Landscapes of the main components, metabolic and microbial signatures, and their correlations during pile-fermentation of Tibetan tea

Microbial fermentation, a key step in Tibetan tea production, plays a pivotal role in forming the tea's unique quality. In our study, we mapped out the landscapes of major components, metabolomic signatures, and microbial features of Tibetan tea using component content determination, untargeted metabolomic analysis, and ITS and 16S rRNA sequencing. The results reveal that theabrownin content demonstrated a consistent growth trend post-fermentation, increasing from 41.96 ± 1.64 mg/g to 68.75 ± 2.58 mg/g. However, the content of epigallocatechin gallate (EGCG) significantly dwindled from 80.02 ± 0.51 mg/g to 8.12 ± 0.07 mg/g. Additionally, 518 metabolites were pinpointed as pivotal to the metabolic variation induced by microbial fermentation. The microbiome analysis exhibited a considerable shift in the microbiota signature, with Aspergillus emerging as the dominant microorganism. To conclude, these findings offer novel perspectives for enhancing the quality of Tibetan tea and abbreviating fermentation time through the regulation of microbiota structure.

Changes in sensory characteristics, chemical composition and microbial succession during fermentation of ancient plants Pu-erh tea

"Ancient tea plants" are defined as tea trees > 100 years old, or with a trunk diameter > 25 cm; their leaves are manufactured to high - quality, valuable ancient plants pu-erh tea (APPT). In this study, a fermentation of APPT were developed, and outstanding sweetness of APPT infusion was observed. During fermentation, the content of soluble sugars, theabrownins (p < 0.05), as well as 41 metabolites were increased [Variable importance in projection (VIP) > 1.0; p < 0.05 and Fold-change (FC) FC > 2]; While relative levels of 72 metabolites were decreased (VIP > 1.0, p < 0.05 and FC < 0.5. Staphylococcus, Achromobacter, Sphingomonas, Thermomyces, Rasamsonia, Blastobotrys, Aspergillus and Cladosporium were identified as dominant genera, and their relative levels were correlated with contents of characteristic components (p < 0.05). Together, changes in sensory characteristics, chemical composition and microbial succession during APPT fermentation were investigated, and advanced the formation mechanism of its unique quality.

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.

Changes of fungal community and non-volatile metabolites during pile-fermentation of dark green tea

Fungal community and non-volatile metabolites changes during the pile-fermentation are key factors to organoleptic qualities of dark green tea. However, the correlation between fungal succession and non-volatile compounds has never been satisfactorily explained. The purpose of the present study was to investigate fungal succession and its correlation with flavor compounds by multi-omics. Illumina Miseq sequencing of ITS1 region was conducted to analyze the fungal succession, a total of 78 OTUs which consisted of one phyla, nine classes, 15 orders, 26 families, 37 genera were identified, with Ascomycota as dominant phyla. Cluster analysis and non-metric multidimensional scaling of samples demonstrated the distribution of OTUs in multi-dimensional space, the pile-fermentation process of dark green tea can be divided into four periods according to the generated trajectory of fungal population, S0, S1-S3, S4-S5, and S6. Aspergillus is the dominant genus. Penicillium, Cyberlindnera, Debaryomyces, Candida, Thermomyces, Rasamsonia, Thermoascus, and Byssochlamys appear in different periods. three alkaloids, seven catechins, nine amino acids, five organic acids, five flavones and flavonoid glycosides were identified by UPLC-QTOF-MS/MS, and the contents were all decreasing. Caffeine, EGC, EGCG, L-theanine, kaempferitrin, L-phenylalanine, gallic acid, and myricetin-3-O-galactoside are important ingredients which contribute to the flavor of dark green tea. This study demonstrated the fungal succession, non-volatile flavor compounds and their relationships during pile-fermentation of dark green tea, and provides new insights into evaluating pivotal role of fungal succession in the manufacturing process of dark green tea.

Aged fragrance formed during the post-fermentation process of dark tea at an industrial scale

Although aged fragrance is the most outstanding quality characteristic of dark tea, its formation still is not much clear. Thus, the volatiles of Qingzhuan tea (QZT) during the whole post-fermentation process were investigated at an industrial scale. The results showed that most of volatiles increased during pile-fermentation of QZT and weakened during aging storage, but some new volatiles were produced through aging storage. Hexanal, (E)-2-hexenal, (E)-2-decenal, 2,6,6-trimethyl-1-cyclohexene-1-carboxaldehyde, heptanal, (E)-2-octenal, (R)-5,6,7,7a-tetrahydro-4,4,7a-trimethyl-2(4H)-benzofuranone, ionone, 2-heptanone, 3-ethyl-4-methyl-1H-pyrrole-2,5-dione, (R,S)-5-ethyl-6-methyl-3-hepten-2-one, cis-5-ethenyltetrahydro-5-trimethyl-2-furanmethanol, and linalool generated by pile-fermentation should be the basic volatiles of aged fragrance in QZT, and 4-(2,4,4-trimethyl-cyclohexa-1,5-dienyl)-but-3-en-2-one, 6-methyl-5-heptene-2-one, safranal, guaiene, trans-2-(2-propynyloxy)-cyclohexanol, nonanal, and 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2-butanone formed during aging storage should be the transformed volatiles of aged fragrance in QZT, which together constitute the characteristic components of aged fragrance. Notably, 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2-butanone, 6-methyl-5-heptene-2-one, and safranal were selected as the key volatiles of QZT. These results contribute to understand better the formation of agedfragrance in dark tea.

Characterization of thermophilic fungal community associated with pile fermentation of Pu-erh tea

This study aimed to characterize the thermophilic fungi in pile-fermentation process of Pu-erh tea. Physicochemical analyses showed that the high temperature and low pH provided optimal conditions for propagation of fungi. A number of fungi, including Blastobotrys adeninivorans, Thermomyces lanuginosus, Rasamsonia emersonii, Aspergillus fumigatus, Rhizomucor pusillus, Rasamsonia byssochlamydoides, Rasamsonia cylindrospora, Aspergillus tubingensis, Aspergillus niger, Candida tropicalis and Fusarium graminearum were isolated as thermophilic fungi under combination of high temperature and acid culture conditions from Pu-erh tea pile-fermentation. The fungal communities were analyzed by PCR-DGGE. Results revealed that those fungi are closely related to Debaryomyces hansenii, Cladosporium cladosporioides, A. tubingensis, R. emersonii, R. pusillus, A. fumigatus and A. niger, and the last four presented as dominant species in the pile process. These four preponderant thermophilic fungi reached the order of magnitude of 10(7), 10(7), 10(7) and 10(6)copies/g dry tea, respectively, measured by real-time quantitative PCR (q-PCR). The results indicate that the thermophilic fungi play an important role in Pu-erh tea pile fermentation.