Biochemical Components Associated With Microbial Community Shift During the Pile-Fermentation of Primary Dark Tea

Insight into the chemical compositions of Anhua dark teas derived from identical tea materials: A multi-omics, electronic sensory, and microbial sequencing analysis

Anhua dark teas (DTs), including Tianjian tea, Qianliang tea, Hei brick tea, and Fu brick tea, are unique fermented teas from China's Anhua County; yet their chemical composition differences remain unclear. Herein, metabolomics, volatolomics, and electronic sensory assessments were employed to analyze and compare chemical compositions and sensory characteristics of five types of Anhua DTs. All of these teas were derived from identical tea materials. Chemical compositions differed significantly among Anhua DTs, with Tianjian tea remarkable. Long-lasting fermentation and complex processing methods led to transformation of multiple compounds, particularly catechins. Eighteen volatile compounds with OVA > 1 were key aroma contributors in Anhua DTs. Internal transcribed spacer and 16S ribosomal DNA sequencing showed that Eurotium, Pseudomonas, and Bacillus are dominant microorganisms in Anhua DTs. Furthermore, this study unveiled notable differences in chemical compositions between Anhua DTs and five other traditional types of tea. This research enhances our understanding of Anhua DTs processing.

Monitoring the dynamic changes in aroma during the whole processing of Qingzhuan tea at an industrial scale: From fresh leaves to finished tea

Aroma is one of the most outstanding quality characteristics of Qingzhuan tea (QZT), but its formation is still unclear. Thus, the volatile organic compounds (VOCs) during the whole processing of QZT were investigated by headspace solid-phase microextraction/gas chromatography-mass spectrometry. Based on 144 identified VOCs, the results showed that de-enzyming, sun-drying, and piling fermentation were the key processes of QZT aroma formation. Furtherly, 42 differential VOCs (VIP > 1.0 and p < 0.05) and 16 key VOCs (rOAV > 1.0 and/or ROAV > 1.0) were screened. Especially, sulcatone and β-ionone (rOAV > 100 and ROAV > 10) were considered the most important contributors to the aroma of QZT. The metabolisms of key VOCs were mainly involved in oxidative degradation of fatty acids, degradation of carotenoids, and methylation of gallic acid. This study could help to more comprehensively understand the aroma formation in QZT processing at an industrial scale.

Key Factors of Quality Formation in Wuyi Black Tea during Processing Timing

As the most consumed tea in the world, all kinds of black tea are developed from Wuyi black tea. In this study, quality components, regulatory gene expression, and key enzyme activity during the processing were analyzed to illustrate the taste formation of WBT. Withering mainly affected the content of amino acids, while catechins and tea pigments were most influenced by rolling and the pre-metaphase of fermentation. Notably, regulatory gene expression was significantly down-regulated after withering except for polyphenoloxidase1, polyphenoloxidase2, leucoanthocyanidin dioxygenase, chalcone isomerase, and flavonoid 3', 5'-hydroxylase. Co-expression of flavonoid pathway genes confirmed similar expression patterns of these genes in the same metabolic pathway. Interestingly, rolling and fermentation anaphase had a great effect on polyphenol oxidase, and fermentation pre-metaphase had the greatest effect on cellulase. Since gene regulation mainly occurs before picking, the influence of chemical reaction was greater during processing. It was speculated that polyphenol oxidase and cellulase, which promoted the transformation of quality components, were the key factors in the quality formation of WBT. The above results provide theoretical basis for the processing of WBT and the reference for producing high-quality black tea.

Deciphering the underlying core microorganisms and the marker compounds of Liupao tea during the pile-fermentation process

BACKGROUND

Pile fermentation is one of the key steps in developing the Liupao tea (LBT) quality and unique characteristics. The complex biochemical profile of LBT results from microorganisms present during the pile-fermentation process. However, the critical underlying microorganisms and the marker compounds still need to be determined.

RESULTS

Staphylococcus, Brevibacterium, Kocuria, Aspergillus, and Blastobotrys were the common dominant microorganisms at the end of the pile fermentation of LBT. Staphylococcus, Aspergillus, Blastobotrys, and nine other genera carried by raw tea are the core microorganisms in the LBT during pile fermentation. A total of 29 critical compounds contributed to the metabolic changes caused by the processing of LBT. Of these, gallic acid, adenine, hypoxanthine, uridine, betaine, 3,4-dihydroxybenzaldehyde, and α-linolenic acid could be characterized as potential marker compounds. Correlation analysis showed that the core microorganisms, including Sphingomonas, Staphylococcus, Kocuria, Aureobasidium, Blastobotrys, Debaryomyce, and Trichomonascus, were closely related to major chemical components and differential compounds. Moreover, the mutually promoting Staphylococcus, Kocuria, Blastobotrys, and Trichomonascus were correlated with the enrichment of marker compounds. Integrated molecular networking and metabolic pathways revealed relevant compounds and enzymes that possibly affect the enrichment of marker compounds.

CONCLUSION

This study analyzed the LBT fermentation samples by omics analysis to reveal the stable microbial community structure, critical microorganisms, and markers compounds affecting the quality of LBT, which contributes to a better understanding of pile fermentation of LBT and the fermentation theory of dark tea. © 2023 Society of Chemical Industry.

Degradation of Cell Wall Polysaccharides during Traditional and Tank Fermentation of Chinese Liupao Tea

The increase of polysaccharides in the dark tea pile process is thought to be connected to the cell wall polysaccharides' breakdown. However, the relationship between tea polysaccharides (TPSs) and tea cell wall polysaccharides has not been further explored. In this study, the structural changes in the cell wall polysaccharides [e.g., cellulose, hemicellulose (HC), and pectin] in Liupao tea were characterized before and after traditional fermentation and tank fermentation. Additionally, the degradation mechanism of tea cell wall polysaccharides during fermentation was assessed. The results showed that cellulose crystallinity decreased by 11.9-49.6% after fermentation. The molar ratio of monosaccharides, such as arabinose, rhamnose, and glucose in HC, was significantly reduced, and the molecular weight decreased. The esterification degree and linearity of water-soluble pectin (WSP) were reduced. TPS content increases during pile fermentation, which may be due to HC degradation and the increase in WSP caused by cell wall structure damage. Microorganisms were shown to be closely associated with the degradation of cell wall polysaccharides during fermentation according to correlation analyses. Traditional fermentation had a greater effect on the cellulose structure, while tank fermentation had a more noticeable impact on HC and WSP.

Exploring core functional fungi driving the metabolic conversion in the industrial pile fermentation of Qingzhuan tea

The distinct sensory quality of Qingzhuan tea is mainly formed in pile fermentation by a group of functional microorganisms but the core functional ones was poorly characterized. Therefore, this study investigated the dynamic changes in the fungal community and metabolic profile by integrating microbiomics and metabolomics, and explored the core functional fungi driving the metabolic conversion in the industrial pile fermentation of Qingzhuan tea. Indicated by microbiomics analysis, Aspergillus dominated the entire pile-fermentation process, while Thermoascus, Rasamsonia, and Cylindrium successively abounded in the different stages of the pile fermentation. A total of 50 differentially changed metabolites were identified, with the hydrolysis of galloyl/polymeric catechins, biosynthesis of theabrownins, oxidation of catechins, N-ethyl-2-pyrrolidinone substitution of catechins, and deglycosylation of flavonoid glucosides. Nine fungal genera were identified as core functional fungi, in which Aspergillus linked to the hydrolysis of polymeric catechins and insoluble polysaccharides as well as biosynthesis of theabrownins, while Thermoascus participated in the biosynthesis of theabrownins, deglycosylation of flavonoid glucosides, and N-ethyl-2-pyrrolidinone substitution of catechins. These findings would advance our understanding of the quality formation of Qingzhuan tea and provide a benchmark for precise inoculation for its quality improvement.

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.

Chemical components of Fu brick tea and its potential preventive effects on metabolic syndrome

As living standards advance, an escalating emphasis is placed on health, particularly in relation to prevalent chronic metabolic disorders. It is necessary to explore safe and effective functional foods or drugs. Fu brick tea (FBT) is a kind of dark tea fermented by fungi. The extracts are rich in compounds that can effectively relieve metabolic diseases such as hyperglycemia and hyperlipidemia, protect the liver, improve human immunity, enhance antioxidant activity, and regulate intestinal flora. This paper summarizes the biological activities and mechanisms of the extracts, polysaccharides, and small molecular compounds of FBT, which provides a certain theoretical basis for the rational, systematic, comprehensive development and utilization of the FBT resources. It is expected to develop and apply these active substances in health care products and natural medicines and provide more beneficial and diversified FBT products for human beings.

Dynamic Changes in the Microbial Community and Metabolite Profile during the Pile Fermentation Process of Fuzhuan Brick Tea

The pile fermentation process of Fuzhuan brick tea is unique in that it involves preheating without the use of starter cultures. The detailed metabolite changes and their drivers during this procedure are not known. Characterizing these unknown changes that occur in the metabolites and microbes during pile fermentation of Fuzhuan brick tea is important for industrial modernization of this traditional fermented food. Using microbial DNA amplicon sequencing, mass spectrometry-based untargeted metabolomics, and feature-based molecular networking, we herein reveal that significant changes in the microbial community occur before changes in the metabolite profile. These changes were characterized by a decrease in Klebsiella and Aspergillus, alongside an increase in Bacillus and Eurotium. The decrease in lysophosphatidylcholines, unsaturated fatty acids, and some astringent flavan-3-ols and bitter amino acids, as well as the increase in some less astringent flavan-3-ols and sweet or umami amino acids, contributed importantly to the overall changes observed in the metabolite profile. The majority of these changes was caused by bacterial metabolism and the corresponding heat generated by it.

Metabolomic Analysis of Fermented Tibetan Tea Using Bacillus circulans and Their Biological Activity on Mice via the Intestine-Hepatic Axis

The use of Bacillus circulans as the sole starter provides better process control compared to natural fermentation. However, the chemical composition of fermented Tibetan tea by B. circulans and its regulatory effects on the intestine-liver axis has not been reported. For this purpose, a high-resolution liquid chromatography tandem mass spectrometry metabolomics approach was performed. The effects of fermented Tibetan tea on the intestine-liver axis of mice were also evaluated. Untargeted metabolomics analysis showed that the contents of catechin derivatives, flavonoids, phenolic acids, and terpenoids increased by 0.3, 2.38, 2.65, and 3.36%, respectively, compared with those before fermentation. Furthermore, 16S ribosomal RNA sequence analysis revealed that the relative abundance of Lactobacillus spp. in the intestine increased after consumption of fermented tea. Additionally, based on histological and quantitative PCR analyses, fermented Tibetan tea also improved intestinal development and intestinal barrier function in mouse, while increasing the antioxidant capacity of mouse liver. Thus, fermented Tibetan tea could provide beneficial health effects through the intestine-liver axis. These findings have facilitated the study of the chemical composition of Tibetan tea and provided theoretical support for its use as a natural beverage with intestinal probiotic functions.

Exploring the controllability of the Baijiu fermentation process with microbiota orientation

Product quality and stability improvement is important for development of the Baijiu industry. Generally, Baijiu brewing is carried out in a spontaneous fermentation system mediated by microbiota. Thus, complexity and instability are major features. Due to the insufficient understanding of the mechanism for producing Baijiu, the precise control of the fermentation progress has still not been realized, ultimately affecting product quality and stability. The flavor of Baijiu is the most important factor in determining its quality and is formed by microbiota under the driving force of various physicochemical parameters, such as moisture, acidity, and temperature. Therefore, exploring the association among microbiota (core), physicochemical factors (reference) and flavor compounds (target) has become a key point to clarify the formation mechanism for the flavor quality of Baijiu. Daqu fermentation and liquor fermentation are the two major stages of Baijiu brewing. Daqu, distillers' grains, and pit mud, as the most important fermentation substrates of the microbiota respectively, provide a large number of functional microorganisms related to the flavor components. To this end, we reviewed the relevant research progress of microbiota diversity in different fermentation substrates and the interaction mechanisms among microbiota, physicochemical parameters, and flavor components in this paper. Moreover, a research hypothesis of precise control of the Baijiu fermentation process by building fermentation models based on this is proposed. The key point for this idea is the identification of core microbiota closely associated with the formation of key flavor components by multi-omics technology and the acquisition of culturable strains. With this foundation, fermentation models suitable for different brewing environments will be established by constructing synthetic microbiota, designing mathematical models, and determining key fermentation model parameters. The ultimate goal will be to effectively improve the quality and stability of Baijiu products through model regulation.

Tibetan tea reduces obesity brought on by a high-fat diet and modulates gut flora in mice

It has been shown that Tibetan tea (TT) inhibits obesity and controls lipid metabolism. The fundamental processes by which TT prevents obesity are yet entirely unknown. Consequently, this research aimed to ascertain if TT may prevent obesity by modifying the gut flora. Our research demonstrated that TT prevented mice from gaining weight and accumulating fat due to the high-fat diet (HFD), decreased levels of blood total cholesterol (TC), triglycerides (TG), and low-density lipoprotein cholesterol (LDL-C), and raised levels of high-density lipoprotein cholesterol (HDL-C). Adipogenesis-related genes such as acetyl-Coenzyme A carboxylase 1 (ACC1, LOC107476), fatty acid synthase (Fas, LOC14104), sterol regulatory element-binding protein-1c (SREBP-1c, LOC20787), CCAAT/enhancer-binding protein α (C/EBPα, LOC12606), stearoyl-CoA desaturase 1 (SCD1, LOC20249), and peroxisome proliferator-activated receptor γ (PPARγ, LOC19016) had their expression downregulated by lowering the Firmicutes/Bacteroidetes (F/B) ratio and controlling the number of certain gut bacteria. TT also alleviated HFD-induced abnormalities of the gut microbiota. The Muribaculaceae, Lachnospiraceae NK4A136_group, Alistipes, and Odoribacter families were identified as the major beneficial gut microorganisms using Spearman's correlation analysis. Fecal microbiota transplantation (FMT) demonstrated that TT's anti-obesity and gut microbiota-modulating benefits might be transmitted to mice on an HFD, demonstrating that one of TT's targets for preventing obesity is the gut microbiota. TT also increased the amount of short-chain fatty acids (SCFAs) in the feces, including acetic, propionic, and butyric acids. These results indicate the possible development of TT as a prebiotic to combat obesity and associated disorders. These results suggest that TT may act as a prebiotic against obesity and its associated diseases.

Metabolomic and transcriptomic analyses reveal comparisons against liquid-state fermentation of primary dark tea, green tea and white tea by Aspergillus cristatus

Liquid-state fermentation (LSF) of tea leaves is a promising way to obtain tea-based nutraceutical products rich in various bioactive compounds. In the study, the changes of bioactive compounds, tea pigments and complex metabolites from LSF of primary dark tea, green tea and white tea infusions with Aspergillus cristatus were determined. Chemical analyses revealed that soluble sugars, monosaccharide composition, total polyphenols, total flavonoids, free amino acids, soluble proteins and tea pigments were changed in different ways. An untargeted metabolomic analysis and ribonucleic acid sequencing (RNA-seq) based transcriptomic analysis were performed to investigate the metabolic differentiation and clarify the key differentially expressed genes (DEGs, fold change >2 and p < 0.05), showing that amino acid metabolism, carbohydrate metabolism and lipid metabolism were the most enriched pathways during A. cristatus fermentation of primary dark tea, green tea and white tea infusions. In addition, glycerophospholipid metabolism, linoleic acid metabolism and phenylalanine metabolism were greatly accumulated in the fermentation of primary dark tea and white tea infusions; Pyruvate metabolism, glycolysis/gluconeogenesis, fatty acid degradation, tyrosine metabolism, phenylalanine, tyrosine and tryptophan biosynthesis and valine and leucine, isoleucine degradation were greatly accumulated in the fermentation of primary dark tea and green tea infusions; Starch and sucrose metabolism was greatly accumulated in the fermentation of green tea and white tea infusions; Galactose metabolism was significantly enhanced in the fermentation of primary dark tea infusion; Amino sugar and nucleotide sugar metabolism, sphingolipid metabolism and alanine, aspartate and glutamate metabolism were significantly enhanced in the fermentation of green tea infusion. Besides, some other pathways involving aminobenzoate degradation, biosynthesis of cofactors, pyrimidine metabolism, benzoxazinoid biosynthesis and phenazine biosynthesis, tropane, piperidine and pyridine alkaloid biosynthesis and flavone and flavonol biosynthesis also differed from each other. These findings support that A. cristatus plays a vital role in the biochemical and genetic regulation of metabolite profile, and could be considered a potential prospect for better use of A. cristatus on different kinds of tea materials.

Diversifying the Flavor of Black Rice Wines through Three Different Regional Xiaoqus in China and Unraveling Their Core Functional Microorganisms

The flavor of black rice wine (BRW) can be diversified by the Xiaoqus, from different regions; however, the functional microbiota that contributes to its flavor remains unclear. Accordingly, this study selected three regional Xiaoqus from Sichuan Dazhu (Q1), Jiangxi Yingtan (Q2), and Hubei Fangxian (Q3) as starters to investigate flavor compounds and microbial communities during BRW brewing. Results indicated that altogether 61 flavor substances were identified, 16 of which were common characteristic flavor compounds (odor activity value > 0.1). Each BRW possessed unique characteristic flavor compounds. O2PLS and Spearman's correlation analysis determined that characteristic flavor compounds of BRW were mainly produced by Saccharomyces cerevisiae, non-Saccharomyces yeasts, and lactic acid bacteria, with the common core functional strains being Wickerhamomyces and Pediococcus, and with their unique core functional strain likely causing a unique characteristic flavor. This study could promote the high-quality development of the black rice wine industry.

Microbial Diversity and Characteristic Quality Formation of Qingzhuan Tea as Revealed by Metagenomic and Metabolomic Analysis during Pile Fermentation

In order to analyze the changes in the microbial community structure during the pile fermentation of Qingzhuan tea and their correlation with the formation of quality compounds in Qingzhuan tea, this study carried out metagenomic and metabolomic analyses of tea samples during the fermentation process of Qingzhuan tea. The changes in the expression and abundance of microorganisms during the pile fermentation were investigated through metagenomic assays. During the processing of Qingzhuan tea, there is a transition from a bacterial dominated ecosystem to an ecosystem enriched with fungi. The correlation analyses of metagenomics and metabolomics showed that amino acids and polyphenol metabolites with relatively simple structures exhibited a significant negative correlation with target microorganisms, while the structurally complicated B-ring dihydroxy puerin, B-ring trihydroxy galloyl puerlin, and other compounds showed a significant positive correlation with target microorganisms. Aspergillus niger, Aspergillus glaucus, Penicillium in the Aspergillaceae family, and Talaromyces and Rasamsonia emersonii in Trichocomaceae were the key microorganisms involved in the formation of the characteristic qualities of Qingzhuan tea.

Effects and Stress-Relieving Mechanisms of Dark Tea Polysaccharide in Human HaCaT Keratinocytes and SZ95 Sebocytes

A new skincare application scenario for dark tea, a unique and post-fermented tea popular in the health food industry, was developed in this paper. The effects of dark tea polysaccharide (DTP) on stress-induced skin problems and its mechanism of action were investigated by modeling cortisone-induced stress injury in human HaCaT keratinocytes and SZ95 sebaceous gland cells. The results showed a reduced cortisol conversion induced by cortisone under the action of DTP with a concentration of 200 μg/mL, probably by inhibiting the expression of the HSD11B1 enzyme. DTP was also able to suppress the cortisone-induced elevation of lipid levels in SZ95 sebocytes at this concentration. In addition, the composition and structure of DTP were verified by ultrafiltration, ultraviolet-visible spectrophotometry (UV-VIS), high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and infrared spectroscopy. In brief, DTP has a unique and significant stress-relieving effect, which provides new ideas for the development of new ingredients for the skin care industry.

Dynamical changes of tea metabolites fermented by Aspergillus cristatus, Aspergillus neoniger and mixed fungi: A temporal clustering strategy for untargeted metabolomics

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.

Tea (Camellia sinensis (L.) Kuntze) as an emerging source of protein and bioactive peptides: A narrative review

Tea residues represent one of the major agricultural wastes that are generated after the processing of tea. They account for 21-28% of crude protein and are often discarded without the extraction of valuable proteins. Due to various bioactivity and functional properties, tea proteins are an excellent alternative to other plant-based proteins for usage as food supplements at a higher dosage. Moreover, their good gelation capacity is ideal for the manufacturing of dairy products, jellies, condensation protein, gelatin gel, bread, etc. The current study is the first to comprehend various tea protein extraction methods and their amino acid profile. The preparation of tea protein bioactive peptides and hydrolysates are summarized. Several functional properties (solubility, foaming capacity, emulsification, water/oil absorption capacity) and bioactivities (antioxidant, antihypertensive, antidiabetic) of tea proteins are emphasized.

Improving flavor of summer Keemun black tea by solid-state fermentation using Cordyceps militaris revealed by LC/MS-based metabolomics and GC/MS analysis

Cordyceps militaris (C. militaris) has been approved and widely used in healthy food. The present study aimed to improve the flavor of summer Keemun black tea (KBT) using C. militaris solid-state fermentation. Combined with sensory evaluation, the volatile and non-volatile components of solid-state fermentation of KBT (SSF-KBT) and KBT were analyzed. The results showed that after the solid-state fermentation, the contents of total polyphenol, total flavonoid, and total free amino acids were significantly reduced. Further non-targeted metabolomics analysis revealed that the contents of non-galloylated catechins and d-mannitol increased, while the galloylated catechins and flavonoid glycosides decreased as did the bitterness and astringency of KBT. Dihydro-β-ionone and β-ionone (OAV = 59321.97 and 8154.17) were the aroma-active compounds imparting woody and floral odors in SSF-KBT, respectively. Current study provides a new avenue to develop summer-autumn KBT.

A comprehensive review of polyphenol oxidase in tea (Camellia sinensis): Physiological characteristics, oxidation manufacturing, and biosynthesis of functional constituents

Polyphenol oxidase (PPO) is a metalloenzyme with a type III copper core that is abundant in nature. As one of the most essential enzymes in the tea plant (Camellia sinensis), the further regulation of PPO is critical for enhancing defensive responses, cultivating high-quality germplasm resources of tea plants, and producing tea products that are both functional and sensory qualities. Due to their physiological and pharmacological values, the constituents from the oxidative polymerization of PPO in tea manufacturing may serve as functional foods to prevent and treat chronic non-communicable diseases. However, current knowledge of the utilization of PPO in the tea industry is only available from scattered sources, and a more comprehensive study is required to reveal the relationship between PPO and tea obviously. A more comprehensive review of the role of PPO in tea was reported for the first time, as its classification, catalytic mechanism, and utilization in modulating tea flavors, compositions, and nutrition, along with the relationships between PPO-mediated enzymatic reactions and the formation of functional constituents in tea, and the techniques for the modification and application of PPO based on modern enzymology and synthetic biology are summarized and suggested in this article.

Headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS) and odor activity value (OAV) to reveal the flavor characteristics of ripened Pu-erh tea by co-fermentation

Introduction

Pu-erh tea is a geographical indication product of China. The characteristic flavor compounds produced during the fermentation of ripened Pu-erh tea have an important impact on its quality.

Methods

Headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS) and odor activity value (OAV) is used for flavor analysis.

Results

A total of 135 volatile compounds were annotated, of which the highest content was alcohols (54.26%), followed by esters (16.73%), and methoxybenzenes (12.69%). Alcohols in ripened Pu-erh tea mainly contribute flower and fruit sweet flavors, while methoxybenzenes mainly contribute musty and stale flavors. The ripened Pu-erh tea fermented by Saccharomyces: Rhizopus: Aspergillus niger mixed in the ratio of 1:1:1 presented the remarkable flavor characteristics of flower and fruit sweet flavor, and having better coordination with musty and stale flavor.

Discussion

This study demonstrated the content changes of ripened Pu-erh tea’s flavor compounds in the fermentation process, and revealed the optimal fermentation time. This will be helpful to further understand the formation mechanism of the characteristic flavor of ripened Pu-erh tea and guide the optimization of the fermentation process of ripened Pu-erh tea.

Analysis of bacterial community structure of Fuzhuan tea with different processing techniques

The composition and diversity of microbial communities are of considerable significance to the quality development of Camellia sinensis (Fuzhuan tea). In this study, we examined differences in the bacterial community structures of loose, lightly-pressed, hand-made, and machine-pressed Fuzhuan teas and raw dark tea. We observed notable differences in the bacterial communities of the five groups, where there were only 51 consensus sequences. ASV/OTU Venn diagram, Chao1, Ace, Simpson indices, and dilution curve analyses consistently revealed that machine-pressed tea exhibited the highest bacterial diversity. Taxonomically, Actinobacteria, Firmicutes, Proteobacteria, and Cyanobacteria were the dominant bacterial phyla in each group, whereas Corynebacterium, Methylobacterium, and Bifidobacterium were the dominant genera. Our findings revealed significant differences in the bacterial community structures of different Fuzhuan tea products derived from the same raw material, with bacterial diversity rising with increased product compaction.

Dynamic Analysis of the Bacterial Community and Determination of Antioxidant Capacity during the Fermentation of Sour Tea

The imbalance of the redox state caused by extra reactive oxygen species is closely related to many diseases. Therefore, it is necessary for people to ingest antioxidants through food. The safety of some synthetic antioxidants has been questioned. In this context, it is worth exploring natural and safe antioxidants from biological sources. Tea has good antioxidant activity, and the antioxidant activity of fermented sour tea is better than that of other types. It is necessary to clarify the antioxidant capacity of sour tea during fermentation, as well as the microbial community and its sources. Nonculture and culture-dependent methods were adopted to track the changes in the microbial population and community structure during the fermentation of sour tea. Sequence analysis of 16S rRNA gene amplification revealed significant differences in community complexity and structure at different fermentation times. The highest proportion of operational taxonomic units (OTU s) in all samples was Latilactobacillus, which was determined to be Lactiplantibacillus plantarum by further analysis. The second highest proportion of OTUs was Enterobacter. With the fermentation of sour tea, the antioxidant capacity increased, and all isolated Lb. plantarum had good DPPH clearance rates. Our findings suggest that Lb. plantarum plays a crucial role in the fermentation process of sour tea. The possibility of discovering new antioxidants was provided by the determination of the antioxidant capacity and bacterial community during the fermentation of sour tea.

Untargeted metabolomics and quantification analysis reveal the shift of chemical constituents between instant dark teas individually liquid-state fermented by Aspergillus cristatus, Aspergillus niger, and Aspergillus tubingensis

Instant dark teas (IDTs) were individually liquid-state fermented using the fungi Aspergillus cristatus, Aspergillus niger, and Aspergillus tubingensis. To understand how the chemical constituents of IDTs were affected by the fungi, samples were collected and measured by liquid chromatography-tandem mass-tandem mass spectrometry (LC-MS/MS). Untargeted metabolomics analysis revealed that 1,380 chemical constituents were identified in positive and negative ion modes, and 858 kinds of chemical components were differential metabolites. Through cluster analysis, IDTs were different from the blank control, and their chemical constituents mostly included carboxylic acids and their derivatives, flavonoids, organooxygen compounds, and fatty acyls. And the metabolites of IDTs fermented by A. niger and A. tubingensis had a high degree of similarity and were classified into one category, which showed that the fungus used to ferment is critical to the formation of certain qualities of IDTs. The biosynthesis of flavonoids and phenylpropanoid, which involved nine different metabolites such as p-coumarate, p-coumaroyl-CoA, caffeate, ferulate, naringenin, kaempferol, leucocyanidin, cyanidin, and (-)-epicatechin, were significant pathways influencing the quality formation of IDTs. Quantification analysis indicated that the A. tubingensis fermented-IDT had the highest content of theaflavin, theabrownin, and caffeine, while the A. cristatus fermented-IDT had the lowest content of theabrownin, and caffeine. Overall, the results provided new insights into the relationship between the quality formation of IDTs and the microorganisms used in liquid-state fermentation.

Are Fermented Foods Effective against Inflammatory Diseases?

Fermented foods have been used over the centuries in various parts of the world. These foods are rich in nutrients and are produced naturally using various biological tools like bacteria and fungi. Fermentation of edible foods has been rooted in ancient cultures to keep food for preservation and storage for a long period of time with desired or enhanced nutritional values. Inflammatory diseases like rheumatoid arthritis, osteoarthritis, and chronic inflammatory pain are chronic disorders that are difficult to treat, and current treatments for these disorders fail due to various adverse effects of prescribed medications over a long period of time. Fermented foods containing probiotic bacteria and fungi can enhance the immune system, improve gastrointestinal health, and lower the risk of developing various inflammatory diseases. Foods prepared from vegetables by fermentation, like kimchi, sauerkraut, soy-based foods, or turmeric, lack proper clinical and translational experimental studies. The current review has focused on the effectiveness of various fermented foods or drinks used over centuries against inflammation, arthritis, and oxidative stress. We also described potential limitations on the efficacies or usages of these fermented products to provide an overarching picture of the research field.

Development of Candida autochthonous starter for cigar fermentation via dissecting the microbiome

Introduction

The main goal of tobacco fermentation technology is to minimize the alkaloid content while improving flavor substance content.

Methods

This study revealed the microbial community structure and their metabolic functions during cigar leaf fermentation by high-throughput sequencing and correlation analysis, and evaluated the fermentation performance of functional microbes based on in vitro isolation and bioaugmentation fermentation.

Results

The relative abundance of Staphylococcus and Aspergillus increased first but then decreased during the fermentation, and would occupy the dominant position of bacterial and fungal communities, respectively, on the 21st day. Correlation analysis predicted that Aspergillus, Staphylococcus and Filobasidium could contribute to the formation of saccharide compounds, Bacillus might have degradation effects on nitrogenous substances. In particular, Candida, as a co-occurring taxa and biomarker in the later stage of fermentation, could not only degrade nitrogenous substrates and synthesize flavor substances, but also contribute to maintaining the stability of microbial community. Moreover, based on in vitro isolation and bioaugmentation inoculation, it was found that Candida parapsilosis and Candida metapsilosis could significantly reduce the alkaloids content and increase the content of flavor components in tobacco leaves.

Discussion

This study found and validated the critical role of Candida in the fermentation of cigar tobacco leaves through high-throughput sequencing and bioaugmentation inoculation, which would help guide the development of microbial starters and directional regulation of cigar tobacco quality.

Identification of characteristic aroma and bacteria related to aroma evolution during long-term storage of compressed white tea

Compressed white tea (CWT) is a reprocessed tea of white tea. Long-term storage has greatly changed its aroma characteristics, but the material basis and transformation mechanism of its unique aroma are still unclear. In this study, flavor wheel, headspace gas chromatography ion mobility spectroscopy, chemometrics, and microbiomics were applied to study the flavor evolution and important aroma components during long-term storage of CWT, and core functional bacteria were screened. During long-term storage, the aroma of CWT gradually changed from sweet, fruity and floral to stale flavor, woody and herbal. A total of 56 volatile organic compounds (VOCs) were identified, 54 of which were significantly differences during storage. The alcohols content was the highest during 1-5 years of storage, the esters content was the highest during 7-13 years of storage, and the aldehydes content was the highest during 16 years of storage. Twenty-nine VOCs were identified as important aroma components, which were significantly correlated with 6 aroma sub-attributes (P < 0.05). The functional prediction of bacterial community reminded that bacterial community could participate in the transformation of VOCs during storage of CWT. Twenty-four core functional bacteria were screened, which were significantly associated with 29 VOCs. Finally, 23 characteristic differential VOCs were excavated, which could be used to identify CWT in different storage years. Taken together, these findings provided new insights into the changes in aroma characteristics during storage of CWT and increased the understanding of the mechanism of characteristic aroma formation during storage.

Effects of Preharvest Shading on Dynamic Changes in Metabolites, Gene Expression, and Enzyme Activity of Three Tea Types during Processing

Preharvest shading significantly influences tea flavor. However, little attention has been given to the mechanism of shading on metabolites, genes, and enzymes in the processing of different tea types. Our study identified 1028 nonvolatile metabolites covering 10 subclasses using a widely targeted metabolome. The results show that shading had a greater effect on the compositions of amino acids, flavonoids, and theaflavins in tea leaves. The combined transcriptomics and enzyme activity analysis results indicate that the upregulated expression of asparagine, aspartate, and tryptophan synthesis genes and proteolytic enzymes promoted the accumulation of amino acids. The downregulated enzyme genes resulted in the reduction of nongalloylated catechins and flavonoid glycosides. Simultaneously, the accumulation of TFs in shaded tea was due to the enhanced enzymatic activities of polyphenol oxidase and peroxidase during processing. Theaflavin-3-3'-di-O-gallate was also significantly positively correlated with the antioxidant and hypoglycemic activities of shaded tea. The results contribute to a better understanding of how preharvest treatments influence summer tea quality.

Regulation of fungal community and the quality formation and safety control of Pu-erh tea

Pu-erh tea belongs to dark tea among six major teas in China. As an important kind of post-fermented tea with complex microbial composition, Pu-erh tea is highly praised by many consumers owing to its unique and rich flavor and taste. In recent years, Pu-erh tea has exhibited various physiological activities to prevent and treat metabolic diseases. This review focuses on the fungi in Pu-erh tea and introduces the sources, types, and functions of fungi in Pu-erh tea, as well as the influence on the quality of Pu-erh tea and potential safety risks. During the process of fermentation and aging of Pu-erh tea, fungi contribute to complex chemical changes in bioactive components of tea. Therefore, we examine the important role that fungi play in the quality formation of Pu-erh tea. The associations among the microbial composition, chemicals excreted, and potential food hazards are discussed during the pile-fermentation of Pu-erh tea. The quality of Pu-erh tea has exhibited profound changes during the process of pile-fermentation, including color, aroma, taste, and the bottom of the leaves, which are inseparable from the fungus in the pile-fermentation of Pu-erh tea. Specifically, the application prospects of various detection methods of mycotoxins in assessing the safety of Pu-erh tea are proposed. This review aims to fully understand the importance of fungi in the production of Pu-erh tea and further provides new insights into subtly regulating the piling process to improve the nutritional properties and guarantee the safety of Pu-erh tea.

Microbial community succession in the fermentation of Qingzhuan tea at various temperatures and their correlations with the quality formation

With the aim to reveal the microbial community succession at various temperatures in the fermentation of Qingzhuan tea (QZT), the Illumina NovaSeq sequencing was carried out to analyze bacterial and fungal community structure in tea samples collected from the fermentation set at various temperatures, i.e., 25 °C, 30 °C, 37 °C, 45 °C, 55 °C, and room temperature. The results showed that fermentation temperature profoundly affected the microbial community succession in the QZT fermentation. Microbial richness and community diversity decreased along with the increase of fermentation temperature. Despite the differences between microorganisms and their metabolic types among various temperatures, most bacteria and fungi showed positive correlations at the genera level. Klebsiella, Paenibacillus, Cohnella, and Pantoea were confirmed as the main bacterial genera, and Aspergillus and Cyberlindnera were the main fungal genera in QZT fermentation. The microbial genera (i.e. Aspergillus, Rhizomucor, Thermomyces, Ralstonia, Castellaniella, and Vibrio) were positively correlated with fermentation temperature (P < 0.05), while Klebsiella, Paenibacillus, and Aspergillus had good adaptability at different temperatures. Conversely, Pantoea and Cyberlindnera were only suitable for low temperature (≤37 °C) growth, and Thermomyces was only suitable for high temperature (>37 °C) growth. Aspergillus had a significant positive correlation with tea aroma quality (r = 0.64, p < 0.05). This study would help to understand the formation mechanism of QZT from microflora perspective.

Discrimination and characterization of the volatile profiles of five Fu brick teas from different manufacturing regions by using HS–SPME/GC–MS and HS–GC–IMS

Although aroma is one of the most essential factors determining the quality of Fu brick tea (FBT), the aroma profiles of FBTs from different manufacturing areas are rarely investigated. The aroma profiles of FBTs manufactured in five typical provinces of China were comprehensively analyzed on the basis of headspace gas chromatography–ion mobility spectrometry (HS–GC–IMS), headspace solid-phase microextraction gas chromatography–mass spectrometry (HS–SPME–GC–MS), sensory evaluation, odor activity value (OAV), and relative odor activity value (ROAV). HS–GC–IMS and HS–SPME–GC–MS identified 63 and 93 volatile organic compounds (VOCs), respectively. Multivariate statistical analysis indicated that the FBTs from different production regions had remarkably varied aromas. HS–SPME–GC–MS revealed that 27 VOCs (OAV >1) contributed to the overall aroma of the samples, of which 15 key differential compounds can effectively distinguish the aroma profiles of different FBTs. FBT from Shaanxi manifested a strong floral and fruity aroma; that from Hunan had a floral, grassy, and pine-woody aroma; that from Guizhou presented a grassy and herbal aroma; that from Guangxi exhibited a sweet, floral, and minty aroma; and that from Zhejiang possessed various fruit flavors and floral fragrance. OAV analysis identified the biomarkers responsible for the variation in the aroma characteristics of diverse FBTs. These biomarkers included linalool, 6-methyl-5-hepten-2-one, α-ionone, hexanal, and ethyl hexanoate. Sensory evaluation demonstrated that the infusion color and aroma of FBT samples from different provinces also greatly varied. Network correlation analysis revealed that Aspergillus and Eurotium were the crucial microorganisms for the metabolism and formation of VOCs. These findings provide new insight into the VOCs and fragrance features of FBTs produced in different regions of China.

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Volatiles in Fu brick tea from five provinces of China were comprehensively analyzed.

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A total of 63 and 93 VOCs were identified by GC-IMS and GC-MS, respectively.

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Aroma profiles of Fu brick tea from five regions were greatly different.

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15 key volatiles were proposed to discriminate Fu brick tea from different regions.

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The correlations between the key VOCs and fungal community were analyzed.

Succession of Bacterial and Fungal Communities during Fermentation of Medicinal Plants

The fermentation of medicinal plants has been studied very little, as compared to the fermentation of food and beverages. One approach applies fermentation by single bacterial or fungal strains and targets the production of specific compounds or preservation of the fermented material. Spontaneous fermentation by an autochthonous starter community may lead to a more diverse blend of fermentation products because co-occurring microbes may activate the biosynthetic potentials and formation of compounds not produced in single strain approaches. We applied the community approach and studied the fermentation of four medicinal plants (Achillea millefolium, Taraxacum officinale, Mercurialis perennis, and Euphrasia officinalis), according to a standardized pharmaceutical fermentation method. It is based on the spontaneous fermentation by plant-specific bacterial and fungal communities under a distinct temperature regime, with a recurrent cooling during the first week and further fermentation for at least six months. The results revealed both general and plant-specific patterns in the composition and succession of microbial communities during fermentation. Lactic acid bacteria increasingly dominated in all preparations, whereas the fungal communities retained more plant-specific features. Three distinct fermentation phases with characteristic bacterial communities were identified, i.e., early, middle, and late phases. Co-occurrence network analyses revealed the plant-specific features of the microbial communities.

Mycotoxins in Tea ((Camellia sinensis (L.) Kuntze)): Contamination and Dietary Exposure Profiling in the Chinese Population

Tea is popular worldwide with multiple health benefits. It may be contaminated by the accidental introduction of toxigenic fungi during production and storage. The present study focuses on potential mycotoxin contamination in tea and the probable dietary exposure assessments associated with consumption. The contamination levels for 16 mycotoxins in 352 Chinese tea samples were determined by ultra-performance liquid chromatography–tandem mass spectrometry. Average concentrations of almost all mycotoxins in tea samples were below the established regulations, except for ochratoxin A in the dark tea samples. A risk assessment was performed for the worst-case scenarios by point evaluation and Monte Carlo assessment model using the obtained mycotoxin levels and the available green, oolong, black, and dark tea consumption data from cities in China. Additionally, we discuss dietary risk through tea consumption as beverages and dietary supplements. In conclusion, there is no dietary risk of exposure to mycotoxins through tea consumption in the Chinese population.

Microbial Succession and Interactions During the Manufacture of Fu Brick Tea

Fu Brick tea is a very popular post-fermented tea that is known for its “golden flower fungus,” Aspergillus cristatus, which becomes the dominant microbe during the maturation process. This study used both culture-dependent methods and high-throughput sequencing to track microbial succession and interactions during the development of the golden flower fungus, a crucial component of the manufacturing process of Fu Brick tea. Among the bacterial communities, Klebsiella and Lactobacillus were consistently cultured from both fresh tea leaves and in post-fermentation Fu Brick tea. Methylobacterium, Pelomonas, and Sphingomonas were dominant genera in fresh tea leaves but declined once fermentation started, while Bacillus, Kluyvera, and Paenibacillus became dominant after piling fermentation. The abundance of A. cristatus increased during the manufacturing process, accounting for over 98% of all fungi present after the golden flower bloom in the Fu Brick tea product. Despite their consistent presence during culture work, network analysis showed Lactobacillus and Klebsiella to be negatively correlated with A. cristatus. Bacillus spp., as expected from culture work, positively correlated with the presence of golden flower fungus. This study provides complete insights about the succession of microbial communities and highlights the importance of co-occurrence microbes with A. cristatus during the manufacturing process of Fu Brick tea.

Mycotoxins along the tea supply chain: A dark side of an ancient and high valued aromatic beverage

ABSTRACTSTea (Camellia sinensis L.) is a high valued beverage worldwide since ancient times; more than three billion cups of tea are consumed each day. Leaf extracts of the plant are used for food preservation, cosmetics, and medicinal purposes. Nevertheless, tea contaminated with mycotoxins poses a serious health threat to humans. Mycotoxin production by tea fungi is induced by a variety of factors, including poor processing methods and environmental factors such as high temperature and humidity. This review summarizes the studies published to date on mycotoxin prevalence, toxicity, the effects of climate change on mycotoxin production, and the methods used to detect and decontaminate tea mycotoxins. While many investigations in this domain have been carried out on the prevalence of aflatoxins and ochratoxins in black, green, pu-erh, and herbal teas, much less information is available on zearalenone, fumonisins, and Alternaria toxins. Mycotoxins in teas were detected using several methods; the most commonly used being the High-Performance Liquid Chromatography (HPLC) with fluorescence detection, followed by HPLC with tandem mass spectrometry, gas chromatography and enzyme-linked immunosorbent assay. Further, mycotoxins decontamination methods for teas included physical, chemical, and biological methods, with physical methods being most prevalent. Finally, research gaps and future directions have also been discussed.

New insights into metabolomics profile generation in fermented tea: the relevance of bacteria and metabolites in Fuzhuan brick tea

BACKGROUND

The contribution of bacteria to fermented tea is not clear and the associated research is relatively limited. To reveal the role of microorganisms in fermented tea processing, the microbial community and metabolites of Fuzhuan brick tea (FBT), a Chinese traditional fermented tea, were revealed via high-throughput sequencing and liquid chromatography-mass spectrometry (LC-MS).

RESULTS

In FBT, bacterial communities had a higher abundance and diversity, Lactococcus and Bacillus were the main bacteria, and Eurotium was the predominant fungus. The predictive metabolic function indicated the pathways of cellular growth, environmental information, genetics and material metabolism of bacterial communities were abundant, whereas the fungal community predictive metabolic function was almost saprotroph. Using LC-MS, 1143 and 536 metabolites were defined in positive and negative ion mode, respectively. There were essential correlations between bacterial populations and metabolites, such that Bacillus was correlated significantly with 44 metabolites (P < 0.05) and Enterococcus was significantly associated with 15 metabolites (P < 0.05). Some of the main active components were significantly correlated with the bacteria, such as Enterococcus, Lactococcus and Carnobacterium.

CONCLUSION

Not only Eurotium, but also the bacteria were involved in the changes of metabolomics profile in fermented FBT. The present study assists in providing new insights into metabolomics profile generation in fermented tea. The present research lays a foundation for controlling the FBT fermentation by artificial inoculation to improve quality. © 2021 Society of Chemical Industry.

Study on taste characteristics and microbial communities in Pingwu Fuzhuan brick tea and the correlation between microbiota composition and chemical metabolites

Pingwu Fuzhuan brick tea (PWT) is considered the "Sichuan western road" border-selling tea. The taste and quality of Fuzhuan brick tea (FBT) is greatly influenced by microorganisms. Considering the dearth of studies on the taste and microbial community of PWT, this study aimed to investigate the taste characteristics using electronic tongue system and microbial community structures using high-throughput sequencing, followed by comparison with FBT from other regions and determining the correlation between microbial communities and chemical compositions. The taste strengths of sweetness, bitterness, umami and astringency in PWT were all at lower level compared to other regions FBT. Regarding microbial diversity, the fungal communities in PWT were distinct from those of other regions FBT in terms of taxonomic composition and abundance. Unclassified_k_Fungi and Aspergillus were the most dominant fungal genera in PWT. Candidatus_Microthrix, norank_f_Saprospiraceae, and norank_c_C10-SB1A were dominant bacterial genera in PWT, only distinct from those in Hunan FBT (HNT). Principal component analysis results showed that fungal or bacterial community structures of PWT and other regions FBT were distinctly different. Correlation analysis revealed important links between the top 50 microbial populations and metabolites.

SUPPLEMENTARY INFORMATION

The online version of this article contains supplementary material available at (10.1007/s13197-021-04976-y).

A critical review of Fuzhuan brick tea: processing, chemical constituents, health benefits and potential risk

Fuzhuan brick tea (FBT) is a traditional popular beverage in the border regions of China. Nowadays, FBT has been attracted great attention due to its uniquely flavor and various health-promoting functions. An increasing number of efforts have been devoted to the studies on health benefits and chemistry of FBT over the last decades. However, FBT was still received much less attention than green tea, oolong tea and black tea. Therefore, it is necessary to review the current encouraging findings about processing, microorganisms, chemical constituents, health benefits and potential risk of FBT. The fungus fermentation is the key stage for processing of FBT, which is involved in a complex and unique microbial fermentation process. The fungal community in FBT is mainly dominated by "golden flower" fungi, which is identified as Aspergillus cristatus. A great diversity of novel compounds is formed and identified after a series of biochemical reactions during the fermentation process of FBT. FBT shows various biological activities, such as antioxidant, anti-inflammatory, anti-obesity, anti-bacterial, and anti-tumor activities. Furthermore, the potential risk of FBT was also discussed. It is expected that this review could be useful for stimulating further research of FBT.

Determination and Comprehensive Risk Assessment of Dietary Exposure to Ochratoxin A on Fermented Teas

A specialized method for ochratoxin A (OTA) determination on fermented teas was developed and validated using ultraperformance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). Methodology results showed that recovery, relative standard deviation, accuracy, and precision were qualified. The limits of detection and quantification were 0.32 and 0.96 μg/kg, respectively. Two of 158 collected samples were screened for OTA contamination. Comprehensive risk assessment based on OTA contaminations of this study and other peer-reviewed publications was performed. The highest hazard quotient (HQ) value (8.86 × 10^-2) and the highest 1/MoE value (8.61 × 10^-5) in probabilistic assessment were equally below the recommended non-neoplastic and neoplastic thresholds, indicating no health risks. However, the HQ and 1/MoE values of the 95th percentiles in 20-39 and ≥50 years of age were close to thresholds of 1.0 and 1.0 × 10^-4, respectively. Under the extreme case, there were only a few scenarios (e.g., 40-49 years of age) of HQ values below the non-neoplastic threshold, but the 1/MoE value of each group exceeded the neoplastic threshold. This is the first extensive risk assessment on OTA from fermented teas worldwide, but the sample size is still limited, and a large number of samples is encouraged in a future study for a more accurate assessment.

Comparative Metagenomics Reveals Microbial Communities and Their Associated Functions in Two Types of Fuzhuan Brick Tea

Fuzhuan brick tea (FBT) is a unique post-fermented tea product, naturally co-fermented by microorganisms, and has gained global popularity due to its potential health benefits for humans. Considerable efforts have been made toward elucidating the microbial diversity within FBT, but an understanding of the underlying FBT community interactions and functions remains poorly studied. Consequently, the microbial communities of two types of FBT, originating from Hunan and Shaanxi provinces, were investigated using comparative shotgun metagenomic sequencing and functional annotations. Metagenomic analysis indicated that two communities shared similar taxonomic and functional attributes. Two samples shared 486 genera, in which Pseudomonas contributed most to the abundant functions within the two samples. The carbohydrate active enzyme functions of the communities primarily comprised GH (32.92%), GT (26.8%), CEs (20.43%), and AAs (18.04%). Furthermore, the overall metabolic pathways encoded by the metagenomes were largely associated with carbohydrate and amino acid metabolism, with nine metabolic pathways that were differential between two groups including penicillin and cephalosporin biosynthesis. Significantly, a total of 35 potential probiotics were inferred, with Pseudomonas putida being the most abundant inferred probiotic (80%) within the FBT communities. This study provides new insights into FBT microbial communities on their potential functions and roles in FBT characteristics.