Formation and isomerization of (Z)-methyl epijasmonate, the key contributor of the orchid-like aroma, during tea processing.

The elegant orchid-like fragrance of tea has always been tea processors and consumers' top priority. Controlling the production process is very important for tea aroma formation. This study aims to investigate the synthesis of (Z)-methyl epijasmonate (epi-MeJA), a key contributor to orchid-like aroma properties in tea, during tea processing. The changes in content of epi-MeJA were analysed during the processing of two tea varieties (Anxi Tieguanyin and Taiping Houkui) with typical orchid-like fragrance. It was found to be mainly synthesized and accumulated during tea processing, as fresh tea leaves contained little or even no epi-MeJA. Its content was positively correlated with the processing time in the enzyme active stages (before fixation). During the fixation stages, isomerization occurred due to high temperatures, with a degree of epimerization to the much less odor active isomer (Z)-methyl jasmonate. Isomerization could also occurred during the drying process, which is dominated by the drying temperature.

Tea Aroma Analysis Based on Solvent-Assisted Flavor Evaporation Enrichment.

Tea aroma is an important factor in tea quality, but it is challenging to analyze due to the complexity, low concentration, diversity, and lability of the volatile components of tea extract. This study presents a method for obtaining and analyzing the volatile components of tea extract with odor preservation using solvent-assisted flavor evaporation (SAFE) and solvent extraction followed by gas chromatography-mass spectrometry (GC-MS). SAFE is a high-vacuum distillation technique that can isolate volatile compounds from complex food matrices without any non-volatile interference. A complete step-by-step procedure for tea aroma analysis is presented in this article, including the tea infusion preparation, solvent extraction, SAFE distillation, extract concentration, and analysis by GC-MS. This procedure was applied to two tea samples (green tea and black tea), and qualitative as well as quantitative results on the volatile composition of the tea samples were obtained. This method can not only be used for the aroma analysis of various types of tea samples but also for molecular sensory studies on them.

Formation of Volatile Heterocyclic Compounds and Open-Chain Amides of Theanine in Model Systems with Glucose, Tea Leaves, and Tea Extract under Tea-Roasting Conditions.

Theanine is a non-proteinogenic amino acid found in the tea plant Camellia sinensis. At an elevated temperature (>90 °C), it released two major volatile compounds 1-ethyl-1,5-dihydro-2H-pyrrol-2-one and N-ethylsuccinimide. Other products were identified, including 10 pyrroles and 12 amides/imides. The formation of the two major compounds was proposed to be initiated by the deamination of theanine and through the intermediate α-keto acid. In the presence of glucose, the two major products and many other volatiles from theanine thermal degradation were accelerated and further Maillard reactions occurred. A total of 56 compounds were identified in the model system of theanine and glucose, including 12 amides/imides, 16 pyrazines, 16 pyrroles and other N-heterocycles, and 12 furans and other O-heterocycles. Although most of the reaction products were detected in tea leaves and in their aqueous extract with or without the addition of theanine under the same experiment conditions, imides and amides were considerably suppressed, left only minute amounts, or were even no longer detectable. Pyrazines and pyrroles were also shown at reduced concentrations as a result of the interaction with tea components but to a lesser extent. A total of 16 and 12 pyrazines were identified in the theanine/glucose reaction system and tea leaves/aqueous extract after roasting, respectively. The results indicated that pyrazines and other main volatiles in roasted tea leaves were formed from the Maillard reactions of the aqueous fraction of tea leaves. Theanine participated in the formation of pyrazines in tea leaves under roasting conditions.

Use of headspace GC/MS combined with chemometric analysis to identify the geographic origins of black tea.

Some black teas demand high market prices. Black tea samples (306) collected from 10 geographic origins, including China (Guxi, Likou, Jinzipai, Guichi, Dongzhi, Changning, Wuyishan, Shaowu), India (Darjeeling), and Sri Lanka (Kandy), were analyzed using headspace volatilization followed by GC/MS (HS-GC/MS). Forty-eight volatile compounds were identified. The aroma compounds were mainly identified as alcohols, aldehydes, ketones, and esters. Analysis of either full-spectrum data or 22 tea compounds shared among the samples with k-Nearest Neighbor (k-NN) and Random Forest (RF) models discriminated all origins at 100% using KNN and 95% with RF using either data set. The discrimination rates using 2 key aroma compounds (linalool and geraniol) by k-NN were 100% for nine origins, with the rate for Guxi area at 89%, because 3 samples were classified to Jinzipai. The findings support the use of HS-GC/MS combined with chemometrics as a tool to identify the origin of black tea.

Characterizing volatile metabolites in raw Pu'er tea stored in wet-hot or dry-cold environments by performing metabolomic analysis and using the molecular sensory science approach.

The aroma profile of raw pu'er tea (RPT) depends on its storage duration (2-10 years) and storage conditions (wet-hot or dry-cold environment). We analyzed the major odorants of RPT samples by performing metabolomic analysis and by using the molecular sensory science approach. Under dry-cold storage conditions, tea leaves had more carotenoid derivatives, glycoside-derived volatiles, and phenolic volatiles, resulting in "fresh," "floral," and "sweet" aroma. Under wet-hot storage conditions, tea leaves had more methoxybenzenes, which contributed considerably to their "stale" and "woody" aroma. We identified 11 and 4 compounds as the odor markers of RPTs when stored in dry-cold and wet-hot environments, respectively. Our findings provide a scientific basis for optimal storage that yields the desired aroma profile.

Metabolite Profiling and Transcriptome Analysis Revealed the Chemical Contributions of Tea Trichomes to Tea Flavors and Tea Plant Defenses.

Tea trichomes contain special flavor-determining metabolites; however, little is known about how and why tea trichomes produce them. Integrated metabolite and transcriptome profiling on tea trichomes in comparison with that on leaves showed that trichomes contribute to tea plant defense and tea flavor and nutritional quality. These unicellular, nonglandular, and unbranched tea trichomes produce a wide array of tea characteristic metabolites, such as UV-protective flavonoids, insect-toxic caffeine, herbivore-defensive volatiles, and theanine, as evidenced by the expression of whole sets of genes involved in different metabolic pathways. Both dry and fresh trichomes contain several volatiles and flavonols that were not found or at much low levels in trichome-removed leaves, including benzoic acid derivatives, lipid oxidation derivatives, and monoterpene derivatives. Trichomes also specifically expressed many disease signaling genes and various antiherbivore or antiabiotic peptides. Trichomes are one of the domestication traits in tea plants. Tea trichomes contribute to tea plant defenses and tea flavors.

Characterization of the Key Odorants in a High-Grade Chinese Green Tea Beverage (Camellia sinensis; Jingshan cha) by Means of the Sensomics Approach and Elucidation of Odorant Changes in Tea Leaves Caused by the Tea Manufacturing Process.

Sensory-guided analysis of the volatile fraction isolated from a freshly prepared green tea beverage (Camellia sinensis; type Jingshan cha) revealed 58 odor-active compounds after application of an aroma extract dilution analysis. Among them, 3-methylnonane-2,4-dione, (Z)-1,5-octadien-3-one, 3-(methylthio)propanal, trans-4,5-epoxy-(E)-2-decenal, methanethiol, dimethyl sulfide, and indole appeared with the highest flavor dilution factors. A quantitation of 42 aroma compounds by means of stable isotope dilution assays followed by the calculation of odor activity values (OAV; ratio of concentration to odor detection threshold) showed 27 key aroma compounds with OAVs ≥ 1. By far, the highest OAV of 458 was calculated for the asparagus-like/putrid smelling dimethyl sulfide followed by (E,E)-2,4-heptadienal (46). Finally, an aqueous recombinate containing all 27 aroma compounds in the concentrations measured in the beverage successfully mimicked the overall aroma profile of the tea infusion. Quantitative measurements were then performed on authentic tea material to elucidate changes in key aroma compounds during each processing step (fresh leaves, withering, pan-firing, rolling, and drying). The results indicated that dimethyl sulfide, one of the important aroma compounds, was significantly increased by withering of the fresh leaves, however, a major part was lost during drying. Linalool, geraniol, and hexanal showed the highest concentrations in the fresh tea leaves, while significantly lower concentrations were measured in the final tea. The same was observed for all lipid degradation products, such as (E,E)-2,4-heptadienal.

Identification of d-amino acids in tea leaves.

During manufacturing processes and in the storage period of tea, amino acids may undergo enantiomeric isomerization, converting their l- to d-forms. To examine the hypothesis, a method was developed for the analysis of the enantiomers in tea leaves. After enriched by ion-exchange solid-phase extraction, the enantiomeric pairs were separated by a chiral high performance liquid chromatography (HPLC) and subsequently detected and identified by using a high resolution quadrupole time-of-flight mass spectrometry (QTOF MS). Only l-forms of amino acids were found in fresh tea leaves. A total of 11 d-amino acids were found in 19 tea samples, ranging from trace amount to 43 µg/g. The results indicated that the enantioisomerization of amino acids occurred in post-harvest tea leaves, and affected by process conditions and storage time.

Characterization of the orchid-like aroma contributors in selected premium tea leaves.

The orchid-like odor of a tea infusion is regarded as a noble aroma and an essential sensory attribute for certain premium teas. Such tea leaves are difficult to make and the quality is not always reproducible. This study is focused on the molecular sensory basis of the orchid-like attribute in tea brews. The aroma is defined as jasmine- and magnolia-like floral notes with a fruity undertone and found to be closely related to the flower scent of the orchid Cymbidium faberi Rolfe (hui lan) by a sensory panel. Gas chromatography mass spectrometry (GC-MS) and aroma extract dilution analysis revealed that the key contributor was (Z)-methyl epijasmonate (epi-MeJA), which was also one of the main odor compounds in the flower scent of C. faberi and in the infusions of selected high-quality teas. Concentration of epi-MeJA was ranging from 0.09 to 2.2 µg/g in the oolong and green tea leaves.

Tea aroma formation from six model manufacturing processes.

Tea aroma is determined by the nature of the plant, the production processes, and many other factors influencing its formation and release. The objective of this study was to investigate the impact of manufacturing processes on the aroma composition of tea. Fresh tea leaves from the same cultivar and growing area were selected for producing the six types of tea: green, white, yellow, oolong, black, and dark teas. Comprehensive analysis by gas chromatography mass spectrometry (GC/MS) was performed for the volatiles of tea infusion, prepared by solid-phase microextraction (SPME), solid-phase extraction (SPE), and solvent assisted flavor evaporation (SAFE). A total of 168 volatile compounds were identified. Black tea has the highest volatile concentration of 710 µg/g, while green tea has the lowest concentration of 20 µg/g. Significantly affected by these processes, tea aroma molecules are formed mainly from four precursor groups: carotenoids, fatty acids, glycosides, and amino acids/sugars.

A minor, sweet cucurbitane glycoside from Siraitia grosvenorii.

A minor, sweet cucurbitane-glycoside, named iso-mogroside V (1), was isolated from Luo Han Guo (Siraitia grosvenorii (Swingle) C. Jeffery) along with five previously reported mogrosides. The structure of iso-mogroside V, 3-[(4-O-beta-D-glucopyranosyl-beta-D-glucopyranosyl)oxy]-mogrol-24-O-beta-D-glucopyranosyl-(1-->2)-O-[beta-D-glucopyranosyl-(1-->6)]beta-D-glucopyranoside, was established on the basis of extensive 2D-NMR (COSY, TOCSY, NOESY, HSQC, and HMBC) and LCMS analyses. The five known mogrosides were identified as mogroside V (2), 11-oxo-mogroside V (3), siamenoside I (4), mogrosides IVa (5) and IVe (6). Iso-mogroside V was determined to be approximately 500 times sweeter than 0.5% (w/v) sucrose.

Identification of dihydrogalangal acetate in galangal [Alpinia galangal (L.) Swartz] extracts.

Dihydrogalangal acetate has been discovered for the first time in galangal roots [Alpinia galangal (L.) Swartz]. The compound has a taste sensation similar to galangal acetatethe pungent principle of galangalbut it is more stable in food and beverage applications. Therefore, dihydrogalangal acetate provides many advantages as a flavor ingredient for alcohol enhancement and taste modification. Dihydrogalangal acetate is present in approximately 0.0005% of fresh roots and in about 0.004% of dried roots. (S)-Dihydrogalangal acetate is found as the main optical isomer in galangal roots (98%), while its minor (R)-isomer is less abundant (2%). Enantiomers of galangal acetate and dihydrogalangal acetate were separated and evaluated by sensory analysis. (R)-Galangal acetate has a very faint woody and sweet aroma, and (R)-dihydrogalangal acetate is almost odorless, while (S)-galangal acetate has strong and (S)-dihydrogalangal acetate has weak pungent and woody notes. Although the aroma characters of these optical isomers are different, taste sensations were found to have no significant differences among galangal acetate, dihydrogalangal acetate, and their optical isomers.