Aromatic profiles and enantiomeric distributions of chiral odorants in baked green teas with different picking tenderness.

Suitable picking tenderness is an essential prerequisite for manufacturing tea. However, the influence of picking tenderness of fresh tea leaves on the aromatic components is still unclear. In this study, aromatic profiles and chiral odorants in fresh tea leaves and corresponding baked green teas with five levels of tenderness of two representative cultivars were analysed using stir bar sorptive extraction-gas chromatography-mass spectrometry. cis-Linalool oxide (furanoid) and methyl salicylate exhibited significantly increasing trends as samples of all series matured. The content of most chiral odorants was significantly high in the mature samples, and significant content variations of all enantiomers during baked green tea processing could be observed with different trends according to their precursors. In particular, the enantiomeric ratios of most chiral odorants were less influenced by the picking tenderness and processing, while drying (limonene), spreading and fixation (α-terpineol), and spreading (dihydroactinidiolide) influenced the chiral distribution of the aforementioned odorants.

Assessment of the contribution of chiral odorants to aroma property of baked green teas using an efficient sequential stir bar sorptive extraction approach.

Chiral volatile compounds are known to be distributed in teas at various enantiomeric ratios. However, the performance of each enantiomer, including aroma characteristics, aroma intensities, and contribution to the overall flavor of tea, is still unclear. In this study, aroma characteristics and intensities of 38 volatile enantiomers in standards and baked green teas with chestnut-like aroma and clean aroma were evaluated by an efficient sequential headspace-stir bar sorptive extraction (seq-HS-SBSE) approach combined with the enantioselective gas chromatography-olfactometry/mass spectrometry (Es-GC-O/MS) technique. Moreover, aroma recombination results for the two types of baked green teas using 14 chiral odorants and four achiral odorants indicated that the combinations of the detected odorants mainly contributed to the "floral", "sweet", and "chestnut-like" aromas. R-Linalool simultaneously enhanced the "floral", "sweet", and "chestnut-like" aromas; R-limonene mainly contributed to the "sweet" and "clean" aromas; and S-α-terpineol promoted the "sweet" and "floral" aromas of baked green tea.

Identification and quantification of key odorants in the world's four most famous black teas.

Keemun, Assam, Darjeeling and Ceylon black teas are honored as the world's four most famous black teas, and their excellent aroma qualities are well received by people around the world. In this study, aroma components in these four types of teas were analyzed by comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-TOFMS) and gas chromatography-olfactometry (GC-O) technologies. A total of 42 aroma-active compounds were ultimately identified, especially benzeneacetaldehyde, geraniol, (Z)-3-hexen-1-yl hexanoate, trans-ß-ionone, cis-linalool oxide (pyranoid), hotrienol, and methyl salicylate presented the strongest aroma strengths with pleasant scents in all tested teas. The quantification results indicated that 19 compounds including (Z)-3-hexenol, 1-octen-3-ol, linalool, phenylethyl alcohol, hexanal, benzeneacetaldehyde, limonene, heptanoic acid, (Z)-3-hexen-1-ol, acetate, benzyl alcohol, trans-linalool oxide (furanoid), hotrienol, 1-octen-3-one, 2-nonanone, (E)-2-octenal, nonanal, ß-myrcene, 2-pentylfuran, and methylpyrazine were identified as the key compounds with odor activity values (OAVs) higher than 1.0 in the world's four most famous black teas. Notably, the comparison of GC-O and OAV calculation results showed that methyl salicylate (Ceylon), (E)-2-octenal (Assam), benzeneacetaldehyde (Keemun) and linalool and trans-linalool oxide (furanoid) (Darjeeling) might be the most definitive odorants in the corresponding tea categories.

The enantiomeric distributions of volatile constituents in different tea cultivars.

Although the enantiomeric distribution of chiral volatiles presents great potential in discrimination of tea cultivars and their geographic origins, this area has received little attention. Thus, we herein aimed to determine the relationships between tea cultivars and the enantiomeric distributions of their chiral volatile constituents. Headspace solid-phase microextraction (HS-SPME) and enantioselective gas chromatography-mass spectrometry (Es-GC-MS) were employed to quantify 15 volatile components in 22 tea cultivars from different locations within China. The tea cultivars were successfully differentiated by their geographical origins, and the concentrations of R-linalool, S-citronellol, S-E-nerolidol, (1R, 2R)-methyl jasmonate, S-α-ionone, and the two enantiomers of linalool oxide A differed significantly among the different groups. It should also be noted that tea processing methods greatly influenced the formation of volatile enantiomers. Our results demonstrated that the enantiomeric distribution of volatile constituents closely correlates with the geographical origins, leaf types, and manufacturing suitabilities of the tea cultivars examined herein.

Identification of key odorants responsible for chestnut-like aroma quality of green teas.

A chestnut-like aroma is widely considered an important indicator of an excellent-quality green tea; however, the key odorants responsible for chestnut-like aroma have never been systematically studied and remain unknown. In this study, the aroma components of green teas and Chinese chestnuts were analyzed using comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC × GC-TOFMS), and 58 compounds were identified as common aroma components among green teas, boiled Chinese chestnuts, roasted Chinese chestnuts and raw Chinese chestnuts. Subsequently, 17 volatiles, including 3-methylbutanal, (E)-3-penten-2-one, ethylbenzene, heptanal, benzaldehyde, 2-pentylfuran, octanal, benzeneacetaldehyde, (E)-2-octenal, (E,E)-3,5-octadien-2-one, linalool, nonanal, (E)-2-nonenal, decanal, (Z)-hex-3-en-1-yl hexanoate, trans-ß-ionone and (E)-nerolidol, were identified as the key odorants responsible for chestnut-like aroma based on the odor activity value (OAV) calculation method. Besides, the comparison of OAVs of key odorants between fresh tea leaves and finished teas indicated that all key odorants were present in fresh tea leaves and that their contents increased or decreased during tea processing. Moreover, the comparison between results of OAV and gas chromatography-olfactometry (GC-O) methods showed that ethylbenzene, heptanal, benzaldehyde, 2-pentylfuran, (E,E)-3,5-octadien-2-one, linalool, (Z)-hex-3-en-1-yl hexanoate and trans-ß-ionone were the common identified compounds between the two methods. The identification of chestnut-like aroma in green teas will provide a theoretical basis for further research on the directional adjustment and control of tea aroma quality.

Enantiomeric and quantitative analysis of volatile terpenoids in different teas (Camellia sinensis).

Volatile terpenoids play important roles in the formation of tea aroma quality due to their pleasant scents and low odor thresholds. Most volatile terpenoids contain stereogenic centers, which results in various stereo distributions of their enantiomers and diastereoisomers in different types of tea. However, the distribution characteristics of terpenoid enantiomers in teas were still unclear, which poses an obstacle to the scientific understanding of tea aroma. In this work, a new and efficient analysis approach based on headspace solid phase microextraction (HS-SPME)-chiral gas chromatography-mass spectrometry (GC-MS) was established to analyze 12 pairs of familiar terpenoid enantiomers in different teas. The extraction efficiency of the HS-SPME method to extract volatile terpenoids in teas was the greatest when using CAR-DVB-PDMS (50/30µm) fibers and 1:10 proportions between tea and boiling water at a 50°C extraction temperature for 40min, and the stability observation of enantiomeric ratios of the terpenoids well proved the feasibility of the extraction method. The favorable limits of detection, limits of quantitation, repeatability, linearity, and concentration ranges of each terpenoid enantiomer demonstrated the repeatability and reliability of the analytical approach. The enantiomeric and quantitative analyses indicated that S-limonene, S-linalool, (2S, 5S)-linalool oxide A, (2S, 5R)-linalool oxide B, R-4-terpineol, (2S, 5R)-linalool oxide C, (2S, 5S)-linalool oxide D, S-α-terpineol, R-α-ionone, peak 1 of theaspirane A and peak 2 of theaspirane B were the major terpenoid components in most Chinese teas; instead, higher proportions of the opposite enantiomers of the above terpenoids were frequently detected in black teas with large leaf origin and Indonesia white teas. Besides, great diversities of enantiomeric ratios and concentrations among different teas were observed. Furthermore, partial least-squares discriminant analyses were performed to distinguish the concentration differences of the terpenoid enantiomers among different teas; the analysis results indicated that highly significant concentration differences existed between large and small leaf origins of black teas, and significant differences of the concentrations of linalool oxides A-C were observed between green, white and dark teas. The successful application of this chiral analysis technique of tea aroma will lay a scientific foundation for further quality assessment, botanical origin determination and authenticity assessment of teas.

Cloning of a caffeoyl-coenzyme A O-methyltransferase from Camellia sinensis and analysis of its catalytic activity.

Epigallocatechin-3-O-(3-O-methyl) gallate (EGCG3"Me) present in leaves of Camellia sinensis has many beneficial biological activities for human health. However, EGCG3"Me occurs naturally in tea leaves in extremely limited quantities. Finding an enzyme from C. sinensis to catalyze the synthesis of EGCG3"Me is an alternative method to make up for the scarcity of EGCG3"Me in natural situations. In the present study, a complementary DNA (cDNA) encoding region and genomic DNA of the caffeoyl-coenzyme A O-methyltransferase (CCoAOMT) gene were isolated from C. sinensis (designated CsCCoAOMT). Nucleotide sequence analysis of CsCCoAOMT revealed an open reading frame of 738 bp that encodes a polypeptide with a predicted molecular weight of 28 kDa, which correlated well with the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The full-length DNA sequence (2678 bp) contained five exons and four introns. The deduced amino acid sequence of CsCCoAOMT shared 92% identity with CCoAOMTs from Codonopsis lanceolata and Betula luminifera. The catalytic activity of CsCCoAOMT was analyzed. Three monomethylated epigallocatechin-3-O-gallate (EGCG) compounds (EGCG4"Me, EGCG3"Me, and EGCG3'Me) were produced by CsCCoAOMT with K(m) in the micromolar range. Real-time polymerase chain reaction (RT-PCR) experiments indicated that the CsCCoAOMT transcript was present at low levels during the early stages of leaf maturity (the first leaf and bud on a shoot) but the relative expression was augmented at advanced stages of leaf maturity (the third or fourth leaf on a shoot), which accorded well with changes in EGCG3"Me content in fresh leaves. Hence, we concluded that CsCCoAOMT catalyzes the syntheses of methylated EGCGs.