column (MFC) and immunoaffinity column (IAC) in tandem combined to HPLC detection was developed and validated for determining ten mycotoxins of six groups (i.e., aflatoxins of B 1 , B 2 , G 1 and G 2 , ochratoxin A, zearalenone, deoxynivalenol, fumonisins of B 1 , B 2 , and T-2) in dark teas. The interferences from secondary metabolites were effectively reduced, and the sensitivities and recoveries of the method were qualified for tea matrices. Six groups mycotoxins were determined in 108 samples representing the major Chinese dark teas by using the new method. Subsequently, the dietary exposure and health risks were evaluated for different age and gender groups in Kunming and Pu'er in China and Ulan Bator in Mongolia. The occurrence of zearalenone was 4.63% and that of ochratoxin A was 1.85%, with the other four groups mycotoxins were below the limits of quantification. The hazard index values for the five groups' non-carcinogenic mycotoxins were far below 1.0. The deterministic risk assessment indicated no non-carcinogenic risks for dark tea consumption in the three areas. Probabilistic estimation showed that the maximum value of 95th percentile carcinogenic risk value for the aflatoxins was 2.12 × 10−8, which is far below the acceptable carcinogenic risk level (10−6). Hereby, six groups mycotoxins in Chinese dark tea showed no observed risk concern to consumers. Image 1 • A MFC-IAC-HPLC method was developed and validated for mycotoxins determination in dark tea. • Tea matrix interference was effectively reduced by MFC-IAC in tandem. • Multiple mycotoxins contamination was determined. • Multiple mycotoxins dietary risk was assessed on Chinese dark tea. Six groups of mycotoxins determination methods on dark tea matrices were developed, and mycotoxins dietary risk from Chinese dark tea was assessed. [ABSTRACT FROM AUTHOR]

• Microbial fermentation is the key factor controlling the quality of dark tea. • Serial reactions modify the chemical constituents of tea leaves during fermentation. • Multi-omics approaches are used to reveal microbial impact on dark tea quality. Dark tea is a unique fermented tea produced by solid-state fermentation of tea leaves (Camellia sinensis). It includes ripe Pu-erh tea, Fu brick tea, Liupao tea, and other teas. Microbial fermentation is considered to be the key factor controlling the quality of dark tea. It involves a series of reactions that modify the chemical constituents of tea leaves. These chemical conversions during microbial fermentation of dark tea are associated with a variety of functional core microorganisms. Further, Multi-omics approaches have been used to reveal the microbial impact on the conversion of the chemical components in dark tea. In the present review, we provide an overview of the most recent advances in the knowledge of the microbial bioconversion of the chemical components in dark tea, including the chemical composition of dark tea, microbial community composition and dynamics during the fermentation process, and the role of microorganisms in biotransformation of chemical constituents. [ABSTRACT FROM AUTHOR]

• Liupao tea improved diabetes mellitus in rats induced by streptozotocin and high-fat, high-sugar diet. • Liupao tea showed beneficial effect on modulating gut microbiota composition. • SCFA-producing bacteria were up-regulated and SCFAs were increased in feces. • Twenty-nine key phylotypes were significantly correlated with diabetic indices. Liupao tea, a popular dark tea, exhibited anti-diabetic activity in Chinese tea culture. However, the bioavailability of the main active components is low. Therefore, the relationship between the anti-diabetic activity and the modulation of gut microbiota was investigated in this study for the first time. Liupao tea extract (LTE) could significantly alleviate hyperglycemia, insulin-resistance, and plasma lipid profiles. The hypoglycemic effect of high-dose LTE was comparable to that of metformin. Importantly, LTE exhibited beneficial effect on modulating the diabetic-induced disorder of gut microbiota, such as increasing Bacteroidetes/Firmicutes ratio and up-regulating putative short-chain fatty acid (SCFA)-producing bacteria, including Prevotella and Bacteroides. Moreover, 29 key phylotypes were significantly correlated with at least one diabetic index. In conclusion, LTE could be developed into an anti-diabetic functional beverage and its beneficial effects is likely to be associated with the prebiotic effects for increasing the level of SCFAs and the relative abundance of SCFA-producing bacteria. [ABSTRACT FROM AUTHOR]