The Chemistry of Tea - American Chemical Society

Chapter 32

The Chemistry of Tea Chi-Tang H o and Nanqun Z h u

Downloaded by PRINCETON UNIV on November 13, 2014 | http://pubs.acs.org Publication Date: January 15, 2000 | doi: 10.1021/bk-2000-0754.ch032

Department of Food Science, Rutgers University, 65 Dudley Road, New Brunswick, N J 08901-8520

Tea is one of the most popular beverages consumed worldwide. It is the brew prepared from the leaves of the plant Camellia sinensis. Freshly harvested tea leaves require processsing to convert them into green, oolong and black teas. A n estimated 2.5 million metric tons of dried tea are manufactured annually. The distinctive flavor and color of various teas are due to the chemical changes that occur during processing. The two major groups of compounds in tea leaves are catechins and methylxanthins.

Tea refers to the plant Camellia sines is, its leaves, and the extracts and infusions thereof. Camellia sinesis was first cultivated in China and then in Japan. With the opening of ocean routes to the East by European traders during the fifteenth to seventeenth centuries, commercial cultivation gradually expanded to Indonesia and then to the Indian subcontinent, including Sri Lanka (/). Tea is now second only to water in worldwide consumption. Annual production of about 1.8 million tons of dried leaf provides world per capita consumption of 40 liters of beverages (2). Black and green teas are the two main types, defined by their respective manufacturing techniques. Green tea is consumed mostly in Asian countries such as China and Japan, while black tea is more popular in North America and Europe. Oolong tea is an intermediate variant (partially fermented) between green and black tea. Its production is confined to some regions of China including Taiwan.

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© 2000 American Chemical Society

In Caffeinated Beverages; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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Commercial Tea Processing Tea leaf, like all other plant leaf matter, contains carbohydrate, protein, lipids, full complement of genetic material, enzymes and secondary matabolites. In addition, tea leaf is distinguished by its high content of methylxanthins and polyphenols, in particular catechins (flavanols). Table 1 shows a representative analysis of fresh tea leaf. 25% of its dry weight are catechins, 3.0% are flavonols and flavonol glycosides, 3.0% are caffeine and 0.2% are theobromine (2).


Table 1. Composition of Fresh Tea Leaf Components Flavanols Flavonols and flavonol glycosides Polyphenolic acids and depsides Other polyphenols Caffeine Theobromine Amino acids Organic acids Monosaccharides Polysaccharides Cellulose Protein Lignin Lipids Chlorophyll and other pigments Ash Volatiles

% of dry weight 25.0 3.0 5.0 3.0 3.0 0.2 4.0 0.5 4.0 13.0 7.0 15.0 6.0 3.0 0.5 5.0 0.1

S O U R C E : Reproduced from Reference 2. Copyright 1998 C R C Press.

Green Tea In the processing of green tea, the tea flesh is first steamed in the case of Japanese green tea "sen-cha" or pan-fired to produce Chinese green tea (J). These heat treatments inactivate enzymes in the tea leaves. The temperature of pan-firing can reach as high as 230°C which is much higher than the steaming temperature of 100°C. Steaming, therefore, results in fewer chemical changes than pan-firing. Following heat treatment, tea leaves are subjected to subsequent rolling and drying processes to achieve a dry product exhibiting the desired twisted leaf appearance.


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Black Tea More than 75% of the world tea production is black tea. The steps involved in the processing of black tea include withering, leaf disruption, fermentation, drying and grading. A l l steps are designed to achieve optimal oxidation of tea catechins and produce tea products with good flavor and color. Withering step is used for the partial removal of moisture in the tea leaves. The moisture is reduced to about 60-70% of the leaf weight. There are many methods used for leaf disruption such as rolling, cutting, crushing and tearing. The basic requirements are size reduction and cell disruption. This initiates the fermentation process. Fermentation in tea processing simply refers to an enzymatic browning reaction catalyzed by the polyphenol oxidase. After the leaf disruption stage, the leaves are allowed contact with the surrounding air for about 1-3 hours. Fermentaion has great impact on the quality of brew. The final step of black tea processing is drying. In this step, tea is dried by exposing the leaves to a stream of hot air. Moisture is reduced to 2-3%.

Oolong Tea Oolong tea is prepared by firing the leaves after rolling to terminate the oxidaton process. It is only partially oxidized and retains a considrable amount of the original polyphenols (4).

Major Chemical Changes During Tea Processing Catechins are the predominate form of flavonoids in fresh tea leaves. Catechins are characterized by di- or trihydroxy group substitution of the Β ring and the meta5,7-dihydroxy substitution of the A ring of the flavonoid structure (Figure 1). The principal catechins presented in fresh tea leaves are (-)-epicatechin (EC), (-)epigallocatechin (EGC), (-)-epicatechin gallate (ECG) and (-)-epigallocatechin gallate (EGCG) (Figure 2). 3'

Figure 1. Basic flavonoid structure.



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(-)-Epicatechin (-)-Epicatechin gallate (-)-Epigallocatechin (-)-Epigallocatechin gallate

EC ECG EGC EGCG

Ri H Gallate H Gallate

R H H Gallate Gallate 2

Figure 2. Major catechins in tea. The major chemical reaction during tea manufacturing is the oxidative conversion and polymeriation of catechins. The oxidative fermentation of catechins in tea results in the development of appropriate flavor and color of oolong and black teas. It will cause a darking of the leaf and a decrease in astrigency. The initial step of fermentation is the oxidation of catechins to reactive quinones catalyzed by polyphenol oxidase. Polyphenol oxidase can use any of the catechins as a substrate to form the complex polyphenolic constituents found in oolong and black teas. The major condensation compounds are theaflavins and thearubigins. Other products, such as theaflavic acids, theaflagallins, theasinensins, oolongtheanin, and theaflavate A have also been found to be present in black and oolong teas. The possible oxidation pathways of tea catechins during fermentation can be divided into (a) pyrogallolcatechol condensation (Figure 3) and (b) pyrogallol-pyrogallol condensation (Figure 4) (5).

theasinensins

and/or

-COOH

theaflavins

R ' : H or G

Figure 3. Possible pyrogallol-catechol condensation pathway of tea catechins during fermentation



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and/or - C O O H

Figure 4. Possible pyrogallol-pyrogallol during fermentation.

R ' : H or G

condensation pathway of tea catechins

Formation of Theaflavins Theaflavins, which give the characteristic bright orange-red color of black tea, account for 1-2% dry weight of the water extractable fraction. Theaflavins consist of four major components (Figure 5): theaflavin (TF), theafîavin-3-gallate (TF3G), theaflavin-3'-gallate (TF3'G) and theaflavin-3,3-digallate (TFDG), which are formed by the pair oxidation of catechins as follows (6): EC + EGC -» TF E C + E G C G -> TF3G E C G + E G C -> TF3'G E C G + E G C G -> T F D G


321 This series of reactions has been established by the use of model tea fermentation systems (7). Total theaflavin content has been shown to have correlation with the quality of black tea (6).


OH

Theaflavin Theaflavin 3-gallate Theaflavin 3 -gallate Theaflavin 3,3-gallate f

TF TF3G TF3'G TFDG

Ri H Gallate H Gallate

R H H Gallate Gallate 2

Figure 5. Structures of theaflavins.

Formation of Thearubigins Thearubigens are by far the major components of black tea extract. They constitute as much as 10-20% of the dry weight of black tea. They are the major oxidation product of catechins during fermentation, however, due to the difficulty encountered in their separation, the thearubigin chemistry is poorly characterized.

Theasinensins and Oolongtheanin Formation Theasinensins are compounds formed by the coupling of quinone with catechol or the pyrogallol ring of the flavan-3-ol dereivatives. Seven theasinensins (Figure 6) have been isolated and identified from oolong tea (5). They occur only in very small quantities in black tea, presumably because of high reactivity (2). Another condensation product of catechin, with a two fused furan ring structure has been identified in oolong tea and was named oolongtheanin (Figure 7).



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A Β C D E F G

Ri OH OH OH OH OH OH H

R G G H G H G G

2

R G H H G H G G

3

Figure 6. Structures of theasinensins A-G identified in Oolong tea.

Figure 7. Structure of Oolongtheanin identified in Oolong tea.


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Flavor Changes During Tea Processing Flavor is the most important factor on determining the quality of tea. Flavor involves both taste and aroma. The balance of astringency, bitterness and brothy taste is important to the characteristic taste of tea (8). The major contributors to astringency and bitterness of tea are catechins and caffeine. Table II lists the threshold levels for the astringency and bitterness of tea polyphenols (9).


Table II. Threshold Levels for Astringency and Bitterness of Tea Polyphenols

Phenolic compound (-)-EC (-)-ECG (-)-EGC (-)-EGCG Theaflavin Theaflavin monogallate (natural mixture) Theaflavin digallate Gallic acid

Threshold level Astrigency Not astringent 50 Not astringent 60 80 36 12.5 Not astringent

(mg/lOOmL) Bitterness 60 20 35 30 75-100 30-50 Not determined Not bitter

S O U R C E : Modified from Reference 9.

Table III shows the concentrations of catechins, theaflavins, thearubigins and highly polymerized substances in green, oolong and black teas. These differences contribute significantly to the taste differences among these teas.

Table III. Concentrations (%) of Major Polyphenols in Different Teas (8) Compound (-)-EC (-)-ECG (-)-EGC (-)-EGCG Theaflavins Thearubigins HPS

Green tea 0.74-1.00 1.67-2.47 2.60-3.36 7.00-7.53 -

Oolong tea 0.21-0.33 0.99-1.66 0.92-1.08 2.93-3.75 -

-

-

Note: H P S , Highly polymerized substance. Copyright 1995 Marcel Dekker, Inc.

Black tea 0.29-0.42

0.39-0.60 0.98-2.12 7.63-8.03 7.27-7.66

S O U R C E : Reproduced from Reference 8.

Pure caffeine is bitter and the detection threshold is about 3 ppm in water (8). The caffeine concentration in tea brew ranges from 2-4% (10). Part of the caffeine


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Table IV. Odorants of Green and Black Teas Identified on the Basis of AEDA (13) Compound

Odor quality

(Z)-Octa-1,5-dien-3-one

Geranium-like, metallic seasoning-like strawy, fruity biscuit-like leaf-like cucumber-like mushroom-like deep-fried boiled, apple-like caramel-like sweaty metallic, green deep-fried coconut-like honey-like fatty, green burnt vanilla-like coconut-like floral buttery, rancid honey-like boiled, meat-like sweet green malty buttery fatty

3-Hydroxy-4,5-dimethyl-2(5H)-fliranone 3-Methylnonane-2,4-dione (Z)-Hept-4-enal (Z)-Hex-3-enal (E, Z)-Nona-2,6-dienal Oct-l-en-3-one (£,£)-Deca-2,4-dienaI (£)-p-Damascenone 4-Hydroxy-2,5-dimethyl-3(2H)-ruranone 2-/3-Methylbutanoic acid trans-4,5-Epoxydec-2-enal (£,£)-Nona-2,4-dienal Unknown 2-Phenylethanol (£)-Non-2-enal 2-Methoxyphenol Vanillin δ-Decalactone Linalool Butyric acid Phenylacetaldehyde Bis(2-methyl-3-furyl)disulfide Phenylacetic acid Hexanal 3-Methylbutanal Butane-2,3-dione Octanal

FD Green tea 512

Factor Black tea 256

512 512 256 128 128 128 128 128 64 64 64 64 64 32 32 16 16 16 8 8 8

The Chemistry of Tea - American Chemical Society

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