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Aspergillus niger is one of the dominant microorganisms presented in dark tea fermentation. In this study, the biotransformation of steamed green tea leaves fermented by A. niger PW-2 was characterized using metabolomic and proteomic approaches. We observed that, after fermentation, the contents of volatile compounds contributing to the "green" aroma, including linalool, L-α-terpineol and geraniol, decreased significantly. Meanwhile, the astringency taste and contents of metabolites contributing to the taste (catechins) reduced significantly during fermentation. Additionally, the contents of theabrownins, which have health benefits, obviously increased. The bitter and umami tastes were also changed due to the variations in bitter-taste and umami-taste amino acids. We also found that glycoside hydrolases, tannases, catechol oxidases, peroxidases and laccases secreted by A. niger PW-2 were responsible for the metabolism of phenolic compounds and their derivatives (theaflavins, thearubingins and theabrownins). Finally, the metabolic pathways involved in the biosynthesis and degradation of characteristic metabolites were found to reveal the biotransformation characteristics of dark tea fermented with A. niger PW-2. [ABSTRACT FROM AUTHOR]

Aspergillus niger is one of the dominant microorganisms presented in dark tea fermentation. In this study, the biotransformation of steamed green tea leaves fermented by A. niger PW-2 was characterized using metabolomic and proteomic approaches. We observed that, after fermentation, the contents of volatile compounds contributing to the “green” aroma, including linalool, L-α-terpineol and geraniol, decreased significantly. Meanwhile, the astringency taste and contents of metabolites contributing to the taste (catechins) reduced significantly during fermentation. Additionally, the contents of theabrownins, which have health benefits, obviously increased. The bitter and umami tastes were also changed due to the variations in bitter-taste and umami-taste amino acids. We also found that glycoside hydrolases, tannases, catechol oxidases, peroxidases and laccases secreted by A. niger PW-2 were responsible for the metabolism of phenolic compounds and their derivatives (theaflavins, thearubingins and theabrownins). Finally, the metabolic pathways involved in the biosynthesis and degradation of characteristic metabolites were found to reveal the biotransformation characteristics of dark tea fermented with A. niger PW-2.

Fluorosis caused by consumption of dark tea is a major public health problem in the western part of China. In this study, the effects of milk powder, butter, and table salt on the bioavailability of the fluoride in an infusion of dark tea were investigated in a rat model. These substances were examined for their potential as dietary interventions to reduce the risk of tea-induced fluorosis. Fluoride was less bioavailable from the dark tea infusion than from a NaF solution. The addition of milk powder significantly decreased the amount of fluoride absorbed from the dark tea infusion and increased the amount of fluoride in fecal excretion. While butter had a limited effect on fluoride bioavailability, it prolonged the fluoride absorption period. The addition of 4 mg/mL table salt significantly increased the bioavailability of fluoride in the dark tea infusion. The addition of different flavor augmentations to a dark tea infusion had different effects on fluoride bioavailability. Therefore, dietary intervention may be a novel strategy for reducing fluorosis risk. • The bioavailability of fluoride in dark tea and NaF aqueous solution was evaluated. • Milk significantly reduced the bioavailability of fluoride in dark tea. • In rats, butter prolonged the absorption period of fluoride from dark tea. [ABSTRACT FROM AUTHOR]

Post-fermented tea has multiple health-promoting efficacy; however, the molecular mechanisms of quality formation remain largely unknown. Here, we found that leucyl-tRNA synthetase (LeuRS) is essential for the formation of 'golden flowers' in dark teas fermented by A. montevidensis. Metabolomics based on UHPLC-Q-TOF/MS revealed 67, 130, and 98 differential metabolites in the compared groups of fermented teas by wide-type strains (WT)/by Δ leuRS mutants (MT) at 3, 9 and 15 days, respectively. These differential metabolites consisted mainly of flavonoids, sugars, organic acids, and fatty acids. Transcriptome analysis showed that hundreds of genes were significantly differentially expressed compared to WT with MT, which were mainly involved in KEGG pathways such as starch and sucrose metabolism, degradation of aromatic compounds, amino acid metabolism and lipid metabolism. These findings highlighted that LeuRS plays a crucial role in the development of the fungal cleistothecium and the flavor formation of the post-fermented teas with A. montevidensis. • LeuRS plays a key role in the formation of 'golden flowers' in fermented dark tea. • LeuRS influenced the metabolization of flavonoids of fermented dark tea. • LeuRS modulated the metabolization of sugars of fermented dark tea. • LeuRS affected the metabolism of organic acids and fatty acids of fermented dark tea. [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]

This study aimed to optimize the dark tea pile-fermentation conditions through single factor and orthogonal experiments. The optimal conditions consisted of 38 g/100g moisture content of wet green tea, 45 °C of temperature, and 85 g/100g humidity. On this basis, the dynamic changes of flavor components during pile-fermentation were systematically explored by chromatography, mass spectrometry and other methods. During pile-fermentation, changes were observed in the internal ratio of the following various substances, with a decrease in the content of tea polyphenols (especially ester catechins), amino acids, and soluble sugars, as well as the formation of new substances of theaflavin, thearubigin, and theabrownin, thereby affecting the brew color, flavor, and taste of dark tea. After pile-fermentation, a gradual decrease was detected in the floral aroma components, such as phenethyl alcohol, linalool oxides, and linalool, in contrast to a gradual increase in the content of aged fragrance components, including methoxy aromatic compounds like 1,2,3- and 1,2,4-trimethoxybenzene. During pile-fermentation, the important aroma substances such as benzaldehyde, β-ionone, and dihydroactinidiolide showed an uptrend first and then a downtrend. The changes in the ratio of these substances contributed jointly to the aroma quality formation of dark tea. • Dark tea pile-fermentation conditions are optimized by different experiments. • Dynamic changes of quality components during pile-fermentation are explored. • Aged fragrance formation conditions in pile-fermentation are revealed. • Different flavor types of dark tea are obtained by controlling the time point. [ABSTRACT FROM AUTHOR]