The fungal laccase-catalyzed oxidation of EGCG and the characterization of its products

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.

The inhibition effects of chlorogenic acid on the formation of colored oxidation products of (-)-epigallocatechin gallate under enzymatic oxidation

Untargeted Liquid chromatography tandem mass spectrometry (LC-MS) based metabolomics in combination with UV-visible and colorimeter was applied in identifying critical colored enzymatically oxidized products of (-)-epigallocatechin gallate (EGCG). Pearson correlation coefficient analysis between marker compounds and a* value was conducted, and then a series of colored oxidation products were targeted and subsequently identified by diode array detection and mass fragmentation ions. The quinone of oolongtheanin 3-O'-gallate degraded product with quasi-molecular mass ion at m/z 711 was identified as a critical colored oxidation product of single EGCG. To explore the effect of chlorogenic acid on the formation of colored EGCG enzymatic oxidation products, the variation of oxidation products on the oolongtheanin pathway was semi-quantitatively determined. The result showed chlorogenic acid significantly inhibited the formation of colored oxidation products, thus lightened the color of EGCG oxidation mixture. The addition of chlorogenic acid influences the process of tea polyphenols' enzymatic oxidation.

An In Vitro Catalysis of Tea Polyphenols by Polyphenol Oxidase

Tea polyphenol (TPs) oxidation caused by polyphenol oxidase (PPO) in manufacturing is responsible for the sensory characteristics and health function of fermented tea, therefore, this subject is rich in scientific and commercial interests. In this work, an in vitro catalysis of TPs in liquid nitrogen grinding of sun-dried green tea leaves by PPO was developed, and the changes in metabolites were analyzed by metabolomics. A total of 441 metabolites were identified in the catalyzed tea powder and control check samples, which were classified into 11 classes, including flavonoids (125 metabolites), phenolic acids (67 metabolites), and lipids (55 metabolites). The relative levels of 28 metabolites after catalysis were decreased significantly (variable importance in projection (VIP) > 1.0, p < 0.05, and fold change (FC) < 0.5)), while the relative levels of 45 metabolites, including theaflavin, theaflavin-3'-gallate, theaflavin-3-gallate, and theaflavin 3,3'-digallate were increased significantly (VIP > 1.0, p < 0.05, and FC > 2). The increase in theaflavins was associated with the polymerization of catechins catalyzed by PPO. This work provided an in vitro method for the study of the catalysis of enzymes in tea leaves.

Absorption, metabolism, bioactivity, and biotransformation of epigallocatechin gallate

Epigallocatechin gallate (EGCG), a typical flavone-3-ol polyphenol containing eight free hydroxyl groups, is associated with a variety of bioactivities, such as antioxidant, anti-inflammatory, anti-cancer, and antibacterial activities. However, the poor bioavailability of EGCG restricts its use. In this review, we discuss the processes involved in the absorption and metabolism of EGCG, with a focus on its metabolic interactions with the gut microbiota. Next, we summarize the bioactivities of some key metabolites, describe the biotransformation of EGCG by different microorganisms, and discuss its catabolism by specific bacteria. A deeper understanding of the absorption, metabolism, and biotransformation of EGCG may enable its disease-preventive and therapeutic properties to be better utilized. This review provides a theoretical basis for further development and utilization of EGCG and its metabolites for improving the gut microbiota and physiological health.

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.

Improved yield of theaflavin-3,3'-digallate from Bacillus megaterium tyrosinase via directed evolution

Theaflavin-3,3'-digallate (TFDG) in black tea possesses several health benefits. However, low TFDG yields limit its application. Herein, tyrosinases from Bacillus megaterium (Bmtyrc) were used to synthesize TFDG. To improve the catalytic efficiency of tyrosinase, a directed evolution strategy and a high-throughput screening method was employed. Compared with the wild type, mutant Bmtyrc-3 (N205D/D166E/D167G/F197W) showed 6.46 and 4.91-folds higher specific activity and 51.97- and 1.95-folds higher Vmax values towards epigallocatechin gallate (EGCG) and epicatechin gallate (ECG), respectively. Moreover, Bmtyrc-3 displayed significantly enhanced catalytic efficiencies, and the space-time yield of TFDG was 35.35 g L^-1d^-1. Bmtyrc-3 presents a broader substrate binding area, caused by a mutation (N205D) encompassing the active site. Changes in the potential of the substrate binding site and hydrogen bonds, and the electrostatic effect on the protein surface resulted in an increased activity of the substrates EGCG and ECG.

Comprehensive Analysis of Bacterial Community Structure and Diversity in Sichuan Dark Tea (Camellia sinensis)

Bacteria and fungi present during pile-fermentation of Sichuan dark tea play a key role in the development of its aesthetic properties, such as color, taste, and fragrance. In our previous study, high-throughput sequencing of dark tea during fermentation revealed Aspergillus was abundant, but scarce knowledge is available about bacterial communities during pile-fermentation. In this study, we rigorously explored bacterial diversity in Sichuan dark tea at each specific stage of piling. Analysis of cluster data revealed 2,948 operational taxonomic units, which were divided into 42 phyla, 98 classes, 247 orders, 461 families, 1,052 genera, and 1,888 species. Certain members of the family Enterobacteriaceae were dominant at early stages of fermentation YC, W1, and W2; Pseudomonas at middle stage W3; and the highest bacterial diversity was observed at the final quality-determining stage W4. Noticeably, probiotics, such as Bacillus, Lactobacillus, Bifidobacterium, and Saccharopolyspora were also significantly higher at the quality-determining stage W4. Our findings might help in precise bacterial inoculation for probiotic food production by increasing the health benefits of Sichuan dark tea. This research also falls under the umbrella of the "Establish Good Health and Well-Being" Sustainable Development Goals of the United Nations Organization.

Fermentation transforms the phenolic profiles and bioactivities of plant-based foods

Phenolics are a group of compounds derived from plants that have displayed potent biological activities and health-promoting effects. Fermentation is one of the most conventional but still prevalent bioprocessing methods in the food industry, with the potential to increase phenolic content and enhance its nutritive value. This review details the biotransformation of different classes of phenolics (hydroxycinnamic and hydroxybenzoic acids, flavonoids, tannins, stilbenoids, lignans, alkylresorcinols) by various microorganisms (lactic acid bacteria, yeast, filamentous fungi) throughout the fermentation process in plant-based foods. Several researchers have commenced the use of metabolic engineering, as in recombinant Saccharomyces cerevisiae yeast and Escherichia coli, to enhance the production of this transformation. The impact of phenolics on the metabolism of microorganisms and fermentation process, although complex, is reviewed for the first time. Moreover, this paper highlights the general effect of fermentation on the food's phenolic content, and its bioaccessibility, bioavailability and bioactivities including antioxidant capacity, anti-cancer, anti-diabetic, anti-inflammation, anti-obesity properties. Phenolics of different classes are converted into compounds that are often more bioactive than the parent compounds, and fermentation generally leads to a higher phenolic content and antioxidant activity in most studies. However, biotransformation of several phenolic classes is less studied due to its low concentration and apparent insignificance to the food system. Therefore, there is potential for application of metabolic engineering to further enhance the content of different phenolic classes and bioactivities in food.

Status of the application of exogenous enzyme technology for the development of natural plant resources

Exogenous enzymes are extraneous enzymes that are not intrinsic to the subject. The exogenous enzyme industry has been rapidly developing recently. Successful application of recombinant DNA amplification, high-efficiency expression, and immobilization technology to genetically engineered bacteria provides a rich source of enzymes. Amylase, cellulase, protease, pectinase, glycosidase, tannase, and polyphenol oxidase are among the most widely used such enzymes. Currently, the application of exogenous enzyme technology in the development of natural plant resources mainly focuses on improving the taste and flavor of the product, enriching the active ingredient contents, deriving and transforming the structure of a chosen compound, and enhancing the biological activity and utilization of the functional ingredient. In this review, we discuss the application status of exogenous enzyme technology for the development of natural plant resources using typical natural active ingredients from plant, such as resveratrol, steviosides, catechins, mogrosides, and ginsenosides, as examples, to provide basis for further exploitation and utilization of exogenous enzyme technology.

Genome-wide analysis and metabolic profiling unveil the role of peroxidase CsGPX3 in theaflavin production in black tea processing

Plucked tea leaves can be processed into black tea (Camellia sinensis), which is rich in health-promoting molecules, including flavonoid antioxidants. During black tea processing, theaflavins (TFs) and thearubigins (TRs) are generated via the successive oxidation of catechins by endogenous polyphenol oxidase (PPO)- or peroxidase (POD)-mediated reactions. This process must be well controlled to achieve the proper TF/TR ratio, which is an important quality parameter of the tea beverage. However, little is known about the POD/PPO catalyzed TF formation process at the molecular genetic level. Here, we identified and characterized the POD genes responsible for TF production in tea. Genome-wide analysis of POD/PPO family genes, metabolite profiling, and expression analysis of PPO/POD genes in tea leaves enabled us to select several PPO/POD genes potentially involved in TF production. Differential gene expression in plant tissues and enzyme activity in several tea varieties traditionally used for processing of various beverage types indicate that black tea processing primarily depends on PPO/POD activity. Among these POD/PPO genes, the POD CsGPX3 is involved in the generation of TFs during black tea processing. The capacity of PPO/POD-catalysed TF production is potentially used for controlling catechin oxidation during black tea processing and could be used to create molecular markers for breeding of tea plant varieties suitable for the production of high-quality black tea beverages.

Integrated proteomics and metabolomics analysis of tea leaves fermented by Aspergillus niger, Aspergillus tamarii and Aspergillus fumigatus

Post-fermented Pu-erh tea (PFPT) is a microbially-fermented tea with distinct sensory qualities and multiple health benefits. Aspergillus are the dominant fungi in the fermentation and the main contributors to the characteristics of PFPT, so their underlying functions warrant detailed study. Here, tea leaves were fermented by Aspergillus niger, Aspergillus tamarii and Aspergillus fumigatus, and resulting samples (designated as Asn, Ast and Asf, respectively) were analyzed by proteomic and metabolomic methods. Changes to the composition of flavonoids, glycerophospholipids, organo-oxygen compounds and fatty acids resulting from Aspergillus fermentation were observed. Carbohydrate-active enzymes, e.g., endoglucanases and cellulases, for degradation of cellulose, starch, lignin, pectin, xylan and xyloglucan were identified. Glycoside hydrolase, glycosyltransferases, tannase, laccases, vanillyl-alcohol oxidases and benzoquinone reductase were identified and hypothesized to catalyze hydrolysis, oxidation, polymerization and degradation of phenolic compounds. Together, functions of Aspergillius were demonstrated as production of enzymes to change concentrations and compositions of metabolites in tea leaves.

Transcriptome and Metabolic Profiling Unveiled Roles of Peroxidases in Theaflavin Production in Black Tea Processing and Determination of Tea Processing Suitability

Theaflavins (TFs) are generated by endogenous polyphenol oxidase (PPO)- and peroxidase (POD)-catalyzed catechins oxidation during black tea processing, which needs to be well-controlled to obtain a proper TFs/thearubigins (TRs) ratio for better quality. Not all leaves from any tea plant cultivars or varieties are suitable for making high-quality black teas, regardless of the processing techniques. The mechanisms underlying TFs formation and the main factors determining the tea leaf processing suitability are not fully understood. We here integrated transcriptome and metabolite profiling of tea leaves to unveil how enzymes or metabolites in leaves are changed during black tea processing. The information enabled us to identify several PPO and POD genes potentially involved in tea processing for TF production. We characterized a POD gene, whose recombinant enzyme showed TF creation activity. The capacity for POD-catalyzed TF production could be used as a molecular marker for breeding tea plant varieties suitable for high-quality black tea production.

Microbial diversity and component variation in Xiaguan Tuo Tea during pile fermentation

Xiaguan Tuo Tea is largely consumed by the Chinese, but there is little research into the microbial diversity and component changes during the fermentation of this tea. In this study, we first used fluorescence in situ hybridization (FISH), next-generation sequencing (NGS) and chemical analysis methods to determine the microbial abundance and diversity and the chemical composition during fermentation. The FISH results showed that the total number of microorganisms ranges from 2.3×10² to 4.0×10⁸ cells per gram of sample during fermentation and is mainly dominated by fungi. In the early fermentation stages, molds are dominant (0.6×10²~2.8×10⁶ cells/g, 0~35 d). However, in the late stages of fermentation, yeasts are dominant (3.6×10⁴~9.6×10⁶ cells/g, 35~56 d). The bacteria have little effect during the fermentation of tea (10²~10³ cells/g, <1% of fungus values). Of these fungi, A. niger (Aspergillus niger) and B. adeninivorans (Blastobotrys adeninivorans) are identified as the two most common strains, based on Next-generation Sequencing (NGS) analysis. Peak diversity in tea was observed at day 35 of fermentation (Shannon–Weaver index: 1.195857), and lower diversity was observed on days 6 and 56 of fermentation (Shannon–Weaver index 0.860589 and 1.119106, respectively). During the microbial fermentation, compared to the unfermented tea, the tea polyphenol content decreased by 54%, and the caffeine content increased by 59%. Theanine and free amino acid contents were reduced during fermentation by 81.1 and 92.85%, respectively.

Preparation of Biocolorant and Eco-Dyeing Derived from Polyphenols Based on Laccase-Catalyzed Oxidative Polymerization

Natural products have been believed to be a promising source to obtain ecological dyes and pigments. Plant polyphenol is a kind of significant natural compound, and tea provides a rich source of polyphenols. In this study, biocolorant derived from phenolic compounds was generated based on laccase-catalyzed oxidative polymerization, and eco-dyeing of silk and wool fabrics with pigments derived from tea was investigated under the influence of pH variation. This work demonstrated that the dyeing property was better under acidic conditions compared to alkalinity, and fixation rate was the best when pH value was 3. Furthermore, breaking strength of dyed fabrics sharply reduced under the condition of pH 11. Eventually, the dyeing method was an eco-friendly process, which was based on bioconversion, and no mordant was added during the process of dyeing.

Eco-dyeing with biocolourant based on natural compounds

Biomass pigments have been regarded as promising alternatives to conventional synthetic dyestuffs for the development of sustainable and clean dyeing. This investigation focused on in situ dyeing of fabrics with biopigments derived from tea polyphenols via non-enzymatic browning reaction. The average particle size of dyed residual liquor with natural tea polyphenol was 717.0 nm (ranging from 615.5 to 811.2 nm), and the Integ value of dyed wool fabrics was the greatest compared to those of counterparts. In addition, the Integ values of dyed fabrics with residual liquor were much bigger than those with the first reaction solutions when dyed by identical dyeing liquor. As a result, the dyeing process could be carried out many times because the concentration of the residual liquor was relatively superior. All dyed fabrics acquired admirable rubbing as well as washing fastness, and the relevant dyeing mechanism has been analysed in the paper.

High-theabrownins instant dark tea product by Aspergillus niger via submerged fermentation: α-glucosidase and pancreatic lipase inhibition and antioxidant activity

BACKGROUND

Theabrownins (TB) are bioactive components that are usually extracted from Chinese dark tea, in which they are present at low concentrations. The present study aimed to produce an instant dark tea high in theabrownins via submerged fermentation by the fungus Aspergillus niger. Three fermentation parameters that affect theabrownins content (i.e. inoculum size, liquid-solid ratio and rotation speed) were optimized using response surface methodology.

RESULT

Optimum fermentation conditions were modeled to be an inoculum of 5.40% (v/v), a liquid-solid ratio of 27.45 mL g^-1 and a rotation speed of 184 rpm and were predicted to yield 292.99 g kg^-1 TB. Under these experimentally conditions, the TB content of the instant dark tea was 291.93 g kg^-1 . The antioxidant capacity and α-glucosidase and pancreatic lipase inhibitory activities of the high-TB instant black tea were higher than four other typical instant dark tea products.

CONCLUSION

The results of the present study show that careful management of culture conditions can produce a dark tea high in theabrownins. Furthermore, high-theabrownins instant dark tea could serve as a source of bioactive products and be used in functional foods as an ingredient imparting antioxidant properties and the ability to inhibit pancreatic lipase and α-glucosidase. © 2017 Society of Chemical Industry.