A study on tea aroma formation mechanism: alcoholic aroma precursor amounts and glycosidase activity in parts of the tea plant

Widely targeted metabolomics analysis reveals the formation of nonvolatile flavor qualities during oolong tea manufacturing: a case study of Jinguanyin

Background

The manufacturing processes of oolong tea significantly impact its nonvolatile components, leading to the emergence of distinct flavor attributes. Understanding the dynamic changes in nonvolatile components during the manufacturing stages of the Jinguanyin (JGY) cultivar is crucial for unraveling the potential mechanism behind flavor formation.

Methods

Comprehensive metabolomics and sensomics analyses were conducted to investigate the dynamic changes in nonvolatile components throughout various phases of oolong tea processing, focusing on the JGY cultivar.

Results

A total of 1,005 nonvolatile metabolites were detected, with 562 recognized as significant differential metabolites during various phases of oolong tea processing. Notably, the third turning-over, third setting, and high-temperature treatments exhibited the most significant effects on the nonvolatile metabolites of oolong tea. JGY finished tea demonstrated a characteristic flavor profile, marked by mellowness, sweetness in aftertaste, and a significant Yin rhyme. This flavor profile was collectively promoted by the accumulation of amino acids and organic acids, the decrease in flavonols (3-O-glycosides) and sugar substances, the alteration of phenolic acids, and the stabilization of caffeine.

Conclusion

This study contribute to the understanding of the formation of oolong tea flavor qualities. The dynamic changes observed in various types of nonvolatile compounds during oolong tea processing shed light on the intricate interplay of metabolites and their influence on the final flavor characteristics.

Comprehensive investigation on non-volatile and volatile metabolites in four types of green teas obtained from the same tea cultivar of Longjing 43 (Camellia sinensis var. sinensis) using the widely targeted metabolomics

In this study, we produced roasted, baked, steamed, and sun-dried green tea products using the same batch of fresh tea leaves (FTL) of Longjing 43 (Camellia sinensis var. sinensis), and explored processing effects on the metabolic profiles of four types of green teas (FGTs) using the widely targeted metabolomics. Results showed that 146 differential metabolites including flavonoids, amino acids, lipids, and phenolic acids were screened among 1034 non-volatiles. In addition, nineteen differential metabolites were screened among 79 volatiles. Most of non-volatiles and volatiles metabolites changed notably in different manufacturing processes, whereas there were no significant differences (p＞0.05) in the levels of total catechins between FGTs and FTL. The transformation of metabolites was the dominant trend during green tea processing. The results contribute to a better understanding of how the manufacturing process influences green tea quality, and provide useful information for the enrichment of tea biochemistry theory.

Enhancement of Black Tea Aroma by Adding the β-Glucosidase Enzyme during Fermentation on Black Tea Processing

Black tea aroma is one of the essential attributes in determining the quality of black tea. β-Glucosidases were investigated for their ability to enhance the aroma of black tea by hydrolyzing the glycoside compound. The addition of β-glucosidase was done by dissolving the enzyme on a sodium citrate buffer (pH 5.0), which was then sprayed on tea leaves during black tea processing. The β-glucosidase treatment significantly increases the volatile compound from glycoside precursors such as linalool, geraniol, and methyl salicylate. Moreover, the volatile compound from carotenoid and lipid precursors (nerolidol and β-cyclocitral) was also increased with β-glucosidase treatment.

Headspace volatiles influenced by infusion matrix and their release persistence: a case study of oolong tea

The perceived aroma of oolong tea is primarily and directly affected by its infusion matrix, and the release persistence of headspace volatiles can better illustrate the persistent aroma. Headspace solid phase microextraction coupled with gas chromatography-mass spectrometry was performed to analyze the headspace constituents of oolong tea. The presence of the infusion matrix seemed to prevent the headspace release of certain odorants. The release of indole, nerolidol, and α-farnesene was also remarkably enhanced or depressed (2.70, 1.56, and 0.69-fold in tea infusion versus in dry leaves). Moreover, the amount of volatile species gradually decreased with increased water ratio. Eight odorants were determined to be stable and persistent during continuous infusion, whereas six were determined to be less persistent (gradually decreased or stopped releasing). The volatile dilution test further confirmed the persistent release of nerolidol.

Characteristic Fluctuations in Glycosidically Bound Volatiles during Tea Processing and Identification of Their Unstable Derivatives

A recently developed method enabled us to simultaneously characterize and quantitate glycosidically bound volatiles (GBVs) at picomole levels using liquid chromatography-mass spectrometry (LC-MS). On the basis of the analytical data it is possible to screen tea varieties most suitable for black tea processing, in which higher concentrations of primeverosides accumulate. The primeverosides decreased at the rolling step in black tea processing, whereas the glucopyranosides did not change much. The total contents of GBVs gradually increased at the withering steps and then remarkably increased after the fixing step at 230 °C, during oolong tea processing. The presence of 6'-O-malonyl ester type β-D-glucopyranosides in the tea samples suggested a contribution to the increment in glucopyranosides during oolong tea processing. The method was also used to analyze GBVs and their derivatives to understand their possible role in the metabolic pathway of tea.

Understanding Darjeeling tea flavour on a molecular basis

Darjeeling teas are the highest grown teas in the world and preferred for its flavour, aroma and quality. Apart from the genetic makeup of the plant, earlier reports suggest that insect infestation, particularly jassids and thrips triggers the aroma and flavour formation in Darjeeling tea. The present work encompasses the identification of the genes/transcriptomes responsible for the typical flavour of Darjeeling tea, besides understanding the role of jassids and thrips in particular, in producing the best cup character and quality. The quantitative real time PCR analysis was based on a suppression subtractive hybridisation forward library of B157 (tea clone infested with thrips), providing us transcripts related to aroma and flavour formation. We observed the expression of genes like leucine zipper, ntd, nced, geraniol synthase, raffinose synthase, trehalose synthase, amylase, farnesyl transferase, catalase, methyl transferase, linalool synthase, peroxidases, elicitor responsive proteins, linamarase, nerolidol linalool synthase 2, 12-oxophytodienoate reductase, glucosidase, MYB transcription factor, and alcohol dehydrogenase, highly regulated due to insect infestation, manufacturing stresses and mechanical injury. The first report on gene expression dynamics in thrips infested Darjeeling tea leaves can be extrapolated with increase in volatiles which is responsible for enhancing the quality of Darjeeling tea, specially the flavour and aroma of the infusion. We hope to model these responses in order to understand the molecular changes that occur during Darjeeling tea flavour formation.

Expression and biochemical characterization of beta-primeverosidase and application of beta-primeverosylamidine to affinity purification

Beta-primeverosidase (PD) is a family 1 glycosidase catalyzing the hydrolysis of beta-primeverosides (6-O-beta-D-xylopyranosyl-beta-D-glucopyranosides) to release a disaccharide primeverose. To investigate how PD recognizes the disaccharide moiety of beta-primeverosides, the recombinant PD was expressed by a baculovirus-insect cell system. The recombinant PD was secreted from High Five cells and was properly modified with N-glycosylation and correct cleavage at the N-terminal signal peptide. The recombinant PD exhibited high substrate specificity to beta-primeverosides in terms of the glycone moiety, consistently with the substrate specificity of native PD from Camellia sinensis. Next, beta-glycosylamidines were synthesized as substrate analog inhibitors. Beta-primeverosylamidine strongly inhibited PD activity, but beta-glucosylamidine did not. Hence beta-primeverosylamidine is an ideal chemical tool for probing disaccharide recognition in the active site of PD. An affinity adsorbent for PD was prepared using beta-primeverosylamidine as a ligand. Affinity chromatography gave large amounts of PD with high purity, permitting crystallographic study.

Cloning of beta-primeverosidase from tea leaves, a key enzyme in tea aroma formation

A beta-primeverosidase from tea (Camellia sinensis) plants is a unique disaccharide-specific glycosidase, which hydrolyzes aroma precursors of beta-primeverosides (6-O-beta-D-xylopyranosyl-beta-D-glucopyranosides) to liberate various aroma compounds, and the enzyme is deeply concerned with the floral aroma formation in oolong tea and black tea during the manufacturing process. The beta-primeverosidase was purified from fresh leaves of a cultivar for green tea (C. sinensis var sinensis cv Yabukita), and its partial amino acid sequences were determined. The beta-primeverosidase cDNA has been isolated from a cDNA library of cv Yabukita using degenerate oligonucleotide primers. The cDNA insert encodes a polypeptide consisting of an N-terminal signal peptide of 28 amino acid residues and a 479-amino acid mature protein. The beta-primeverosidase protein sequence was 50% to 60% identical to beta-glucosidases from various plants and was classified in a family 1 glycosyl hydrolase. The mature form of the beta-primeverosidase expressed in Escherichia coli was able to hydrolyze beta-primeverosides to liberate a primeverose unit and aglycons, but did not act on 2-phenylethyl beta-D-glucopyranoside. These results indicate that the beta-primeverosidase selectively recognizes the beta-primeverosides as substrates and specifically hydrolyzes the beta-glycosidic bond between the disaccharide and the aglycons. The stereochemistry for enzymatic hydrolysis of 2-phenylethyl beta-primeveroside by the beta-primeverosidase was followed by (1)H-nuclear magnetic resonance spectroscopy, revealing that the enzyme hydrolyzes the beta-primeveroside by a retaining mechanism. The roles of the beta-primeverosidase in the defense mechanism in tea plants and the floral aroma formation during tea manufacturing process are also discussed.

The (3R,9R)-3-hydroxy-7,8-dihydro-beta-ionol disaccharide glycoside is an aroma precursor in tea leaves

The disaccharide glycoside, (3R,9R)-3-hydroxy-7,8-dihydro-beta-ionyl 6-O-beta-D-apiofuranosyl-beta-D-glucopyranoside was isolated as an aroma precursor from the leaves of Camellia sinensis var. sinensis cv. Yabukita. Its stereochemistry was elucidated on the basis of spectral data and chemical synthesis.