Development and Applications of Research on Tea Aroma

This article provides an overview of the methods used to analyze and assess the quality of tea, including the separation of chiral compounds, identification of glycosides, quality assessment, chemotaxonomic classification of tea clones, and functional effects of tea aroma components. Different techniques and instruments such as modified cyclodextrin columns, HPLC, gas chromatography, olfactometry, and chemical sensors are utilized for the analysis and evaluation of tea. The genetic relationship among tea cultivars growing in China and the dispersion routes of tea plants in China and surrounding areas are also explored. Additionally, the functional effects of green tea aroma components such as their antibacterial and antimicrobial activities are discussed.

Separation of Chiral Compounds

Chiral compounds are important in the aroma and biological activity of tea. Chiral discrimination has been made possible by using modified cyclodextrin columns and multidimensional gas chromatographic instrumentation.

Enantiomeric purity and enantiomeric excess (ee) are terms used to describe the results of analyzing enantiomers. Different sensory properties have been found for the enantiomers of certain compounds in tea.

The method of direct capillary gas chromatographic separation of enantiomers has been reported by several researchers. They used heptakis-(2,3,6-tri-O-methyl)-ß-cyclodextrin in polysiloxane and permethylated ß-cyclodextrin as suitable chiral stationary phases.

Different cultivars of tea have been found to have different R/S ratios of linalool and linalool oxides. Enzymatic formation of glycoside precursors leads to the formation of LOs, and their configuration and odor vary for each enantiomer.

The sensory properties of methyl jasmonate and epijasmonate are different for their enantiomers, with methyl jasmonate having a lower odor threshold.

Separation and Identification of Glycosides

The analysis of glycosides in tea extract can be achieved by HPLC. To determine their structure, glycosides must first be isolated and purified. Crude glycosides are extracted with water and then separated on a Sephadex LH-20 column. The purified glycoside can then be analyzed using IR, MS, and NMR. To analyze the related aglycone, isolation is achieved through acid or enzymatic hydrolysis, followed by GC and GC-MS analyses.

Guo et al. (1992) identified the structure of linalyl glycoside as (S)-linalyl ß-primeveroside in plants. Three glycosides were isolated as aroma precursors from tea leaves: 6-O-ß-D-xylopyranosyl-ß-D-glucopyranosides (ß-primeverosides) of the aroma constituents, linalool, 2-phenylethanol, and benzyl alcohol.

Matsumura et al. (1997) investigated the role of diglycosides as tea aroma precursors. They found that the primeverosidase in tea leaves was specific to the glycoside and to the aglycon. Different glycosidase activities in various acetone powders prepared from different tea varieties have been found. Thirteen kinds of glycosides were presented as the precursors of glycosidic aroma in tea leaves.

Quality Assessment

The quality of tea can be assessed by analyzing its volatile flavor components using gas chromatography olfactometry (GCO), and specifically by using a method called aroma extract dilution analysis (AEDA).

The AEDA technique was used to compare the aroma components of green and black tea, and found that (Z)-3-hexenal and (Z)-1,5-octendien-3-one were abundant in green tea, while linalool was present in lower amounts.

Other techniques such as multivariate calibration methods, principal component regression, and partial least squares regression analysis were used to correlate sensory properties of tea with its GC profiles.

Genetic algorithms (GA) were used to optimize the combinations of aroma components in black teas, and unsupervised and supervised pattern recognition techniques were used to differentiate between green tea, oolong tea, and black tea based on their volatile components.

Chemical sensors were also used to evaluate the quality of Japanese green tea objectively.

Finally, various ratios have been used to evaluate the quality of black tea, such as the Wickremasinghe-Yamanishi ratio and the Mahanta ratio.

Chemotaxonomic Method for Classification of Tea Clones

Tea plants have many varieties, but some clones look very similar and can be confused. Takeo (1981a, b, 1983a) proposed a method called the terpene index (TI) to classify tea clones. The TI ratio is the ratio of the gas chromatographic peak areas of linalool to those of linalool plus geraniol, and it is closely related to the place of cultivation, which means it can differentiate clonal teas. A tea with a high TI ratio has a bright and brilliant aroma, while Darjeeling tea that has a low TI ratio has a solemn, rosy, and strong aroma.

Tea flowers have major aroma compounds, such as 2-pentanol, 2-heptanol, bezaldehyde, linalool, geraniol, nerol, and 2-phenylethanol. The total amount of terpenes varied during the blossoming process of tea flowers, but the TI ratio remained more constant than in tea leaves. By using the TI ratio, the genetic relationship among tea cultivars growing in China was studied, and it was found that the TI value is specific for the genetic characteristics. The dispersion routes of tea plants in China and to surrounding areas were also shown. You and Wang (1993) investigated the changes in aroma pattern of the Fuding cultivar after it was transplanted to other places in China, and the result showed that the difference of the aroma pattern of related green tea depends on the distance between the original place and the site of transplantation.

Functional Effects

Green tea aroma components have been found to exhibit antibacterial and antimicrobial activities. A study conducted by Kubo et al. (1992) examined the antimicrobial activity of the 10 most abundant volatile components of green tea flavor, and found that most of the volatiles tested inhibited the growth of Streptococcus mutans, a cariogenic bacterium. Among them, nerolidol was the most potent, while linalool was the least effective. Indole significantly enhanced the activity of δ-cadinene and caryophyllene against S. mutans and inhibited the growth of all of the gram-negative bacteria tested.

Further research by Muroi and Kubo (1993) found that combining sesquiterpene hydrocarbons (δ-cadinene and β-caryophyllene) with indole increased the bactericidal activities against S. mutans. Synergetic effects were also found for linalool, geraniol and nerolidol when they were combined with indole. 1-tridecanol was found to be the most effective for controlling S. mutans among the long-chain alcohols tested.

In addition, Kubo and Morimitsu (1995b) investigated the cytotoxicity of green tea aroma components against two human carcinoma cell lines and found that nerolidol, β-ionone, δ-cadinene, and β-caryophyllene exhibited moderate cytotoxicity in vitro.

Lastly, Young et al. (1994) found that thujone, caryophyllene and farnesol in tea flavors showed bactericidal effects on various bacteria including Escherichia coli, Enterobacter aerogenes, Vibrio parahaemolyticus, Pseudomonas aeruginosa, Bacillus subtilis and Staphylococcus aureus using the paper disc method (8 mm diameter). The mixture of caryophyllene and farnesol was more bactericidal than the mixture of thujone, caryophyllene and farnesol or each compound separately. Caryophyllene and farnesol showed a strong bactericidal effect for V. parahaemolyticus, E. aerogenes and B. subtilis.

These findings suggest that green tea essential oil may have potential as an antimicrobial agent.

Chemical Communication among Tea Plant, Tea Geometrid, and Apanteles sp. Wasp

The study looked at how chemicals released by tea plants attract insects that eat other insects. The researchers found that tea plant volatiles attracted adult tea geometrid moths, and some alcohols and C6 compounds elicited the strongest responses. Apanteles wasps, which attack tea geometrids, were more attracted to aliphatic aldehydes than alcohols, and C5-C6 compounds were the most attractive to them. The study’s results could help develop better ways to control pests in tea plantations.