Study of the dynamic changes in the non-volatile chemical constituents of black tea during fermentation processing by a non-targeted metabolomics approach

Effects of fermentation duration on the flavour quality of large leaf black tea based on metabolomics

Fermentation durations are crucial in determining the quality of black tea flavour. The mechanism underlying the degradation of black tea flavour caused by inappropriate fermentation duration remains unclear. In this study, the taste of black teas with different fermentation durations (BTFs) was analysed using sensory evaluation, electronic tongue, and metabolomics. The results revealed significant differences in 46 flavour profile components within the BTFs. Notably, metabolites such as gallocatechin gallate, gallocatechin, and epigallocatechin were found to be primarily reduced during fermentation, leading to a reduction in the astringency of black tea. Conversely, an increase in d-mandelic acid and guanine among others was observed to enhance the bitter flavour of black tea, while 3-Hydroxy-5-methylphenol nucleotides were found to contribute to sweetness. Furthermore, succinic acid and cyclic-3',5'-adenine nucleotides were associated with diminished freshness. This study offers a theoretical foundation for the regulation of flavour quality in large leaf black tea.

Changes and biotransformation mechanism of main functional compounds during kombucha fermentation by the pure cultured tea fungus

Kombucha was fermented by the pure cultured tea fungus, and the changes of functional compounds and their transformation were explored. After fermentation, the contents of total polyphenols, total flavonoids, quercetin, kaempferol and catechins respectively enhanced by 77.14%, 69.23%, 89.11%, 70.32% and 45.77% compared with the control, while flavonol glycosides reduced by 38.98%. The bioavailability of polyphenols and flavonoids respectively increased by 29.52% and 740.6%, and DPPH and ABTS respectively increased by 43.81% and 35.08% compared with the control. Correlation analysis showed that microorganisms and the antioxidant activity were highly positive correlation with total polyphenols, total flavonoids, EGC, EC, EGCG, ECG, quercetin and kaempferol, and negative correlation with kaempferol-3-glucoside. The constructed models confirmed that organic acids were more likely to damage the structure of tea leaves, and enzymes (polygalacturonidase and tannase) and solvents (acids, alcohols and esters) had a synergistic effect on the biotransformation of functional compounds.

Investigation of non-volatile metabolite variations during round green tea processing and effect of pan-frying degree using untargeted metabolomics and objective quantification

Round green tea (RGT) presents unique properties and is widely distributed in China, and during processing, it undergoes dynamic changes in non-volatile metabolites (NVMs), which are poorly understood. Utilizing UHPLC-Q-Exactive/MS analysis, this study comprehensively characterized 216 NVMs during RGT processing and identified fixation and pan-frying as key processes influencing NVMs. Additionally, 23 key differential NVMs were screened, with amino acid and flavonoid metabolism highlighted as key metabolic pathways for RGT taste and color quality. The impact of pan-frying degree on shape, color, and taste was also explored. Moderate pan-frying led to optimal results, including a tight and round shape, green and bright color, mellow and umami taste, and reduced astringent and bitter taste NVMs, including epigallocatechin gallate, procyanidin B2, myricetin 3-O-galactoside, quinic acid, strictinin, phenylalanine, and theobromine. This study addresses the NVM research gap in RGT processing, thus providing a technical foundation for the precision-oriented processing of high-quality tea.

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.

Exploring the flavor changes in mung bean flour through Lactobacillus fermentation: insights from volatile compounds and non-targeted metabolomics analysis

BACKGROUND

Mung beans are highly nutritious but their leguminous flavor limits their development. Lactic acid bacteria (LAB) fermentation can decrease unwanted bean flavors in legumes and enhance their flavor. This study examined the influence of Lactobacillus fermentation on the flavor characteristics of mung bean flour (MBF) using volatile compounds and non-targeted metabolomics.

RESULTS

Lactobacillus plantarum LP90, Lactobacillus casei LC89, and Lactobacillus acidophilus LA85 eliminated 61.37%, 48.29%, and 43.73%, respectively, of the primary bean odor aldehydes from MBF. The relative odor activity value (ROAV) results showed that fermented mung bean flour (FMBF) included volatile chemicals that contributed to fruity, flowery, and milky aromas. These compounds included ethyl acetate, hexyl formate, 3-hydroxy-2-butanone, and 2,3-butanedione. The levels of amino acids with a fresh sweet flavor increased significantly by 93.89, 49.40, and 35.27% in LP90, LC89, and LA85, respectively. A total of 49 up-regulated and 13 down-regulated significantly differential metabolites were annotated, and ten metabolic pathways were screened for contributing to the flavor. The correlation between important volatile compounds and non-volatile substances relies on two primary metabolic pathways: the citric acid cycle pathway and the amino acid metabolic system.

CONCLUSION

The flavor of MBF was enhanced strongly by the process of Lactobacillus fermentation, with LP90 having the most notable impact. These results serve as a reference for identifying the flavor of FMBF. © 2024 Society of Chemical Industry.

Rolling forms the diversities of small molecular nonvolatile metabolite profile and consequently shapes the bacterial community structure for Keemun black tea

The detailed dynamics of small molecular nonvolatile chemical and bacterial diversities, as well as their relationship are still unclear in the manufacturing process of Keemun black tea (KMBT). Herein, mass spectrometry-based untargeted metabolomics, Feature-based Molecular Networking (FBMN) and bacterial DNA amplicon sequencing were used to investigate the dense temporal samples of the manufacturing process. For the first time, we reveal that the pyrogallol-type catechins are oxidized asynchronously before catechol-type catechins during the black tea processing. Rolling is the key procedure for forming the small molecular nonvolatile metabolite profile (SMNMetProf), increasing the metabolite richness, and then shaping the bacterial community structure in the KMBT manufacturing process, which decreases both molecular weight and molecular polarity of the small molecular nonvolatile metabolites. The SMNMetProf of black tea is formed by the endogenous enzymatic oxidation of tea leaves, rather than bacterial fermentation.

Re-rolling treatment in the fermentation process improves the taste and liquor color qualities of black tea

Fermentation is a vital process occurred under the premise of rolling affecting black tea quality. Theoretically, re-rolling during fermentation will remodel the biochemical conditions of tea leaves, and thus influence black tea quality. Herein, we studied the effect of re-rolling on black tea taste and liquor color. Sensory evaluation showed that re-rolling significantly weakened the astringency taste and improved the redness and luminance of liquor. With re-rolling, the color attributes of a* and L* and the contents of theaflavins and thearubigins were significantly improved. Metabolomics analysis showed that the contents of 110 non-volatile compounds were significantly different among black teas with different rolling treatments. In summary, re-rolling accelerated the oxidation of polyphenols into pigments, the hydrolysis of proteins into amino acids, and the metabolism of alkaloids, organic acids, glycosidically-bound volatiles, and lipids during the fermentation period. Our study provided a novel and simple way to improve black tea quality.

Identifying the structures and taste characteristics of two novel Maillard reaction products in tea

Maillard reaction products (MRPs) produced during thermal processing of tea are intimately related to its flavor. Our recent work revealed that both levels of l-theanine and d-galacturonic acid in tea leaves decreased dramatically during drying, whereas the specific MRPs from l-theanine and d-galacturonic acid remain elusive. Here, the MRPs formed from l-theanine and d-galacturonic acid were investigated and their taste characteristics and the involved mechanisms were explored. Two novel MRPs from l-theanine and d-galacturonic acid were identified as 1-(1-carboxy-4-(ethylamino)-4-oxobutyl)-3-hydroxypyridin-1-ium (MRP 1) and 2-(2-formyl-1H-pyrrole-1-yl) theanine (MRP 2). MRP 1 and MRP 2 accumulated in dark tea and black tea and were associated with sour (threshold, 0.25 mg/mL) and astringent tastes and an umami taste (threshold, 0.18 mg/mL), respectively. Molecular docking revealed that the taste characteristics of MRPs may be due to strong binding to umami taste receptor proteins (CASR, T1R1/T1R3) and the sour taste protein OTOP1 via hydrogen bonds and hydrophobic interactions.

Comparison of Volatile and Nonvolatile Metabolites in Black Tea under Four Second-Drying Methods Using Widely Targeted Metabolomics

Second-drying has an impact on the development of flavor and aroma in black tea. However, the effect of the shape changes of the tea leaves during second-drying on the quality of black tea has yet to be evaluated. In this study, GC-TOFMS and UPLC-HRMS identified 411 volatile metabolites and 253 nonvolatile metabolites. Additionally, 107 nonvolatile compounds and 21 different volatiles were screened. Significant alterations (p < 0.01) were found in 18 amino acid derivatives, 17 carbohydrates, 20 catechins, 19 flavonoids, 13 phenolic acids, and 4 organic acids. The content of certain amino acids and carbohydrates correlated with the shape of black tea. Furthermore, sweet aroma compound formation was facilitated by hot-air second-drying while the remaining second-drying approaches encouraged the formation of the fruity aroma compound. The results of the study provide a theoretical basis and technical instructions for the accurate and precise processing of premium black tea.

The influence of rolling pressure on the changes in non-volatile compounds and sensory quality of congou black tea: The combination of metabolomics, E-tongue, and chromatic differences analyses

Rolling represents an essential stage in congou black tea processing. However, the influence of rolling pressure on tea flavor and non-volatile compounds remains unclear. Herein, a combination of untargeted metabolomics, tea pigments quantification, E-tongue, colorimeter and sensory evaluation was used to evaluate the effect of rolling pressure on black tea quality. As the rolling pressure increased, theaflavins (TFs), thearubigins (TRs), and theabrownins (TBs) significantly elevated. The tea metabolic profiles fluctuated and 47 metabolites were identified as key differential metabolites including flavan-3-ols, flavonol/flavone glycosides, phenolic acids, amino acids. These substances altered possibly due to the variations in enzymatic oxidation of tea phenolics and amino acids. Overall, black tea with moderate rolling pressure presented higher sweetness, lower bitterness, and higher quality index (10 TFs + TRs)/TBs. The results were verified by a validation batch. This study provided new insights into the regulation of rolling pressure and a guidance for black tea processing.

Comprehensive applications of metabolomics on tea science and technology: Opportunities, hurdles, and perspectives

With the development of metabolomics analytical techniques, relevant studies have increased in recent decades. The procedures of metabolomics analysis mainly include sample preparation, data acquisition and pre-processing, multivariate statistical analysis, as well as maker compounds' identification. In the present review, we summarized the published articles of tea metabolomics regarding different analytical tools, such as mass spectrometry, nuclear magnetic resonance, ultraviolet-visible spectrometry, and Fourier transform infrared spectrometry. The metabolite variation of fresh tea leaves with different treatments, such as biotic/abiotic stress, horticultural measures, and nutritional supplies was reviewed. Furthermore, the changes of chemical composition of processed tea samples under different processing technologies were also profiled. Since the identification of critical or marker metabolites is a complicated task, we also discussed the procedure of metabolite identification to clarify the importance of omics data analysis. The present review provides a workflow diagram for tea metabolomics research and also the perspectives of related studies in the future.

Revealing the chemical differences and their application in the storage year prediction of Qingzhuan tea by SWATH-MS based metabolomics analysis

It's generally believed that the longer the storage, the better the quality of dark tea, but the chemical differences of Qingzhuan tea (QZT) with different storage years is still unclear. Herein, in this work, an untargeted metabolomic approach based on SWATH-MS was established to investigate the differential compounds of QZT with 0-9 years' storage time. These QZT samples were roughly divided into two categories by principal component analysis (PCA). After orthogonal projections to latent structures discriminant analysis (OPLS-DA), 18 differential compounds were putatively identified as chemical markers for the storage year variation of QZT. Heatmap visualization showed that the contents of catechins, fatty acids, and some phenolic acids significantly reduced, flavonoid glycosides, triterpenoids, and 8-C N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs) increased with the increase of storage time. Furthermore, these chemical markers were verified by the peak areas corresponding to MS2 ions from SWATH-MS. Based on the extraction chromatographic peak areas of MS and MS2 ions, a duration time prediction model was built for QZT with correlation coefficient R2 of 0.9080 and 0.9701, and RMSEP value of 0.85 and 1.24, respectively. This study reveals the chemical differences of QZT with different storage years and provides a theoretical basis for the quality evaluation of stored dark tea.

Manufacturing process differences give Keemun black teas their distinctive aromas

Traditional Keemun black tea is also known as Congou black tea (CF). Over the last three decades, three other types of Keemun black tea (Jinzhen, JZ; Maofeng, MF; Xiangluo, XL) made by different processing have been introduced into the tea market. Total CF volatile concentrations ranged from 1666.3 to 2185.7 μg/L, followed by XL (1193.5-1916.1 μg/L), JZ (1058.9-1811.0 μg/L), and MF (987.5-1518.0 μg/L) tea infusions. A total of 79 volatiles in tea infusions was identified by two methods, among which fourteen with OAVs > 1 were identified and OAVs proportion of volatiles with flowery, fruity, or sweet notes to those with other notes differed in four Keemun black teas (CF = 6.58:1, MF = 5.16:1, JZ = 4.04:1, XL = 5.11:1). Phenylethyl alcohol oxidation resulted in phenylacetaldehyde formation which is the characteristic odorant in Keemun black tea. We clearly show that changes in tea processing gives the distinctive aroma to different Keemun black teas.

Characterizing and Decoding the Effects of Different Fermentation Levels on Key Aroma Substances of Congou Black Tea by Sensomics

Fermentation is the key technology for black tea aroma formation. The key aroma substances of black tea at different fermentation stages (unfermented (WDY), underfermented (F1H), fully fermented (F4H), and overfermented (F8H)) were characterized by the methodology of Sensomics. Aroma extract dilution analysis was performed on volatile fractions extracted by using solvent-assisted flavor evaporation and solid-phase microextraction, yielding 93 odor-active areas. Internal standard method plus stable isotope dilution analysis was used for quantitative analysis. The omission experiment identified 23 aroma substances. Further reduction and addition experiments revealed phenylacetaldehyde, (E,E)-2,4-heptadienal, geraniol, linalool, β-damascenone, 2-methylbutyraldehyde, dimethyl sulfide, and isovaleraldehyde with odor activity values (OAV) > 100 as the characteristic aroma components of F4H and also as the main contributors to aroma differences between different fermentation degrees. The green odor of (E,E)-2,4-heptadienal was highlighted in WDY and F1H relative to that in F4H due to the lower contribution of phenylacetaldehyde and β-damascenone in the former two samples. Additionally, excessive OAV increase of fatty aldehydes in F8H masked its similar floral and fruity aroma.

Effects of Key Components on the Antioxidant Activity of Black Tea

Many components (such as tea polyphenols, catechins, theaflavins, theasinensins, thearubigins, flavonoids, gallic acid, etc.) in black tea have antioxidant activities. However, it is not clear which components have a greater influence on the antioxidant activity of black tea. In this study, the antioxidant activity and contents of tea polyphenols, catechins, theaflavins, thearubigins, theabrownins, TSA, total flavonoids, amino acids, caffeine, and total soluble sugar were analyzed in 51 black teas. Principal component analysis (PCA), orthogonal partial least-squares discrimination analysis (OPLS-DA), and the correlation analysis method were used for data analysis. The results showed that catechins in tea polyphenols were the most important components that determine the antioxidant activity of black tea. Among them, epicatechin gallate (ECG), epi-gallocatechin gallate (EGCG), epicatechin (EC), and epi-gallocatechin (EGC) were significantly positively correlated with the antioxidant activity of black tea, and theabrownin was negatively correlated with the antioxidant activity of black tea. Furthermore, this study analyzed the correlation between the changes in catechin and its oxidized polymers with antioxidant activity during black tea fermentation; it verified that catechins were significantly positively correlated with the antioxidant activity of black tea, and theabrownin showed a negative correlation. And the antioxidant activity of catechins and their oxidation products in vitro and their correlation in black tea processing were used as validation. This study provides a comparison method for comparing the antioxidant activity of black tea.

Quantitative Analysis of Bioactive Compounds in Commercial Teas: Profiling Catechin Alkaloids, Phenolic Acids, and Flavonols Using Targeted Statistical Approaches

Tea, an extensively consumed and globally popular beverage, has diverse chemical compositions that ascertain its quality and categorization. In this investigation, we formulated an analytical and quantification approach employing reversed-phase ultra-high-performance liquid chromatography (UHPLC) methodology coupled with diode-array detection (DAD) to precisely quantify 20 principal constituents within 121 tea samples spanning 6 distinct variants. The constituents include alkaloids, catechins, flavonols, and phenolic acids. Our findings delineate that the variances in chemical constitution across dissimilar tea types predominantly hinge upon the intricacies of their processing protocols. Notably, green and yellow teas evinced elevated concentrations of total chemical moieties vis à vis other tea classifications. Remarkably divergent levels of alkaloids, catechins, flavonols, and phenolic acids were ascertained among the disparate tea classifications. By leveraging random forest analysis, we ascertained gallocatechin, epigallocatechin gallate, and epicatechin gallate as pivotal biomarkers for effective tea classification within the principal cadre of tea catechins. Our outcomes distinctly underscore substantial dissimilarities in the specific compounds inherent to varying tea categories, as ascertained via the devised and duly validated approach. The implications of this compositional elucidation serve as a pertinent benchmark for the comprehensive assessment and classification of tea specimens.

The Integration of Metabolomics, Electronic Tongue, and Chromatic Difference Reveals the Correlations between the Critical Compounds and Flavor Characteristics of Two Grades of High-Quality Dianhong Congou Black Tea

Tea's biochemical compounds and flavor quality vary depending on its grade ranking. Dianhong Congou black tea (DCT) is a unique tea category produced using the large-leaf tea varieties from Yunnan, China. To date, the flavor characteristics and critical components of two grades of high-quality DCT, single-bud-grade DCT (BDCT), and special-grade DCT (SDCT) manufactured mainly with single buds and buds with one leaf, respectively, are far from clear. Herein, comparisons of two grades were performed by the integration of human sensory evaluation, an electronic tongue, chromatic differences, the quantification of major components, and metabolomics. The BDCT possessed a brisk, umami taste and a brighter infusion color, while the SDCT presented a comprehensive taste and redder liquor color. Quantification analysis showed that the levels of total polyphenols, catechins, and theaflavins (TFs) were significantly higher in the BDCT. Fifty-six different key compounds were screened by metabolomics, including catechins, flavone/flavonol glycosides, amino acids, phenolic acids, etc. Correlation analysis revealed that the sensory features of the BDCT and SDCT were attributed to their higher contents of catechins, TFs, theogallin, digalloylglucose, and accumulations of thearubigins (TRs), flavone/flavonol glycosides, and soluble sugars, respectively. This report is the first to focus on the comprehensive evaluation of the biochemical compositions and sensory characteristics of two grades of high-quality DCT, advancing the understanding of DCT from a multi-dimensional perspective.

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.

Analyzing the influence of withering degree on the dynamic changes in non-volatile metabolites and sensory quality of Longjing green tea by non-targeted metabolomics

Withering is an important processing stage in green tea, which contributes to the tea flavor quality. The aim of this work was to comprehensively investigate the changes of chemical features and flavor attributes in Longjing green teas produced with five different withering degrees (moisture content of 75.05, 72.53, 70.07, 68.00, and 64.78%, w.b.). Combined with human sensory evaluation, electronic tongue and chromatic differences analysis, an assessment of the relationship between the withering degree and the sensory quality of Longjing tea was obtained. By using a non-targeted metabolomics approach, 69 significantly differential metabolites were screened. As the withering degree increased, most free amino acids and catechin dimers were increased, largely attributed to the hydrolysis of proteins and catechin oxidative polymerization, respectively. The contents of organic acids as well as phenolic acids and derivatives were reduced. Interestingly, flavone C-glycosides decreased overall while flavonol O-glycosides increased. The correlation analysis revealed that metabolites such as theasinensin F, theasinensin B, theaflavin, theaflavin-3,3′-gallate, theaflavin-3′-gallate, malic acid, succinic acid, quinic acid, theanine glucoside and galloylglucose had a greater influence on the taste and color of tea infusion (|r| &gt; 0.6, p &lt; 0.05). Overall, an appropriate withering degree at a moisture content of around 70% is more favorable to enhance the Longjing tea quality. These results may enhance the understanding of green tea flavor chemistry associated with withering and provide a theoretical basis for green tea processing.

Integration of lipidomics and metabolomics approaches for the discrimination of harvest time of green tea in spring season by using UPLC-Triple-TOF/MS coupled with chemometrics

The production season is one of the judgment standards of the green tea quality and spring tea is generally considered of higher quality. Moreover, early spring tea is usually more precious and sells for a higher price. Therefore, a multifaceted strategy that integrates lipidomics and metabolomics, based on UPLC-Triple-TOF/MS coupled with chemometrics, was developed to discriminate early spring green tea (ET) and late spring green tea (LT). Twenty-six lipids and forty-five metabolites were identified as characteristic components. As for characteristic lipids, most of glycerophospholipids and acylglycerolipids have higher contents in ET. By contrast, glycoglycerolipids, sphingolipids and hydroxypheophytin a were shown higher levels in LT samples. Most of the differential metabolites identified were more abundant in ET samples. LT samples have much higher catechin, procyanidin B2, and 3',8-dimethoxyapigenin 7-glucoside contents. Based on the integration of differential lipids and metabolites, the reconstructed orthogonal partial least squares discriminant analysis (OPLS-DA) model displayed 100% correct classification rates for harvest time discrimination of green tea samples. These results demonstrated that the integration of lipidomics and metabolomics approaches is a promising method for the discrimination of tea quality.

Structural insight into polyphenol oxidation during black tea fermentation

Polyphenol oxidation during fermentation plays a critical role in the formation of flavor and function of black tea. However, how the specific structures of tea polyphenols affect their oxidation kinetics during black tea fermentation is still unknown. Here, we found that the oxidations of tea polyphenols, including 7 catechins, 4 phenolic acids and 11 flavonoid glycosides followed pseudo-first-order kinetics during fermentation. Molecular structure and oxygen concentration collaboratively regulated the oxidation rate of different polyphenols. Pyrogallol structure was more easily to be oxidized than catechol and monophenol structure in B-ring, the gallic group in C-ring could inhibit oxidation of catechins, while the role of sugar moiety of flavonoid glycosides was differentiating. In addition, oxygen was found to be the key factor limiting the oxidation rate of polyphenols in regular black tea fermentation, and the oxidation rate constants of tea polyphenols were linearly and positively correlated with oxygen concentration.

Analysis of metabolites in green tea during the roasting process using non-targeted metabolomics

BACKGROUND

Roasting plays an important role in the formation of flavor of roasted green tea; however, the changes in chemicals during this process have not been systematically studied until now. To reveal the dynamic changes in chemicals in green tea during roasting, non-targeted metabolomics, coupled with chemometrics, was employed.

RESULTS

A total of 101 non-volatile metabolites were identified in tea samples, and 29 metabolites were identified as characteristic metabolites of roasting. A significant increase in catechins and their derivatives, organic acids, and flavonoid glycosides was observed, while the content of some amino acids and their derivatives decreased over 50% during roasting. The content of theanine glucoside increased dramatically (by 21.23-fold at the roasting stage), and Maillard-derived compounds also increased to varying degrees.

CONCLUSION

Glycosylation, oxidative polymerization, and pyrolysis were important reactions responsible for the formation and transformation of flavor compounds in roasted green tea during roasting. © 2022 Society of Chemical Industry.

Optimization of Cordyceps sinensis fermentation Marsdenia tenacissima process and the differences of metabolites before and after fermentation

In this paper, we explored the interaction of factors which influenced the Cordyceps sinensis fermentation Marsdenia tenacissima (Roxb.) Wight et Arn, a Dai (a national minority of China) medicine, and the optimal fermentation conditions. The differences of C. sinensis metabolites in normal state (CN) and products of two-way liquid fermentation of C. sinensis and Marsdenia tenacissima (CM) and Marsdenia tenacissima (MT). The interactive effect of factors was analyzed and the best conditions are obtained through the box-behnken design (BBD) in response surface methodology (RSM). All metabolites were determined by ultra high performance liquid chromatography quadrupole time of flight mass spectrometer (UHPLC-Q-TOF-MS), analyzed and identified by metabonomics technology. Results showed that the optimum fermentation conditions were the concentration of raw medicinal materials is 160 g/L, the fermentation time is 6 days, the inoculation volume is 9.5%, the rotating speed is 170 rpm. 197 metabolites were identified in both positive ion and negative ion. 119 metabolites were significantly different between CN and CM. 43 metabolites were significantly different between CM and MT. Differential metabolic pathways were enriched. In conclusion, this paper optimizes the bidirectional fermentation process of M. tenacissima and C. sinensis through response surface methodology, and analyzes the changes of components from the level of metabonomics, so as to provide reference for exploring medicinal fungi fermentation of traditional Chinese medicine.

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.

Metagenomic insight into the microbial degradation of organic compounds in fermented plant leaves

Microbial degradation of organic compounds is an environmentally benign and energy efficient part in product processing. Fermentation of plant leaves involves enzymatic actions of many microorganisms. However, microbes and enzymes discovered from natural degradation communities were still limited by cultural methods. In this study, we used a metagenomics sequence-guided strategy to identify the microbes and enzymes involved in compound degradation and explore the potential synergy among community members in fermented tobacco leaves. The results showed that contents of protein, starch, pectin, lignin, and cellulose varied in fermented leaves from different growing sites. The different compound contents were closely related to taxonomic composition and functional profiles of foliar microbial communities. Microbial communities showed significant correlations with protein, lignin, and cellulose. Vital species for degradations of protein (Bacillus cereus and Terribacillus aidingensis), lignin (Klebsiella pneumoniae and Pantoea ananatis) and cellulose (Pseudomonas putida and Sphingomonas sp. Leaf20) were identified and relating hydrolytic enzymes were annotated. Further, twenty-two metagenome-assembled genomes (MAGs) were assembled from metagenomes and six potential cellulolytic genomes were used to reconstruct the cellulose-degrading process, revealing the potential metabolic cooperation related to cellulose degradation. Our work should deepen the understanding of microbial roles in plant fermentation and provide a new viewpoint for applying microbial consortia to convert plant organic components to small molecules.

Characterisation of catechins and their oxidised derivatives in Ceylon tea using multi-dimensional liquid chromatography and high-resolution mass spectrometry

Tea is a complex food matrix comprising of many structurally diverse compounds, of which catechins and their oxidised derivatives are of particular interest due to their nutritional functionality. However, these catechins and derivatives exist in various isomeric forms with few or no pure standards available, rendering their analysis challenging. A method combining multi-dimensional liquid chromatography (MDLC) and high-resolution mass spectrometry (HRMS) was developed for the characterisation of these compounds using Ceylon tea as a model. Based on a Plackett-Burman (PB) design, flow rate and initial methanol percentage were identified as the most significant factors (p < 0.05) affecting chromatogram coverage and resolution (Rs) for comprehensive two-dimensional LC (LCxLC) and heart-cutting two-dimensional LC (LC-LC) respectively. Central composite design (CCD) was then applied using these parameters for method optimisation and to identify second-order relationships between screened parameters. The optimised LCxLC (flow rate: 2.18 mL/min and initial methanol percentage: 28.0%) and LC-LC (flow rate: 0.86 mL/min and initial methanol percentage for different cuts: A- 10.0%; B- 15.8%; and C- 18.7%) methods were applied to the analysis of Ceylon tea samples from seven regions of Sri Lanka and demonstrated an improved separation of co-eluting isomeric compounds. Finally, with the mass spectral information from HRMS, a total of 31 compounds (eight monomers, 17 dimers, five trimers and one tetramer) were detected and putatively identified in Ceylon tea.

Emerging applications of nano-optical sensors combined with near-infrared spectroscopy for detecting tea extract fermentation aroma under ultrasound-assisted sonication

The current innovative work combines nano-optical sensors with near-infrared spectroscopy for rapid detection and quantification of polyphenols and investigates the potential of the nano-optical sensor based on chemo-selective colorants to detect the dynamic changes in aroma components during the fermentation of tea extract. The procedure examined the influence of different ultrasound-assisted sonication factors on the changes in the consumption rate of polyphenols during the fermentation of tea extract versus non-sonication as a control group. The results showed that the polyphenol consumption rate improved under the ultrasound conditions of 28 kHz ultrasound frequency, 24 min treatment time, and 40 W/L ultrasonic power density. The metal-organic framework based nano-optical sensors reported here have more adsorption sites for enhanced adsorption of the volatile organic compounds. The polystyrene-acrylic microstructure offered specific surface area for the reactants. Besides, the employed porous silica nanospheres with higher porosity administered improved gas enrichment effect. The nano-optical sensor exhibits good performance with a "chromatogram" for the identification of aroma components in the fermentation process of tea extract. The proposed method respectively enhanced the consumption rate of polyphenol by 35.57%, 11.34% and 16.09% under the optimized conditions. Based on the established polyphenol quantitative prediction models, this work demonstrated the feasibility of using a nano-optical sensor to perform in-situ imaging of the fermentation degree of tea extracts subjected to ultrasonic treatment.

Effects of Fermentation Temperature and Time on the Color Attributes and Tea Pigments of Yunnan Congou Black Tea

Yunnan Congou black tea (YCBT) is a typical black tea in China, and is rich in theaflavins (TFs), thearubigins (TRs), and theabrownins (TBs). However, the influence of the fermentation temperature and time on the liquor and appearance color and the correlation between the tea pigments and its color attributes remain unclear. We investigated the effects of the fermentation temperature and time on the color attributes and tea pigments of YCBT. A low fermentation temperature was beneficial to maintain a bright orange-red liquor color and promote the accumulation of TFs and TRs. In contrast, a high temperature gave the liquor a glossy appearance and was beneficial for the formation of TBs. A correlation analysis showed that the 10TFRB index best represented the contribution of tea pigments to the quality of black tea. Moreover, TRs and TBs content prediction models were established based on the liquor L and H values, where the former value can be used as an important index to judge the fermentation process. This study will further enrich the theory of black tea processing chemistry and provide technical support for the precise and directional production of black tea.

Tea (Camellia sinensis): A Review of Nutritional Composition, Potential Applications, and Omics Research

Tea (Camelliasinensis) is the world’s most widely consumed non-alcoholic beverage with essential economic and health benefits since it is an excellent source of polyphenols, catechins, amino acids, flavonoids, carotenoids, vitamins, and polysaccharides. The aim of this review is to summarize the main secondary metabolites in tea plants, and the content and distribution of these compounds in six different types of tea and different organs of tea plant were further investigated. The application of these secondary metabolites on food processing, cosmetics industry, and pharmaceutical industry was reviewed in this study. With the rapid advancements in biotechnology and sequencing technology, omics analyses, including genome, transcriptome, and metabolome, were widely used to detect the main secondary metabolites and their molecular regulatory mechanisms in tea plants. Numerous functional genes and regulatory factors have been discovered, studied, and applied to improve tea plants. Research advances, including secondary metabolites, applications, omics research, and functional gene mining, are comprehensively reviewed here. Further exploration and application trends are briefly described. This review provides a reference for basic and applied research on tea plants.

Improvement of Biosynthetic Ansamitocin P-3 Production Based on Oxygen-Vector Screening and Metabonomics Analysis

A novel approach involving exogenous oxygen vectors was developed for improving the production of biosynthetic Ansamitocin P-3 (AP-3). Four types of oxygen vectors including soybean oil, n-dodecane, n-hexadecane, and Tween-80 were applied to explore the effect of exogenous oxygen vectors on AP-3 yield. It was observed that soybean oil exhibited a better ability for promoting AP-3 generation than the other three oxygen vectors. Based on the results of the single-factor experiment, response surface methodology was employed to obtain the optimal soybean oil addition method. The optimum soybean oil concentration was 0.52%, and the addition time was 50 h. Under this condition, the yield of AP-3 reached 106.04 mg/L, which was 49.48% higher than that of the control group without adding oxygen vectors. To further investigate the influence of dissolved oxygen on precious orange tufts actinomycetes variety A. pretiosum strain metabolism and AP-3 yield, metabolomics analysis was carried out by detecting strain intermediate metabolites at various stages under different dissolved oxygen levels. Moreover, differential metabolite screening and metabolic pathway enrichment analysis were combined to exploit the effect mechanism of soybean oil on AP-3 production. Results suggested that primary metabolic levels of the TCA cycle and amino acid metabolism increased with the increase in dissolved oxygen level, which was beneficial to the life activities of bacteria and the synthesis of secondary metabolic precursors, thus increasing the production of AP-3.

Metabolomics combined with proteomics provides a novel interpretation of the compound differences among Chinese tea cultivars (Camellia sinensis var. sinensis) with different manufacturing suitabilities

Different tea cultivars differ in their manufacturing suitability. In this study, metabolomics and proteomics were applied to investigate the metabolite and protein differences in fresh leaves from 23 Chinese tea cultivars suitable for manufacturing green, white, oolong, and black teas. The combined analysis revealed 115 differential metabolites and significant differences in the biosynthesis pathways for amino acids, phenylpropanoids, flavonoids, and terpenoids, and in the peroxidases abundances among these four groups. Green tea cultivars had higher abundances of amino acids and amino acids biosynthesis-related enzymes but lower abundances of flavanols and flavonoids biosynthesis-related enzymes. Black tea cultivars presented higher abundances of flavanols, flavanol-O-glycosides, flavonoids biosynthesis-related enzymes, and peroxidases. Oolong tea cultivars showed higher abundances of enzymes involved in terpenoids biosynthesis. Our study provides a novel interpretation of the manufacturing suitability of tea cultivars from the perspective of both metabolites and proteins and will be helpful for cultivar breeding.

Directed Accumulation of Nitrogen Metabolites through Processing Endows Wuyi Rock Tea with Singular Qualities

The execution of specific processing protocols endows Wuyi rock tea with distinctive qualities produced through signature metabolic processes. In this work, tea leaves were collected before and after each of three processing stages for both targeted and untargeted metabolomic analysis. Metabolic profiles of processing stages through each processing stage of rotation, pan-firing and roasting were studied. Overall, 614 metabolites were significantly altered, predominantly through nitrogen- enriching (N) pathways. Roasting led to the enrichment of 342 N metabolites, including 34 lipids, 17 organic acids, 32 alkaloids and 25 amino acids, as well as secondary derivatives beneficial for tea quality. This distinctive shift towards enrichment of N metabolites strongly supports concluding that this directed accumulation of N metabolites is how each of the three processing stages endows Wuyi rock tea with singular quality.

Metabolomics Combined with Proteomics Provide a Novel Interpretation of the Changes in Flavonoid Glycosides during White Tea Processing

In this study, nonvolatile metabolomics and proteomics were applied to investigate the change mechanism of flavonoid glycoside compounds during withering processing of white tea. With the extension of withering time, the content of the main flavonoid glycoside compounds significantly decreased, and then the flavonoid aglycones and water-soluble saccharides contents increased. However, the change trends of these compounds were inconsistent with the expression pattern of related biosynthesis pathway proteins, indicating that the degradation of flavonoid glycosides might exist in the withering process of white tea. One co-expression network that was highly correlated with variations in the flavonoid glycosides’ component contents during the withering process was identified via WGCNA. Further analysis revealed that the degradation of flavonoid glycosides may be related to the antioxidant action of tea leaves undergoing the withering process. Our results provide a novel characterization of white tea taste formation during processing.