Mechanism of oolongtheanin formation via three intermediates

Revealing the variances in color formation and bioactivities of seven catechin monomers throughout the enzymatic reaction by colorimetric and mass spectrometry

The capacity differences of seven catechin monomers to produce colors after treating with catechin-free extract were investigated. After 240-min reaction, only (-)-epicatechin (EC) and (+)-catechin (C) presented obvious luminous red color with L* values of 63.32-71.73, a* values of 37.13-46.44, and b* values of 65.64-69.99. Meanwhile, the decrease rate of EC and C was 43.52 %-50.35 %, which were significantly lower than those of other catechin monomers (85.91 %-100 %). The oxidized products of catechin monomers were analyzed by ultra-high performance liquid chromatography-quadrupole-time of flight-mass spectrometry coupled with diode array detector, wherein dehydro-dimers and -trimers (oxidative coupling products of catechins' A-B ring) were found to be the major chromogenic compounds of EC and C. Additionally, the antioxidant capacity of catechin monomers only decreased after 30-min reaction, while along with further enzymatic reaction, catechin monomers presented comparable oxyradical scavenging ability (e.g., the DPPH inhibitory rates of catechin monomers were in the range of 24.42 %-50.77 %) to vitamin C (positive control, DPPH inhibitory rate was 27.66 %). Meanwhile, the inhibitory effects of most catechin monomers on α-glucosidase were enhanced in different degrees. These results provided basis for the development of enzymatically-oxidized catechin monomers as functional food color additives.

Oxidation and degradation of (epi)gallocatechin gallate (EGCG/GCG) and (epi)catechin gallate (ECG/CG) in alkali solution

The oxidative decomposition/degradation of two main tea flavanols, EGCG/GCG and ECG/CG, was studied in alkaline solution under ultrasonic-assisted thermal conditions. The study employed HPLC-ESI-ToF-MS to identify the products generated by atmospheric oxygen oxidation and various base-catalyzed reactions. Strong basic condition led to accelerated hydrolysis and oxidation of EGCG/GCG and ECG/CG and yielded gallic acid, de-galloyl flavanols and corresponding o-quinone derivatives. Meanwhile, peroxidation or base-catalyzed cleavage and rearrangement occurred extensively on C- and B-rings of flavanol and generated various simpler aldehydes or acids. Besides, a number of dimers/trimers were produced. This contribution provides empirical proof of oxidative degradation of flavanols under strong alkaline condition. Meanwhile, detailed reaction mechanisms of C-/B-ring degradation and dimerization/polymerization phenomena are proposed to help understand the structural changes of flavanols under strong alkaline conditions.

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.

The molecular mechanisms of quality difference for Alpine Qingming green tea and Guyu green tea by integrating multi-omics

Introduction

Harvest time represents one of the crucial factors concerning the quality of alpine green tea. At present, the mechanisms of the tea quality changing with harvest time have been unrevealed.

Methods

In the current study, fresh tea leaves (qmlc and gylc) and processed leaves (qmgc and gygc) picked during Qingming Festival and Guyu Festival were analyzed by means of sensory evaluation, metabolomics, transcriptomic analysis, and high-throughput sequencing, as well as their endophytic bacteria (qm16s and gy16s).

Results

The results indicated qmgc possessed higher sensory quality than gygc which reflected from higher relative contents of amino acids, and soluble sugars but lower relative contents of catechins, theaflavins, and flavonols. These differential metabolites created features of light green color, prominent freshness, sweet aftertaste, and mild bitterness for qmgc.

Discussion

Flavone and flavonol biosynthesis and phenylalanine metabolism were uncovered as the key pathways to differentiate the quality of qmgc and gygc. Endophytic bacteria in leaves further influence the quality by regulating the growth of tea trees and enhancing their disease resistance. Our findings threw some new clues on the tea leaves picking to pursue the balance when facing the conflicts of product quality and economic benefits.

Revealing the profound meaning of pan-firing of oolong tea - A decisive point in odor fate

The delicate aroma of Bao-chung tea comes from oxidation, followed by fixation in the pan-firing step. Traditionally, the timing of pan-firing has been based on odor perception by tea masters and lacks relevant scientific research. Pan-firing at three different green-note intensities and three stirring sequences was used to explore the relationship between the compositions of volatile organic compounds (VOCs) before pan-firing and in the finished tea. Pan-firing decreased green leaf volatiles and increased the ratio of terpenoid volatiles. The characteristic VOCs of the finished tea were highly related to VOCs before pan-firing (R² = 0.97). Principal component analysis revealed that the traditional judgment of the pan-firing step is based on nonanal, β-linalool, and cis- and trans-linalool oxides. The timing of pan-firing is crucial for VOCs, and VOC composition before pan-firing can be used to predict desired tea aroma.

Production Mechanisms of Black Tea Polyphenols

Black tea accounts for 70-80% of world tea production, and the polyphenols therein are produced by enzymatic oxidation of four tea catechins during tea fermentation. However, only limited groups of dimeric oxidation products, such as theaflavins, theasinensins, and theacitrins, have been isolated from black tea and chemically characterized. This is largely because of the complexity and heterogeneity of the oxidation products. To determine structures and production mechanisms of uncharacterized black tea polyphenols, in vitro model fermentation experiments using pure catechins and polyphenol oxidase have been applied, and basic oxidation mechanisms have been established. Contemporary methods, such as LC-MS, are also effective to identify catechin oxidation products in black tea. Despite ongoing efforts, almost 60% of the solids in black tea infusion remain uncharacterized. These compounds include the so-called thearubigins, which are a heterogeneous mixture of uncharacterized catechin oxidation products with oligomeric structures. This review summarizes the current knowledge of the production mechanisms of representative black tea polyphenols and presents recent progress in characterization of thearubigins.