Bioactive Volatile Compounds from Plants - American Chemical Society

Chapter 5

Formation of Some Volatile Components of Tea

Downloaded by UNIV OF GUELPH LIBRARY on June 28, 2012 | http://pubs.acs.org Publication Date: April 6, 1993 | doi: 10.1021/bk-1993-0525.ch005

Akio Kobayashi, Kikue Kubota, and Motoko Yano Laboratory of Food Chemistry, Ochanomizu University, 2—1—1, Ohtsuka, Bunkyo-ku, Tokyo, Japan

A cluster analysis showing the complex gas chromatograms of tea volatiles correlates to aroma character, which is the main factor in classifying various teas. Some of the main volatiles were formed by enzymatic hydrolysis of the nonvolatile fraction from a hot-water extract of green tea, from which two glycosides were separated and identified as ß-D-glucosides. Optical isomers of linalool and 3,7dimethyl-1,5,7-octatrien-3-ol were present with different R-,S-ratios among the various tea volatiles. This suggests different formation mechanisms among these structurally related compounds.

Tea is probably the most popular beverage all over the world. When we use the word "tea", it means a product from only one plant species, Camellia sinensis. With development of the tea drinking custom, various types of tea have been manufactured in different countries. According to their manufacturing processes, tea can be classified into three types, i.e., fermented, semifermented and nonfermented, which are generally called black, oolong and green teas, respectively. The fermentation process in tea manufacturing does not mean microbial fermentation, but involves changes in the taste, aroma and color by an enzymatic action in tea leaves. For the production of Japanese green tea, freshly plucked tea leaves are steamed to stop enzyme activity; therefore, its aroma resembles the fresh green plant. By crushing and tearing withered tea leaves, the various enzyme activities cooperate to produce the color, taste and aroma of black tea. Contrary to these two extreme processes, oolong tea is manufactured under milder fermentation conditions which proceed in keeping tea leaves almost intact through the withering and rolling processes. Therefore, "semi-fermented" does not mean an incomplete fermentation process (7) in black tea, but involves a different enzymatic action to produce the characteristic oolong tea aroma. 0097-6156/93/0525-0049$06.00/0 © 1993 American Chemical Society

In Bioactive Volatile Compounds from Plants; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

50

BIOACTIVE VOLATILE COMPOUNDS FROM PLANTS

Table I. Tea Sample Used in a Clustar Analysis Sample

Country

Commercial Name Sample or Cultiver


Number Black Tea

Β 1 -B 8 B 9 - -B12 B13

Sri Lanka India Japan

Ceilon Tea Darjeeling Tea Benihikari

8 4 1

Oolong Tea

014 -023 024 -027 028 -029 G30 -G31 G32 G33 -G38 G39 -G44

China Taiwan Taiwan Japan Japan Japan Japan

Oolong Tea Oolong Tea Pouchong Tea Sencha Gyokuro Sencha Aracha*

10 4

Green Tea

Total

2 1 6 5 44

* made from five different cultivars

Cluster Analysis of the Tea Volatiles The development of analytical techniques has made possible the identification of more than 300 compounds as tea volatile constituents (2), and studies on these volatiles have been directed to correlate such complicated constituents to their aroma characters. The chemometric approach has often been applied to correlate chemical analysis data to a sensory evaluation of some food products. Tea is thought to be a good example for the application of this method because there are some characteristic commercially available products which have been internationally evaluated. As shown in Table I, 44 samples of typical black, oolong and green teas were collected from their producing countries for a multivariate analysis, the tea aroma volatiles being isolated under the same conditions by simultaneous distillation and extraction apparatus, and then analysed by high-resolution gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Comparison of the Gas Chromatograms. The gas chromatograms of the volatiles from three different teas are shown in Fig 1. Green tea shows the simplest gas chromatogram with about 150 separate peaks. The most complicated one is that of oolong tea with more than 250 peaks, and the gas chromatogram of black tea is characterized by the presence of several main peaks. However, almost all the peaks appearing on the green tea gas chromatogram are commonly present on the other two. Therefore, it is possible to apply a multivariate analysis to these gas chromatographic data.


5.

KOBAYASHI ET AL.

51 Formation of Some Volatile Components of Tea

2

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Black Tea 10 LS. 11


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JUJLJJUL

Oolong Tea

1

12

te

Green Tea

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10

20

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30

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40

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70

80(rrtn)

Figure 1. Gas Chromatograms of Typical Tea Volatiles Gas chromatographic conditions: 50m χ 0.25mm FS-WCOT column coated with PEG 20M. Programming of oven temperature; 60°C(4min hold) up to 180°C programmed with 2°C/min. Carrier gas; Nitrogen flow rate 1.2ml/min, Split ratio; 33:1. Compounds: l.hexanal, 2.(Z)-2-hexenal, 3.(Z)-3-hexenol, 4.1inalool oxide I, 5.1inalool oxide II, 6.1inalool, 7.3,7-dimethyl-l,5,7-octatrien-3-ol, 8.1inalool oxide IV, 9.methyl salicylate, lO.geraniol, 11.benzyl alcohol, 12.phenylethanol. Grouping of Commercial Tea by a Cluster Analysis. We picked up 77 peaks present commonly throughout the gas chromatograms, and the ratios of their peak areas to the internal standard (hexadecane) were used as variables for the analysis. The cluster analysis shown in Fig 2 (Togari,N., Ochanomizu University, Master Thesis) indicates that the GC pattern can clearly classify three different types of tea and, moreover, the grouping of a distinct tea can be correlated to a sensory evaluation or empirical knowledge about commercial tea. For example, black tea has two semi-clusters containing mainly Assam and Darjeeling teas, respectively. Green tea also has two major semi-clusters, and one of them is grouped very tightly. This implies that the green tea aroma is similar among commercially available products, and this similarity can be explained by the fact that it is impossible to change the aroma of non-fermented tea during the manufacturing process. Contrary to these two different types of tea, there are three semi-clusters


52

BIOACTIVE VOLATILE COMPOUNDS FROM PLANTS

Β : Black tea Ο : Oolong tea G : Green tea

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Bioactive Volatile Compounds from Plants - American Chemical Society

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