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TEA | Chemistry
Tea imports for consumption for selected countries for the year 1995−97 and 1998
TEA/Chemistry 300 Yr. 1995−97
275 250 225 200 175
aa
150 125
Total
436.7 115.1 16.7 16.7 409.9 215.1 22.2 1215.7
866.7 215.4 32.6 33.3 773.7 418.8 44.0 2306.3
75 50 25
a
aa 9.4
Europe
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1998
430.0 100.3 15.9 16.6 363.8 203.7 21.8 1090.6
100
UK CIS G Po Fr
0043
1995-97
Region
Yr. 1998
Europe North America/West Indies Latin America Oceania Asia Africa Other Countries World
0
fig0003
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a 1.3 1.4
aa
a
a
aa
2.8 2.6
Ir OE Ca US WI Ch OL Au Du Su Af North America/ Latin Oceania West Indies America
aa
a 7.1
Ir
a
a
aa 5.2 5.5
Iq Jp Jo Pk Sy OAs Eg Lb Mo SA Su Tu OAf Oth Asia
Africa
Other countries
Figure 3 Tea imports for consumption for selected countries for the 1995–97 period and 1998 ( 1000 Mt) (imports adjusted for reexports). aFigures shown are provisional or estimated. UK: United Kingdom, CIS: Commonwealth of independent states (former Union of Soviet Socialist Republics), G: Germany, Po: Poland, Fr: France, Ir: Ireland, OE: Other European countries, Ca: Canada, US: United States of America, WI: West Indies, Ch: Chile, OL: Other Latin American countries, Au: Australia, Du: Dubai, Su: Saudi Arabia, Af: Afghanistan, Ir: Iran, Iq: Iraq, Jp: Japan, Jo: Jordan, Pk: Pakistan, Sy: Syria, OAs: Other Asian countries, Eg: Egypt, Lb: Libya, Mo: Morrocco, SA: South Africa, Su: Sudan, Tu: Tunisia, OAf: Other African countries. Data from the Annual Bulletin of Statistics (2000).
The FAO projections suggest an imbalance in the international market, with a surplus of export availabilities over import requirements, reaching 24 000 Mt by 2005 from an almost balanced market in 1993–95. This possible imbalance implies that world market prices would be under downward pressure if there were no additional increase in demand and/or downward adjustments in production. One of the ways to narrow the trade deficit and improve prices is to expand consumption through promotion. Over the last few years, the IGG on Tea has made major efforts to create awareness of the health benefits of tea drinking and to work out a generic promotion program that would stimulate consumption of tea. The FAO reports that auction prices of black tea rose significantly during the first quarter of 1998; however they failed to maintain this vigor during the remainder of the year. Tea prices were boosted in 1997 and early 1998 by reports of drought-induced damage to the crops in Kenya and Indonesia, as well as strong import demand in the former Soviet Union. However, following the production recovery in Kenya, and to a lesser extent in Indonesia, coupled with a sharp drop in import demand from the Russian Federation due to deterioration of economic conditions, prices weakened significantly during 1998. See also: Coffee: Decaffeination; Essential Oils: Properties and Uses; Phenolic Compounds; Tea: Chemistry; Processing
Further Reading Anonymous (1999) Committee on Commodity Problems. Intergovernmental Group on Tea. Thirteenth Session. Food and Agriculture Organization of the United Nations. Ottawa, Canada, 27–29 September 1999. Anonymous (2000) Annual Bulletin of Statistics. London: International Tea Committee. Purseglove JW (1986) Tropical Crops. Dicotyledons. London: Longman. Willson KC and Clifford MN (1992) Tea. London: Chapman & Hall.
Chemistry P O Owuor, Tea Research Foundation of Kenya, Kericho, Kenya Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Tea Tea beverages are processed from the young tender shoots of Camellia sinensis (L) O. Kuntze. The plant biosynthesizes several chemicals during growth. Table 1 summarizes the approximate chemical composition of dry tea leaves. The polyphenols, dominated by catechins (or flavan-3-ols) constitute up to 32% of the dry weight of young tender shoots. The tender tea plant shoots also contain both saturated and unsaturated fatty acids, either as glycosides or as
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5744 TEA/Chemistry tbl0001
Table 1 Approximate chemical composition of young shoots of Camellia sinensis var. Assamica Type
Components
Dry weight (%)
Water-soluble
Phenolic compounds Flavanols Epigallocatechin gallate Epigallocatechin Epicatechin gallate Epicatechin Gallocatechin Catechin Flavonol glycosides Proanthocyanidins Phenolic acids Caffeine Amino acids Theanine Others Carbohydrates Organic acids Starch Other polysaccharides Proteins Ash Cellulose Lignin Lipids Pigments Volatiles
40 18–30 9–13
Partially water-soluble
Water-insoluble
0002
3–6 3–6 1–3 1–2 1–2 3–4 2–3 4 3–4 4 2 2 4 0/5 2–5 12 15 5 7 6 4–9 0.5 0.01–0.02
free acids. There are also methlyxanthines (mainly caffeine), amino acids and proteins, terpenes, and terpene glycosides, and many endogenous primary volatile flavor compounds. Although the biosynthetic pathways of these compounds are interesting, only the chemical and biochemical processes (after the young tender shoots have been plucked to make various kinds of tea beverages) are discussed here. Of the many tea beverages, the most extensive chemical transformations occur during black tea processing. The article shall therefore reflect the chemistry of tea as represented by black tea manufacture, although, these chemical reactions partially occur to different degrees in the processing of other tea beverages. Polyphenols
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Polyphenols play a key role in the chemistry of the formation of the nonvolatile components of black tea. Although attempts to understand the chemistry of the formation of the nonvolatile black tea components started over half a century ago, to date, the chemistry is only partially understood. The young tender shoots of the tea plant used to process tea beverages contain high amounts of polyphenols,
comprising flavanols (catechins), flavanol glycosides, leucoanthocyanins, and phenolic acids, etc. The dominant polyphenols in green leaf are flavanol (Figure 1) comprising (þ)-catechin (C), (þ)-gallocatechin (GC), ()-epicatechin (EC), ()-epigallocatechin (EGC) and ()-epigallocatechin gallate (EGCG). These compounds dominate the chemistry of nonvolatile compounds in tea. Several factors, including climate, genetic make-up, age of shoots and agronomic practices, cause variations in the total amounts and ratio of these compounds in young tea shoots. Although, recently, ()-epicatechin-3,5-digallate and 3-methylgallates of ()-epicatechin and epigallocatechin have been isolated in green tea leaves, their chemistry with respect to tea processing remains unknown. (þ)-Catechin-3-gallate (CG) and (þ)-gallocatechin-3-gallate (GCG) have also been isolated in manufactured teas, but not from fresh leaves. They are likely products of epimerization or racemization caused by firing (drying). The other important component in the chemistry of nonvolatile components of tea is polyphenol oxidase. This is an o-dihidroxyphenolic oxygen reductase enzyme leading to the production of o-quinones. The enzyme is abundant in young tender shoots of green tea leaves, especially those cultivars suitable for making black teas. The most noticeable chemical transformations/ changes occur during the fermentation phase of black tea processing. Although this stage is called fermentation, in the true sense, the reactions that occur are those of oxidation. Following cell matrix destruction brought about by maceration, the catechins (flavanols) and other polyphenols undergo polyphenol oxidase initiated reactions, forming brown-colored products. The 1,2-dihydoxyphenols are oxidized to the quinones in the presence of oxygen, reacting further to form various brownish compounds. A simple catechin and a gallocatechin, for example, undergo oxidative reactions, which involve a loss of carbon dioxide to form a benzotropolone ring system. The compound formed is known as ’theaflavin.’ Figure 2 outlines the formation of theaflavin. From various combinations of the flavanols, it is possible to produce several theaflavins, as shown in Table 2. However, the dominant theaflavins found in black tea are theaflavin, theaflavin-3-gallate, theaflavin-30 -gallate and theaflavin-3,30 -digallate. The proportions of these four theaflavins in black tea vary with processing conditions and the composition of the individual precursor flavanols in green leaf. Traces of epitheaflavic acid, theaflavin acid, and epitheaflavic acid-30 -gallate have also been found in black tea. Normally, there is very little gallic acid in green leaf. However, during fermentation, its levels
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TEA/Chemistry tbl0002
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Table 2 Synthesis of theaflavins from pairs of flavanols Parent flavanols
Theaflavin
Dihydroxy ()-Epicatechin ()-Epicatechin ()-Epicatechin gallate ()-Epicatechin gallate ()-Epicatechin (þ)-Catechin ()-Epicatechin ()-Epicatechin gallate (þ)-Catechin (þ)-Catechin gallate
Trihydoxy ()-Epigallocatechin ()-Epigallocatechin gallate ()-Epigallocatechin ()-Epigallocatechin gallate (þ)-Gallocatechin (þ)-Gallocatechin (þ)-Gallic acid (þ)-Gallic acid (þ)-Gallic acid (þ)-Gallic acid
Theaflavin Theaflavin-3-gallate Theaflavin-30 -gallate Theaflavin-3,30 -digallate Isotheaflavin Neotheaflavin Epitheaflavic acid Epitheaflavic acid-30 -gallate Theaflavic acid Theaflavic acid-30 -gallate
OH
OH
OH
OH H
H HO
HO
O
O
OH
OR
OR H
R 2O
H
R2O
i (−)-Epicatechin; R1 = R2 = H
iv (−)-Epigallocatechin; R = R2 = H
ii (−)-Epicatechin-3-gallate; R1 = 3,4,5-trihydroxy-benzoyl, R2 = H
v (−)-Epigallocatechin-3-gallate; R1 = 3,4,5trihydroxybenzoyl, R2 = H
iii (−)-Epicatechin-3,5-digallate;
vi (−)-Epigallocatechin-3,5-digallate; R1 = R2 = 3,4,5-
R1 = R2 = 3,4,5-trihydroxy-benzoyl
trihydroxybenzoyl OH
OH OH
OH
H HO
H
O
H
O
HO
H
H
H HO
OR
vii (+)-Catechin; R=H viii (+)-Catechin-3-gallate; R = 3,4,5-trihydroxy-benzoyl fig0001
HO
OR
ix (+)-Gallocatechin; R=H x (+)-Gallocatechin-3-gallate; R = 3,4,5-trihydroxy-benzoyl
Figure 1 Structures of dominant flavanols in young tender shoots of tea plants.
increase, probably from the hydrolysis of galloyl esters from the catechins or gallated theaflavins. Although the rate of oxidation of gallic acid to the quinone acid is low, the presence of theaflavic acids is suspected to originate from gallic acid. Additional isomers of the theaflavins can also arise from
racemization and epimerization of the flavanols. However, since racemization and/or epimerization of flavanols occur mainly during firing, very few isomers of theaflavins are normally found in black tea. Theaflavins account for 0.3–1.8% dry weight of black tea and 1–6% of total solids in tea. They are
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5746 TEA/Chemistry HO
O HO H O O
O
O
O
HO
HO
(O)
HO
H H
HO HO
O
O
OH
O
HO
OH
O
O
OH
O
HO
OH
HO HO
HO HO
HO
HO OH HO HO
HO
HO O
HO
HO H
HO O
O HO
HO OH
O
O HOOC
O
HO
OH
HO HO fig0002
0010
Figure 2 Mechanism of theaflavin formation. From Takino Y, Imagawa H, Horikawa H, and Tanaka A (1964) Studies on the mechanism of oxidation of tea leaf catechins – formation of a reddish/orange pigment and its spectral relationship to some benzotropolone derivatives. Agricultural Biology and Chemistry 28: 64–71, with permission.
bright red pigments that give the tea liquor its characteristic ’brightness’ and ’briskness’. The contributions of the individual theaflavins to quality differ, with the gallated theaflavins being more astringent. The other set of brownish compounds formed during black tea processing are called ’thearubigins.’ Despite over 60 years’ research, a full chemical characterization of thearubigins has yet to be achieved. Indeed, even their contribution to tea quality is not fully understood. The thearubigins are a group of compounds, of which some fractions have a large molecular weight and are nondialyzable compounds. The mixture of groups of compounds normally called thearubigins has been identified to have fractions of polysaccharides, proteins, nucleic acid anthocyanidins, cyanidin, and delphinidin. Indeed, the thearubigin structure could be a polymeric proanthocyanidin. The thearubigins are thought to be polymeric products of the various polyphenols. Indeed, a 4–8 interflavanoid bonding has been considered possible after oxidation of catechin. Over 40 peaks representing pigmented components have been separated by HPLC from black tea liquor, in the range of expected thearubigins. With prolonged fermentation, theaflavins undergo oxidative degradation. Thus, apart from thearubigins being formed directly from the catechins, and breakdown of thearubigins, theaflavin intermediates may also be involved in the coupled oxidation leading to more thearubigins. Thearubigins may not be a group of compounds with a common basic structure. They may be colored compounds
resulting from catechin oxidation, catechin coupling without benzotropolone ring formation and catechin– anthocyanidin interactions. Other high-molecularweight compounds formed by the interactions between flavanol quinones with other macromolecules such as proteins, carbohydrates, and nucleic acids are also classified as thearubigins. The proposed strategy for the formation of theaflavins and thearubigins from flavanols is summarized in Figure 3. The formation of one theaflavin requires one molecule of a simple catechin and a gallocatechin. Tea shoots normally have higher amounts of the gallocatechins than of the simple catechins. This imbalance in the ratio affects the ability of particular leaves to make theaflavins, and may direct most gallocatechins to make thearubigins. The redox potentials of the individual flavanols to quinones also affect the amounts of the particular quinone available for reaction to produce theaflavin. The availability of a high polyphenol oxidase activity and oxygen are therefore critical in the direction of theaflavins to thearubigins formed under ideal conditions. However, even under ideal conditions, after some time, the formed theaflavins start degrading to thearubigins. Resultant thearubigins also degrade further or polymerize to other thearubigins. Oxygen is required in three key steps: the oxidation of catechins to quinone, benzotropolone ring formation, and oxidative degradation of theaflavins. The first two are necessary for theaflavin formation. For the formation of high amounts of theaflavins, excess
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TEA/Chemistry TRc r4
TRb
O2 EC
ECQ
r3
fig0003
0013
0014
O2 r2
EGCQ TRa
Figure 3 The formation of theaflavin and thearubigin from the catechin and possible role played by the simple catechins in coupled oxidations. EC, epicatechin; EGC, epigallocatechin; ECQ, epicatechin quinones; EGCQ, epigallocatechin quinone; TF, theaflavin; TR, thearubigin.
oxygen should therefore be available. The flavanols have a high affinity for oxygen. When oxygen is limiting, the benzotropolone ring system formation becomes inhibited, and fewer theaflavins are formed. However, during the theaflavin formation, there is also competitive degradation of theaflavins. With excess oxygen and/or when the catechin levels have been depleted, this reaction can be significant, reducing the amounts of theaflavins, but increasing thearubigins. However, this may not be the only way the thearubigins are formed. As in any chemical reaction system, the reaction temperature is critical. Fermentation of temperatures between 15 and 30 C is beneficial to theaflavin formation. However, a higher temperature fermentation facilitates thearubigin formation at the expense of theaflavins. Firing terminates the chemical reactions associated with the formation of the nonvolatile black tea components. Firing heat denatures polyphenol oxidase, reduces the moisture content to a product that can be stored, and enhances reactions responsible for black tea aroma. Firing black teas at temperatures above 100 C increase the blackness, which may be due to pyrolytic reactions. Thearubigins comprise 9–19% black tea dry weight and 30–60% soluble solids in black tea. They contribute to the ’liquor color’ and ’thickness’ or ’body.’ Methyl Xanthines
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the levels of caffeine increase slightly during withering. However, the levels do not change during fermentation and may decrease slightly during firing. A good ratio of caffeine and theaflavins in black tea imparts a brisk character to the liquors. High levels of caffeine in tea lead to the so-called ’creaming down’ in the liquors, an indication of high quality.
r1
TF
EGC
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The caffeine is the major purine alkaloid in tea leaves, but theobromine and theophylline are also found, albeit in low quantities. Dry fresh tea shoots contain about 3–4% caffeine. After the shoots are plucked,
Other Tea Components Many aroma compounds have been identified in various teas. Their biogenetic pathways in green leaf have been worked out. However, here, only the chemistry of formation of volatile flavor compounds during tea processing shall be highlighted. The aroma compounds in tea can be classified broadly into primary or secondary products. The primary products are biosynthesized by the plant, whilst the secondary products are produced during tea manufacture via enzymatic, redox, or pyrolytic reactions of carotenes, amino acids, unsaturated fatty acids plus other lipids, and terpene glycosides. Some aroma compounds constitute both primary and secondary products. The primary compounds that have been identified in fresh green teas include Z-2-penten-1-ol, n-hexanol, Z-3-hexen-1-ol, E-2hexen-1-ol, linalool plus its oxides, nerol, geraniol, benzyl alcohol, 2-phenylethanol, and nerolidol. Their quantities change after plucking and during tea processing. In the course of manufacture, the concentrations of some of the alcohols increase, possibly due to hydrolysis of their glycosides, whereas for others, there is a decrease due to oxidative reactions and glycosidation.
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Amino Acids
After a tea leaf has been plucked from the plant, the concentration of amino acids in the leaf increases as the proteins in the fresh green tea leaf break down in a process catalyzed by peptidase. The dominant amino acid in tea is theanine (5-N-ethylglutamine), accounting for 2% dry weight or 50% of total amino acids in tea. In green tea, theanine is associated with a ’brothy’ taste. High levels improve the green tea quality. In black tea, however, high levels reduce the quality. This amino acid does not undergo the chemical transformations that the a-amino acids undergo to make volatile compounds. In black tea processing, the amino acid levels, decrease during fermentation, and this is accompanied by the production of aldehydes, as outlined in Figure 4. Valine, leucine, isoleucine, and phenylalanine are converted to 2methylpropanal, 2-methylbutanal, pentanal, and phenyl acetaldehyde, respectively. These reactions
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5748 TEA/Chemistry OH HO
OH
O
Proteins
OH (−)-Epicatechin OH
Peptidase
Polyphenol oxidase or peroxidase R
O HO
H2N
O
O
Polymerized polyphenol
CH CO2H Amino acid
OH OH
Quinone −H2O O
HO
N
O
−NH3
R CO2H
CH
OH Schiff base I
OH
CO2 HO
H2O
OH
R
N
O
CH
OH OH
NH
O
OH OH
fig0004
HO
Peroxidase or polyphenol oxidase OH
NH2
O OH
R
CHO Aldehyde Alcohol dehydrogenase RCH2OH alcohol
Figure 4 Formation of aldehydes, alcohols, and carboxylic acids from amino acids.
are catalyzed by polyphenol oxidase and/or peroxidase in the presence of oxygen and catechins. Some of the formed aldehydes are further oxidized to carboxylic acids during firing (as the concentration of acids is higher in fired teas) or storage, although some of the aldehydes are certainly reduced to their respective primary alcohols. Lipids 0019
Acid Heat and/or autooxidation
OH
O HO
RCO2H
Schiff base II H2O
Lipids make up between 4 and 9% dry weight of the fresh tea leaf and are composed mainly of free fatty acids and fatty acid esters. Linolenic acid is the major fatty acid in tea, but variations in this observation have been noted. The fatty acid profile changes with the geographical area of tea production, agronomic
practices, and variety. The levels of fatty acids change throughout tea manufacture. During withering, the fatty acid esters are hydrolyzed to free fatty acids. The unsaturated free fatty acids degrade to form aroma compounds, but the fate of saturated fatty acids during tea processing is unknown. The mechanism for the degradation of unsaturated fatty acids to aroma compounds is outlined in Figure 5. Linoleic and linolenic acids produce hexanal and E-2-hexenal, respectively, when the acids are added to tea leaf extracts. Z-3-Hexenal formed from linolenic acid easily isomerizes to E-2-hexenal, and also is the precursor of Z-3-hexenol in macerated tea leaves. Alcohol dehydrogenase reduces the aldehydes to alcohols.
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Free falty acids and lipids Lipolytic acylhydrolase
CO2H Linolenic acid O2
Linolenic acid O2 Lipoxygenase
Lipoxygenase
OOH
OOH CO2H 13-Hydroperoxylinolenic acid
CO2H 13-hydroperoxylinolenic acid CO2H
Hydroperoxide lyase CO2H
OHC
CH2OH ADH
IF
Z-3-hexenal
CO2H Pyrolytic 12-OXO-(10E) or -dodecenoic acid autooxidation
Hexanoic acid
E-2-Hexenoic acid IF CO2H
Z-3-hexenoic acid Pyrolytic or autooxidation CHO ADH CH2OH
CHO Hexanal O2 OHC
CO2H
Hydroperoxide lyase O2
12-OXO-(9Z ) -dodecenoic acid
Hexenal
CO2H + other free fatty acids
O2
Z-3-hexenal
IF
IF
CHO E-2-hexenal
Pyrolytic or autooxidation
CHO E-3-hexenal
O2
ADH
Pyrolytic or autooxidation HO2C Diacid
CO2H
O2
CH2OH
E-3-hexenal Pyrolytic E-2-hexenoic acid or autooxidation CO2H
CO2H E-3-hexenoic acid fig0005
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Figure 5 Production of volatile flavor compounds from linoleic and linolenic acids.
The linoleic acid forms 13-hydroperoxy acid, which is an intermediate in the production of C6 aldehydes and alcohols in tea leaves. The hydroperoxidation of the acid occurs in the presence of lipoxygenase enzyme in a highly stereospecific manner forming only l-hydroperoxy acid. Hydroperoxide lyase breaks down the 13-hydroperoxide acid to C6 aldehyde and 12-oxo-acid. The action of this enzyme is enentioselective, breaking down only the l-hydroperoxide acids. The formation of 9-oxo-nonanoic acid from linolenic acid in tea chloroplasts, by cleavage at C-10, suggests that Z-3,Z-6-nonadienal, Z-3,Z-6-nonadienol, E-2,Z-6-nonadienal, and E-2,Z-6-nonadienol
may also be derived from linolenic acid via a similar intermediate. Similarly, cleavage at the C-10 carbon of linoleic acid might be expected to produce Z-3nonenal, E-2-nonenal and E-2-nonenol. However, only minor amounts of E-2,Z-6-nonadienal and E-2 nonenal have been detected in tea, implying that the hydroperoxidation of linoleic and linolenic acids occurs predominantly at the C-13 carbon to produce the C6 aldehydes and alcohols. Low levels of palmitoleic and oleic acids have been detected in fresh tea leaves. These fatty acids break down to form heptanal and heptanol, nonanal, and nonanol, respectively, during tea processing. The relationship between precursor fatty acid in fresh leaf
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and derived aroma compound in the processed product is rarely linear due to the various interactions that take place during processing. Linolenic acid and 13hydroperoxylinolenic acid, for example, inhibit the formation of n-hexanal from linoleic acid during tea manufacture. In addition, the aroma compounds formed have different boiling points, and more of the lower boiling compounds are lost by volatilization during processing. Terpene Glycosides 0025
There has been considerable speculation on the mechanism of formation of monoterpene alcohols during tea manufacture. It was originally thought that linalool was a product of carotene degradation. Later, it was suggested that terpene alcohols were produced from oxygenated isoprenoid hydrocarbons. However, it has now been demonstrated that linalool and geraniol are hydrolytic breakdown products of bd-terpene glycosides during tea manufacture. Indeed, many alcohols in the tea aroma are products of glycoside hydrolysis. In recent studies, several alcohol glycosides have been isolated from tea leaves. These include identified glycosides of 2-phenylethanol, all the four isomers of linalool, geraniol, benzyl alcohol, nerolidol, etc. These glycosides are hydrolyzed during tea manufacture to form their respective alcohols. Pigments
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Fresh tea leaves contain appreciable amounts of pigments, mainly chlorophyls and carotenes. Fresh tea leaves contain about 1.4 mg g1 dry weight chlorophyls a and b. During tea processing, the chlorophyls degrade to pheophytins and pheophorbides. These compounds play an important role in giving black tea its shade of color. A number of breakdown products from the phytol side-chain contribute to the aroma complex of tea. More than 15 carotenoid pigments, dominated by neoxanthin, violaxanthin, lutein and b-carotene, have been identified in fresh tea leaf. These carotenoid compounds account for about 0.5% dry weight of tea leaves. The carotenes decrease during tea processing with the resultant production of various aroma compounds. b-Carotene degrades to b-ionone, whilst b-ionone, a-ionone, 3-hydroxy-b-ionone, 3hydroxy-5,6-epoxyionone, 3,5-dihydroxy-4,5-dihydro-6,7-didehydro-a-ionone, and other terpenoid aldehydes and ketones are degradation products of other carotenes present in tea leaves. Dihydroactinidiolide, 2,2,6-trimethylcyclohexanone, 5,6epoxyionone, 2,2,6-trimethyl-6-hydroxycylohexanone, and theaspirone and possibly formed form the primary oxidation products of carotenes,
i.e., b-ionone. b-Damascenone, a-damascone, bdamascone, 3-oxo-b-ionone, 1,2-epoxy-10 ,20 dihydrob-ionone, loliolide, dehydrovomifoliol, and 3, 7-dimethyl-1,5-octadien-3,7-diol are speculated to be derived from carotenes via oxidative enzymatic reactions that take place during withering and fermentation, and pyrolytic reactions during firing. The mechanisms of these reactions have, however, not been fully worked out. The formation of these compounds is affected by the amounts of catechins present, oxidase activity, degree of mixing of the cell contents, and concentrations of the reactants. These factors change with degree of wither. Loss of carotenes has been demonstrated to increase with physical withering and fermentation process. Further pyrolytic and photo- and/or autooxidative reactions of carotenes occur during firing to produce more aroma compounds. The compounds produced from carotenes have a major effect on the aroma of tea. Flavory teas are normally produced from green leaf with high carotene contents. As research on tea aroma continues, it is inevitable that more mechanisms and pathways for the formation of tea aroma compounds will be identified. These will likely involve nonvolatile precursors, which currently are largely ignored with respect to tea aroma and quality. For example, it is known that chlorophyl degrades to phytol and other products, but the contribution of chlorophyl degradation products to tea aroma is not known. Considerable research has been directed into determining how the aroma complex changes with variations in agronomic, cultural, and manufacturing practices. Many studies have indicated the changes that occur in aroma composition by varying one parameter or the other without any attempts to quantify and classify the contribution of the aroma compounds to quality. Generally, the aroma compounds can be classified into two groups, i.e., those although important for the characteristic black tea smell, are deleterious to black tea quality when present at higher concentrations (group I compounds), and those that impart a sweet flowery aroma to tea, the presence of which is considered to be highly desirable (group II compounds). The classification of aroma compounds in group I and group II compounds has been based on either the odor characteristics or the retention time of the aroma compounds during gas chromatographic analysis. The ratio of group II to group I aroma compounds has been used to classify teas in order of flavor quality. In other studies, the ratio of terpenoid to nonterpenoid compounds has been used. Although these ratios provide the basis of a semiquantitative method for classifying teas in order of their aroma quality, the ratios must be used with
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caution as the olfactory perception limits of the aroma compounds differ widely. Some compounds may be present at very low levels yet have a large impact on aroma and vice versa. For example, methyl epijasmonate has an aroma that is 400 times stronger than that of methyl jasmonate at the same concentration. In addition, some of the compounds considered deleterious to black tea quality are important for green tea quality.
Chemistry of Tea Manufacture 0031
Several tea beverages exist. These beverages include green teas, several semifermented teas, and black teas. The chemistry occurring during their processing varies depending on the desired final product. In green tea processing, oxidative reactions, discussed above, are completely discouraged. In black tea processing, there is more extensive oxidation of the catechins, other polyphenols, amino acids, and unsaturated fatty acids. Fewer oxidative reactions occur in the processing of the semifermented teas.
Black Tea Processing Withering 0032
Processing of tea beverages starts as soon as the leaf is detached (plucked or harvested) from the plant. The polyphenol oxidase activity decreases, while the catechins levels vary. ()-Epicatechin, ()-epigallocatechin gallate, and ()-epicatechin gallate levels decline. This decline is associated with oxidative transformations. Caffeine levels rise, while protein levels decline. This decline is caused by an increase in the activity of proteolytic enzyme activity, which hydrolyzes the proteins to amino acids, with a concomitant increase in the level of free amino acids. Carotenoid compounds degrade due to photoisomerization to volatile flavor compounds. Fatty acid esters are hydrolyzed to free fatty acids that oxidize during fermentation through a lipoxygenase-initiated reaction to volatile flavor compounds associated with the green notes in tea. Terpene and other alcohol glycosides hydrolyze to simple alcohols that contribute to tea aroma. These transformations, which continue up to the point at which the leaf is macerated, are collectively called ’chemical wither.’ Usually, the leaf is subjected to moisture loss to make it more flaccid, so that maceration is easy. Moisture loss, the most visible change in the leaf before maceration, is referred to as ’physical wither.’ Chemical wither benefits mostly flavory black teas, as it improves the black tea aroma and to some extent benefits plain
5751
black tea as it reduces the level of green taste. Physical wither benefits both flavory and plain black tea quality. Hard physically withered leaves are easier to macerate and make more aromatic black teas. Withering therefore plays an important role in black tea processing. Maceration
Maceration ruptures the leaf cell structure, exposing the chemical constituents of the cells, mainly polyphenols, oxidative and degradative enzymes, lipids, amino acids, etc., to oxygen. Most importantly, catechins come into contact with the polyphenol oxidase enzyme, initiating ’tea oxidation,’ which is erroneously referred to as ’fermentation.’ Several methods are used, but the most common are the orthodox rolling, crush, tear, and curl (CTC), and Laurie tea processor (LTP) methods. The method used has a significant effect on the resultant black tea. There is less cell matrix destruction with the use of orthodox rollers than the other two. The orthodox maceration therefore leads to fewer oxidative reactions, and fermentation is slow. Cell matrix destruction is greater in the CTC and LTP maceration methods, leading to more extensive oxidation, but these teas are less aromatic, with higher plain tea quality parameters. LTP manufacture requires a softer physical wither than CTC processing.
0033
Fermentation
Most chemical transformations occur during the fermentation phase of black tea processing. These transformations are responsible for the characteristic taste and aroma products of black tea. As illustrated in Figures 2 and 3, the catechins and polyphenol oxidase form the theaflavins and thearubigins. The amino acids and fatty acids also oxidize to various volatile flavor components. For LTP and CTC manufacture, the process is complete after 60–120 min.
0034
Firing (Drying)
Firing is necessary to terminate fermentation and to dry tea for storage and transport. As a result of the temperature rise, some reactions are accelerated until the rise is adequate to denature the enzymes or moisture has been adequately removed to prevent reactions occurring, but a lot of changes occur to give black tea its character. As a result of firing, the color changes as a result of the transformation of chlorophyls to pheophytins and pheophorbides. Some caffeine is lost while the amount of volatile flavor compounds is reduced. The volatile flavor compounds that result from various pyrolytic reactions are formed during firing.
0035
0036
5752 TEA/Processing Sorting 0037
0038
0039
The fired black tea is sorted first by removal of fiber then by separation into different particle sizes. The various particle sizes define the various grades. Generally, although the grades have different chemical compositions, quality is not solely dictated by grade.
its spectral relationship to some benzotropolone derivatives. Agriculture and Biological Chemistry 28: 64–71. Yamanishi T (1999) Tea flavour. In: Jain NK (ed.) Global Advances in Tea Science, pp. 707–722. New Delhi: Aravali Books.
Other Tea
Processing
Green and Semifermented Teas
P O Owuor, Tea Research Foundation of Kenya, Kericho, Kenya
There are fewer chemical transformations in the processing of the other teas compared with black teas. In green tea processing, attempts are made to insure that there is no oxidation, especially that of the catechins. The process therefore starts with steaming or roasting to deactivate polyphenol oxidase activity. The green teas are made from tea plant varieties with a lower catechin content than those for black tea, but this level is sufficient to create astringency. The volatile components of green tea are basically those of the primary products. Partially fermented teas undergo incomplete fermentation. Several types exist. Some are processed by roasting, whereas others are subjected to hightemperature rolling. See also: Amino Acids: Properties and Occurrence; Caffeine; Carotenoids: Occurrence, Properties, and Determination; Chlorophyl; Sensory Evaluation: Aroma; Tannins and Polyphenols
Further Reading Owuor PO (1992) A comparison of gas chromatographic volatile profiling methods for assessing the flavour quality of Kenyan black teas. Journal of the Science of Food and Agriculture 59: 189–197. Owuor PO (1995) Results from factory processing and black tea quality research in Kenya (1982–1994). Tea 16: 62–69. Robertson A (1992) The chemistry and biochemistry of black tea production – the non-volatiles. In: Willson KC and Clifford MN (eds) Tea: Cultivation to Consumption, 1st edn, pp. 555–601. London: Chapman & Hall. Robinson JM and Owuor PO (1992) Tea aroma. In: Willson KC and Clifford MN (eds) Tea: Cultivation to Consumption, 1st edn, pp. 603–647. London: Chapman & Hall. Sakata K, Gou W and Moon JH (1999) Tea chemistry with special reference to tea aroma precursors. In: Jain NK (ed.) Global Advances in Tea Science, pp. 693–706. New Delhi: Aravali Books International. Takino Y, Imagawa H, Horikawa H and Tanaka A (1964) Studies on the mechanism of oxidation of tea leaf catechins – formation of a reddish/orange pigment and
Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Tea Beverages produced from tea leaves include black tea, green tea, and various partially fermented teas such as oolong and pouchong. All of these products are cultivated and harvested using similar procedures, but variations in manufacturing methods determine the final product. This article deals with tea processing from cultivation through to packaging.
0001
Cultivation Tea was introduced into many countries of the world from South-east Asia and grows in climates ranging from the Mediterranean to the hot humid tropics. Commercially viable plantations have been established between as far north as Turkey and Georgia (24 N), and as far south as Argentina (27 S), at altitudes ranging from sea level to 2700 m. Successful commercial cultivation of tea requires a minimum annual rainfall of about 1400 mm when irrigation is not carried out. Rainfall needs to be well distributed with at least 120 mm per month. Prolonged drought adversely affects tea growth, and in such conditions, irrigation is advocated. The interaction of soil texture and rainfall distribution is an important factor to be considered when assessing the suitability of an area for tea. Tea does not tolerate water-logged conditions. Ambient temperatures of 12–30 C are considered ideal for growing tea. Temperatures above 30 C, accompanied by low humidity, have been shown to inhibit active growth. The optimum soil temperature for active growth within the feeder root depth is 20–25 C. Using long-term average yield data from Kenyan tea estates, situated at altitudes between 1500 and 2250 m, it has been shown that annual tea production falls by 200 kg of black tea per hectare for every 100 m rise in altitude. This has been attributed to decreases in air and soil temperature. The tea plant cannot withstand frost conditions. Night
0002
0003
0004
TEA/Chemistry 300 Yr. 1995−97
275 250 225 200 175
aa
150 125
Total
436.7 115.1 16.7 16.7 409.9 215.1 22.2 1215.7
866.7 215.4 32.6 33.3 773.7 418.8 44.0 2306.3
75 50 25
a
aa 9.4
Europe
0044
1998
430.0 100.3 15.9 16.6 363.8 203.7 21.8 1090.6
100
UK CIS G Po Fr
0043
1995-97
Region
Yr. 1998
Europe North America/West Indies Latin America Oceania Asia Africa Other Countries World
0
fig0003
5743
a 1.3 1.4
aa
a
a
aa
2.8 2.6
Ir OE Ca US WI Ch OL Au Du Su Af North America/ Latin Oceania West Indies America
aa
a 7.1
Ir
a
a
aa 5.2 5.5
Iq Jp Jo Pk Sy OAs Eg Lb Mo SA Su Tu OAf Oth Asia
Africa
Other countries
Figure 3 Tea imports for consumption for selected countries for the 1995–97 period and 1998 ( 1000 Mt) (imports adjusted for reexports). aFigures shown are provisional or estimated. UK: United Kingdom, CIS: Commonwealth of independent states (former Union of Soviet Socialist Republics), G: Germany, Po: Poland, Fr: France, Ir: Ireland, OE: Other European countries, Ca: Canada, US: United States of America, WI: West Indies, Ch: Chile, OL: Other Latin American countries, Au: Australia, Du: Dubai, Su: Saudi Arabia, Af: Afghanistan, Ir: Iran, Iq: Iraq, Jp: Japan, Jo: Jordan, Pk: Pakistan, Sy: Syria, OAs: Other Asian countries, Eg: Egypt, Lb: Libya, Mo: Morrocco, SA: South Africa, Su: Sudan, Tu: Tunisia, OAf: Other African countries. Data from the Annual Bulletin of Statistics (2000).
The FAO projections suggest an imbalance in the international market, with a surplus of export availabilities over import requirements, reaching 24 000 Mt by 2005 from an almost balanced market in 1993–95. This possible imbalance implies that world market prices would be under downward pressure if there were no additional increase in demand and/or downward adjustments in production. One of the ways to narrow the trade deficit and improve prices is to expand consumption through promotion. Over the last few years, the IGG on Tea has made major efforts to create awareness of the health benefits of tea drinking and to work out a generic promotion program that would stimulate consumption of tea. The FAO reports that auction prices of black tea rose significantly during the first quarter of 1998; however they failed to maintain this vigor during the remainder of the year. Tea prices were boosted in 1997 and early 1998 by reports of drought-induced damage to the crops in Kenya and Indonesia, as well as strong import demand in the former Soviet Union. However, following the production recovery in Kenya, and to a lesser extent in Indonesia, coupled with a sharp drop in import demand from the Russian Federation due to deterioration of economic conditions, prices weakened significantly during 1998. See also: Coffee: Decaffeination; Essential Oils: Properties and Uses; Phenolic Compounds; Tea: Chemistry; Processing
Further Reading Anonymous (1999) Committee on Commodity Problems. Intergovernmental Group on Tea. Thirteenth Session. Food and Agriculture Organization of the United Nations. Ottawa, Canada, 27–29 September 1999. Anonymous (2000) Annual Bulletin of Statistics. London: International Tea Committee. Purseglove JW (1986) Tropical Crops. Dicotyledons. London: Longman. Willson KC and Clifford MN (1992) Tea. London: Chapman & Hall.
Chemistry P O Owuor, Tea Research Foundation of Kenya, Kericho, Kenya Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Tea Tea beverages are processed from the young tender shoots of Camellia sinensis (L) O. Kuntze. The plant biosynthesizes several chemicals during growth. Table 1 summarizes the approximate chemical composition of dry tea leaves. The polyphenols, dominated by catechins (or flavan-3-ols) constitute up to 32% of the dry weight of young tender shoots. The tender tea plant shoots also contain both saturated and unsaturated fatty acids, either as glycosides or as
0001
5744 TEA/Chemistry tbl0001
Table 1 Approximate chemical composition of young shoots of Camellia sinensis var. Assamica Type
Components
Dry weight (%)
Water-soluble
Phenolic compounds Flavanols Epigallocatechin gallate Epigallocatechin Epicatechin gallate Epicatechin Gallocatechin Catechin Flavonol glycosides Proanthocyanidins Phenolic acids Caffeine Amino acids Theanine Others Carbohydrates Organic acids Starch Other polysaccharides Proteins Ash Cellulose Lignin Lipids Pigments Volatiles
40 18–30 9–13
Partially water-soluble
Water-insoluble
0002
3–6 3–6 1–3 1–2 1–2 3–4 2–3 4 3–4 4 2 2 4 0/5 2–5 12 15 5 7 6 4–9 0.5 0.01–0.02
free acids. There are also methlyxanthines (mainly caffeine), amino acids and proteins, terpenes, and terpene glycosides, and many endogenous primary volatile flavor compounds. Although the biosynthetic pathways of these compounds are interesting, only the chemical and biochemical processes (after the young tender shoots have been plucked to make various kinds of tea beverages) are discussed here. Of the many tea beverages, the most extensive chemical transformations occur during black tea processing. The article shall therefore reflect the chemistry of tea as represented by black tea manufacture, although, these chemical reactions partially occur to different degrees in the processing of other tea beverages. Polyphenols
0003
Polyphenols play a key role in the chemistry of the formation of the nonvolatile components of black tea. Although attempts to understand the chemistry of the formation of the nonvolatile black tea components started over half a century ago, to date, the chemistry is only partially understood. The young tender shoots of the tea plant used to process tea beverages contain high amounts of polyphenols,
comprising flavanols (catechins), flavanol glycosides, leucoanthocyanins, and phenolic acids, etc. The dominant polyphenols in green leaf are flavanol (Figure 1) comprising (þ)-catechin (C), (þ)-gallocatechin (GC), ()-epicatechin (EC), ()-epigallocatechin (EGC) and ()-epigallocatechin gallate (EGCG). These compounds dominate the chemistry of nonvolatile compounds in tea. Several factors, including climate, genetic make-up, age of shoots and agronomic practices, cause variations in the total amounts and ratio of these compounds in young tea shoots. Although, recently, ()-epicatechin-3,5-digallate and 3-methylgallates of ()-epicatechin and epigallocatechin have been isolated in green tea leaves, their chemistry with respect to tea processing remains unknown. (þ)-Catechin-3-gallate (CG) and (þ)-gallocatechin-3-gallate (GCG) have also been isolated in manufactured teas, but not from fresh leaves. They are likely products of epimerization or racemization caused by firing (drying). The other important component in the chemistry of nonvolatile components of tea is polyphenol oxidase. This is an o-dihidroxyphenolic oxygen reductase enzyme leading to the production of o-quinones. The enzyme is abundant in young tender shoots of green tea leaves, especially those cultivars suitable for making black teas. The most noticeable chemical transformations/ changes occur during the fermentation phase of black tea processing. Although this stage is called fermentation, in the true sense, the reactions that occur are those of oxidation. Following cell matrix destruction brought about by maceration, the catechins (flavanols) and other polyphenols undergo polyphenol oxidase initiated reactions, forming brown-colored products. The 1,2-dihydoxyphenols are oxidized to the quinones in the presence of oxygen, reacting further to form various brownish compounds. A simple catechin and a gallocatechin, for example, undergo oxidative reactions, which involve a loss of carbon dioxide to form a benzotropolone ring system. The compound formed is known as ’theaflavin.’ Figure 2 outlines the formation of theaflavin. From various combinations of the flavanols, it is possible to produce several theaflavins, as shown in Table 2. However, the dominant theaflavins found in black tea are theaflavin, theaflavin-3-gallate, theaflavin-30 -gallate and theaflavin-3,30 -digallate. The proportions of these four theaflavins in black tea vary with processing conditions and the composition of the individual precursor flavanols in green leaf. Traces of epitheaflavic acid, theaflavin acid, and epitheaflavic acid-30 -gallate have also been found in black tea. Normally, there is very little gallic acid in green leaf. However, during fermentation, its levels
0004
0005
0006
0007
0008
TEA/Chemistry tbl0002
5745
Table 2 Synthesis of theaflavins from pairs of flavanols Parent flavanols
Theaflavin
Dihydroxy ()-Epicatechin ()-Epicatechin ()-Epicatechin gallate ()-Epicatechin gallate ()-Epicatechin (þ)-Catechin ()-Epicatechin ()-Epicatechin gallate (þ)-Catechin (þ)-Catechin gallate
Trihydoxy ()-Epigallocatechin ()-Epigallocatechin gallate ()-Epigallocatechin ()-Epigallocatechin gallate (þ)-Gallocatechin (þ)-Gallocatechin (þ)-Gallic acid (þ)-Gallic acid (þ)-Gallic acid (þ)-Gallic acid
Theaflavin Theaflavin-3-gallate Theaflavin-30 -gallate Theaflavin-3,30 -digallate Isotheaflavin Neotheaflavin Epitheaflavic acid Epitheaflavic acid-30 -gallate Theaflavic acid Theaflavic acid-30 -gallate
OH
OH
OH
OH H
H HO
HO
O
O
OH
OR
OR H
R 2O
H
R2O
i (−)-Epicatechin; R1 = R2 = H
iv (−)-Epigallocatechin; R = R2 = H
ii (−)-Epicatechin-3-gallate; R1 = 3,4,5-trihydroxy-benzoyl, R2 = H
v (−)-Epigallocatechin-3-gallate; R1 = 3,4,5trihydroxybenzoyl, R2 = H
iii (−)-Epicatechin-3,5-digallate;
vi (−)-Epigallocatechin-3,5-digallate; R1 = R2 = 3,4,5-
R1 = R2 = 3,4,5-trihydroxy-benzoyl
trihydroxybenzoyl OH
OH OH
OH
H HO
H
O
H
O
HO
H
H
H HO
OR
vii (+)-Catechin; R=H viii (+)-Catechin-3-gallate; R = 3,4,5-trihydroxy-benzoyl fig0001
HO
OR
ix (+)-Gallocatechin; R=H x (+)-Gallocatechin-3-gallate; R = 3,4,5-trihydroxy-benzoyl
Figure 1 Structures of dominant flavanols in young tender shoots of tea plants.
increase, probably from the hydrolysis of galloyl esters from the catechins or gallated theaflavins. Although the rate of oxidation of gallic acid to the quinone acid is low, the presence of theaflavic acids is suspected to originate from gallic acid. Additional isomers of the theaflavins can also arise from
racemization and epimerization of the flavanols. However, since racemization and/or epimerization of flavanols occur mainly during firing, very few isomers of theaflavins are normally found in black tea. Theaflavins account for 0.3–1.8% dry weight of black tea and 1–6% of total solids in tea. They are
0009
5746 TEA/Chemistry HO
O HO H O O
O
O
O
HO
HO
(O)
HO
H H
HO HO
O
O
OH
O
HO
OH
O
O
OH
O
HO
OH
HO HO
HO HO
HO
HO OH HO HO
HO
HO O
HO
HO H
HO O
O HO
HO OH
O
O HOOC
O
HO
OH
HO HO fig0002
0010
Figure 2 Mechanism of theaflavin formation. From Takino Y, Imagawa H, Horikawa H, and Tanaka A (1964) Studies on the mechanism of oxidation of tea leaf catechins – formation of a reddish/orange pigment and its spectral relationship to some benzotropolone derivatives. Agricultural Biology and Chemistry 28: 64–71, with permission.
bright red pigments that give the tea liquor its characteristic ’brightness’ and ’briskness’. The contributions of the individual theaflavins to quality differ, with the gallated theaflavins being more astringent. The other set of brownish compounds formed during black tea processing are called ’thearubigins.’ Despite over 60 years’ research, a full chemical characterization of thearubigins has yet to be achieved. Indeed, even their contribution to tea quality is not fully understood. The thearubigins are a group of compounds, of which some fractions have a large molecular weight and are nondialyzable compounds. The mixture of groups of compounds normally called thearubigins has been identified to have fractions of polysaccharides, proteins, nucleic acid anthocyanidins, cyanidin, and delphinidin. Indeed, the thearubigin structure could be a polymeric proanthocyanidin. The thearubigins are thought to be polymeric products of the various polyphenols. Indeed, a 4–8 interflavanoid bonding has been considered possible after oxidation of catechin. Over 40 peaks representing pigmented components have been separated by HPLC from black tea liquor, in the range of expected thearubigins. With prolonged fermentation, theaflavins undergo oxidative degradation. Thus, apart from thearubigins being formed directly from the catechins, and breakdown of thearubigins, theaflavin intermediates may also be involved in the coupled oxidation leading to more thearubigins. Thearubigins may not be a group of compounds with a common basic structure. They may be colored compounds
resulting from catechin oxidation, catechin coupling without benzotropolone ring formation and catechin– anthocyanidin interactions. Other high-molecularweight compounds formed by the interactions between flavanol quinones with other macromolecules such as proteins, carbohydrates, and nucleic acids are also classified as thearubigins. The proposed strategy for the formation of theaflavins and thearubigins from flavanols is summarized in Figure 3. The formation of one theaflavin requires one molecule of a simple catechin and a gallocatechin. Tea shoots normally have higher amounts of the gallocatechins than of the simple catechins. This imbalance in the ratio affects the ability of particular leaves to make theaflavins, and may direct most gallocatechins to make thearubigins. The redox potentials of the individual flavanols to quinones also affect the amounts of the particular quinone available for reaction to produce theaflavin. The availability of a high polyphenol oxidase activity and oxygen are therefore critical in the direction of theaflavins to thearubigins formed under ideal conditions. However, even under ideal conditions, after some time, the formed theaflavins start degrading to thearubigins. Resultant thearubigins also degrade further or polymerize to other thearubigins. Oxygen is required in three key steps: the oxidation of catechins to quinone, benzotropolone ring formation, and oxidative degradation of theaflavins. The first two are necessary for theaflavin formation. For the formation of high amounts of theaflavins, excess
0011
0012
TEA/Chemistry TRc r4
TRb
O2 EC
ECQ
r3
fig0003
0013
0014
O2 r2
EGCQ TRa
Figure 3 The formation of theaflavin and thearubigin from the catechin and possible role played by the simple catechins in coupled oxidations. EC, epicatechin; EGC, epigallocatechin; ECQ, epicatechin quinones; EGCQ, epigallocatechin quinone; TF, theaflavin; TR, thearubigin.
oxygen should therefore be available. The flavanols have a high affinity for oxygen. When oxygen is limiting, the benzotropolone ring system formation becomes inhibited, and fewer theaflavins are formed. However, during the theaflavin formation, there is also competitive degradation of theaflavins. With excess oxygen and/or when the catechin levels have been depleted, this reaction can be significant, reducing the amounts of theaflavins, but increasing thearubigins. However, this may not be the only way the thearubigins are formed. As in any chemical reaction system, the reaction temperature is critical. Fermentation of temperatures between 15 and 30 C is beneficial to theaflavin formation. However, a higher temperature fermentation facilitates thearubigin formation at the expense of theaflavins. Firing terminates the chemical reactions associated with the formation of the nonvolatile black tea components. Firing heat denatures polyphenol oxidase, reduces the moisture content to a product that can be stored, and enhances reactions responsible for black tea aroma. Firing black teas at temperatures above 100 C increase the blackness, which may be due to pyrolytic reactions. Thearubigins comprise 9–19% black tea dry weight and 30–60% soluble solids in black tea. They contribute to the ’liquor color’ and ’thickness’ or ’body.’ Methyl Xanthines
0015
the levels of caffeine increase slightly during withering. However, the levels do not change during fermentation and may decrease slightly during firing. A good ratio of caffeine and theaflavins in black tea imparts a brisk character to the liquors. High levels of caffeine in tea lead to the so-called ’creaming down’ in the liquors, an indication of high quality.
r1
TF
EGC
5747
The caffeine is the major purine alkaloid in tea leaves, but theobromine and theophylline are also found, albeit in low quantities. Dry fresh tea shoots contain about 3–4% caffeine. After the shoots are plucked,
Other Tea Components Many aroma compounds have been identified in various teas. Their biogenetic pathways in green leaf have been worked out. However, here, only the chemistry of formation of volatile flavor compounds during tea processing shall be highlighted. The aroma compounds in tea can be classified broadly into primary or secondary products. The primary products are biosynthesized by the plant, whilst the secondary products are produced during tea manufacture via enzymatic, redox, or pyrolytic reactions of carotenes, amino acids, unsaturated fatty acids plus other lipids, and terpene glycosides. Some aroma compounds constitute both primary and secondary products. The primary compounds that have been identified in fresh green teas include Z-2-penten-1-ol, n-hexanol, Z-3-hexen-1-ol, E-2hexen-1-ol, linalool plus its oxides, nerol, geraniol, benzyl alcohol, 2-phenylethanol, and nerolidol. Their quantities change after plucking and during tea processing. In the course of manufacture, the concentrations of some of the alcohols increase, possibly due to hydrolysis of their glycosides, whereas for others, there is a decrease due to oxidative reactions and glycosidation.
0016
0017
Amino Acids
After a tea leaf has been plucked from the plant, the concentration of amino acids in the leaf increases as the proteins in the fresh green tea leaf break down in a process catalyzed by peptidase. The dominant amino acid in tea is theanine (5-N-ethylglutamine), accounting for 2% dry weight or 50% of total amino acids in tea. In green tea, theanine is associated with a ’brothy’ taste. High levels improve the green tea quality. In black tea, however, high levels reduce the quality. This amino acid does not undergo the chemical transformations that the a-amino acids undergo to make volatile compounds. In black tea processing, the amino acid levels, decrease during fermentation, and this is accompanied by the production of aldehydes, as outlined in Figure 4. Valine, leucine, isoleucine, and phenylalanine are converted to 2methylpropanal, 2-methylbutanal, pentanal, and phenyl acetaldehyde, respectively. These reactions
0018
5748 TEA/Chemistry OH HO
OH
O
Proteins
OH (−)-Epicatechin OH
Peptidase
Polyphenol oxidase or peroxidase R
O HO
H2N
O
O
Polymerized polyphenol
CH CO2H Amino acid
OH OH
Quinone −H2O O
HO
N
O
−NH3
R CO2H
CH
OH Schiff base I
OH
CO2 HO
H2O
OH
R
N
O
CH
OH OH
NH
O
OH OH
fig0004
HO
Peroxidase or polyphenol oxidase OH
NH2
O OH
R
CHO Aldehyde Alcohol dehydrogenase RCH2OH alcohol
Figure 4 Formation of aldehydes, alcohols, and carboxylic acids from amino acids.
are catalyzed by polyphenol oxidase and/or peroxidase in the presence of oxygen and catechins. Some of the formed aldehydes are further oxidized to carboxylic acids during firing (as the concentration of acids is higher in fired teas) or storage, although some of the aldehydes are certainly reduced to their respective primary alcohols. Lipids 0019
Acid Heat and/or autooxidation
OH
O HO
RCO2H
Schiff base II H2O
Lipids make up between 4 and 9% dry weight of the fresh tea leaf and are composed mainly of free fatty acids and fatty acid esters. Linolenic acid is the major fatty acid in tea, but variations in this observation have been noted. The fatty acid profile changes with the geographical area of tea production, agronomic
practices, and variety. The levels of fatty acids change throughout tea manufacture. During withering, the fatty acid esters are hydrolyzed to free fatty acids. The unsaturated free fatty acids degrade to form aroma compounds, but the fate of saturated fatty acids during tea processing is unknown. The mechanism for the degradation of unsaturated fatty acids to aroma compounds is outlined in Figure 5. Linoleic and linolenic acids produce hexanal and E-2-hexenal, respectively, when the acids are added to tea leaf extracts. Z-3-Hexenal formed from linolenic acid easily isomerizes to E-2-hexenal, and also is the precursor of Z-3-hexenol in macerated tea leaves. Alcohol dehydrogenase reduces the aldehydes to alcohols.
0020
0021
TEA/Chemistry
5749
Free falty acids and lipids Lipolytic acylhydrolase
CO2H Linolenic acid O2
Linolenic acid O2 Lipoxygenase
Lipoxygenase
OOH
OOH CO2H 13-Hydroperoxylinolenic acid
CO2H 13-hydroperoxylinolenic acid CO2H
Hydroperoxide lyase CO2H
OHC
CH2OH ADH
IF
Z-3-hexenal
CO2H Pyrolytic 12-OXO-(10E) or -dodecenoic acid autooxidation
Hexanoic acid
E-2-Hexenoic acid IF CO2H
Z-3-hexenoic acid Pyrolytic or autooxidation CHO ADH CH2OH
CHO Hexanal O2 OHC
CO2H
Hydroperoxide lyase O2
12-OXO-(9Z ) -dodecenoic acid
Hexenal
CO2H + other free fatty acids
O2
Z-3-hexenal
IF
IF
CHO E-2-hexenal
Pyrolytic or autooxidation
CHO E-3-hexenal
O2
ADH
Pyrolytic or autooxidation HO2C Diacid
CO2H
O2
CH2OH
E-3-hexenal Pyrolytic E-2-hexenoic acid or autooxidation CO2H
CO2H E-3-hexenoic acid fig0005
0022
0023
Figure 5 Production of volatile flavor compounds from linoleic and linolenic acids.
The linoleic acid forms 13-hydroperoxy acid, which is an intermediate in the production of C6 aldehydes and alcohols in tea leaves. The hydroperoxidation of the acid occurs in the presence of lipoxygenase enzyme in a highly stereospecific manner forming only l-hydroperoxy acid. Hydroperoxide lyase breaks down the 13-hydroperoxide acid to C6 aldehyde and 12-oxo-acid. The action of this enzyme is enentioselective, breaking down only the l-hydroperoxide acids. The formation of 9-oxo-nonanoic acid from linolenic acid in tea chloroplasts, by cleavage at C-10, suggests that Z-3,Z-6-nonadienal, Z-3,Z-6-nonadienol, E-2,Z-6-nonadienal, and E-2,Z-6-nonadienol
may also be derived from linolenic acid via a similar intermediate. Similarly, cleavage at the C-10 carbon of linoleic acid might be expected to produce Z-3nonenal, E-2-nonenal and E-2-nonenol. However, only minor amounts of E-2,Z-6-nonadienal and E-2 nonenal have been detected in tea, implying that the hydroperoxidation of linoleic and linolenic acids occurs predominantly at the C-13 carbon to produce the C6 aldehydes and alcohols. Low levels of palmitoleic and oleic acids have been detected in fresh tea leaves. These fatty acids break down to form heptanal and heptanol, nonanal, and nonanol, respectively, during tea processing. The relationship between precursor fatty acid in fresh leaf
0024
5750 TEA/Chemistry
and derived aroma compound in the processed product is rarely linear due to the various interactions that take place during processing. Linolenic acid and 13hydroperoxylinolenic acid, for example, inhibit the formation of n-hexanal from linoleic acid during tea manufacture. In addition, the aroma compounds formed have different boiling points, and more of the lower boiling compounds are lost by volatilization during processing. Terpene Glycosides 0025
There has been considerable speculation on the mechanism of formation of monoterpene alcohols during tea manufacture. It was originally thought that linalool was a product of carotene degradation. Later, it was suggested that terpene alcohols were produced from oxygenated isoprenoid hydrocarbons. However, it has now been demonstrated that linalool and geraniol are hydrolytic breakdown products of bd-terpene glycosides during tea manufacture. Indeed, many alcohols in the tea aroma are products of glycoside hydrolysis. In recent studies, several alcohol glycosides have been isolated from tea leaves. These include identified glycosides of 2-phenylethanol, all the four isomers of linalool, geraniol, benzyl alcohol, nerolidol, etc. These glycosides are hydrolyzed during tea manufacture to form their respective alcohols. Pigments
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Fresh tea leaves contain appreciable amounts of pigments, mainly chlorophyls and carotenes. Fresh tea leaves contain about 1.4 mg g1 dry weight chlorophyls a and b. During tea processing, the chlorophyls degrade to pheophytins and pheophorbides. These compounds play an important role in giving black tea its shade of color. A number of breakdown products from the phytol side-chain contribute to the aroma complex of tea. More than 15 carotenoid pigments, dominated by neoxanthin, violaxanthin, lutein and b-carotene, have been identified in fresh tea leaf. These carotenoid compounds account for about 0.5% dry weight of tea leaves. The carotenes decrease during tea processing with the resultant production of various aroma compounds. b-Carotene degrades to b-ionone, whilst b-ionone, a-ionone, 3-hydroxy-b-ionone, 3hydroxy-5,6-epoxyionone, 3,5-dihydroxy-4,5-dihydro-6,7-didehydro-a-ionone, and other terpenoid aldehydes and ketones are degradation products of other carotenes present in tea leaves. Dihydroactinidiolide, 2,2,6-trimethylcyclohexanone, 5,6epoxyionone, 2,2,6-trimethyl-6-hydroxycylohexanone, and theaspirone and possibly formed form the primary oxidation products of carotenes,
i.e., b-ionone. b-Damascenone, a-damascone, bdamascone, 3-oxo-b-ionone, 1,2-epoxy-10 ,20 dihydrob-ionone, loliolide, dehydrovomifoliol, and 3, 7-dimethyl-1,5-octadien-3,7-diol are speculated to be derived from carotenes via oxidative enzymatic reactions that take place during withering and fermentation, and pyrolytic reactions during firing. The mechanisms of these reactions have, however, not been fully worked out. The formation of these compounds is affected by the amounts of catechins present, oxidase activity, degree of mixing of the cell contents, and concentrations of the reactants. These factors change with degree of wither. Loss of carotenes has been demonstrated to increase with physical withering and fermentation process. Further pyrolytic and photo- and/or autooxidative reactions of carotenes occur during firing to produce more aroma compounds. The compounds produced from carotenes have a major effect on the aroma of tea. Flavory teas are normally produced from green leaf with high carotene contents. As research on tea aroma continues, it is inevitable that more mechanisms and pathways for the formation of tea aroma compounds will be identified. These will likely involve nonvolatile precursors, which currently are largely ignored with respect to tea aroma and quality. For example, it is known that chlorophyl degrades to phytol and other products, but the contribution of chlorophyl degradation products to tea aroma is not known. Considerable research has been directed into determining how the aroma complex changes with variations in agronomic, cultural, and manufacturing practices. Many studies have indicated the changes that occur in aroma composition by varying one parameter or the other without any attempts to quantify and classify the contribution of the aroma compounds to quality. Generally, the aroma compounds can be classified into two groups, i.e., those although important for the characteristic black tea smell, are deleterious to black tea quality when present at higher concentrations (group I compounds), and those that impart a sweet flowery aroma to tea, the presence of which is considered to be highly desirable (group II compounds). The classification of aroma compounds in group I and group II compounds has been based on either the odor characteristics or the retention time of the aroma compounds during gas chromatographic analysis. The ratio of group II to group I aroma compounds has been used to classify teas in order of flavor quality. In other studies, the ratio of terpenoid to nonterpenoid compounds has been used. Although these ratios provide the basis of a semiquantitative method for classifying teas in order of their aroma quality, the ratios must be used with
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TEA/Chemistry
caution as the olfactory perception limits of the aroma compounds differ widely. Some compounds may be present at very low levels yet have a large impact on aroma and vice versa. For example, methyl epijasmonate has an aroma that is 400 times stronger than that of methyl jasmonate at the same concentration. In addition, some of the compounds considered deleterious to black tea quality are important for green tea quality.
Chemistry of Tea Manufacture 0031
Several tea beverages exist. These beverages include green teas, several semifermented teas, and black teas. The chemistry occurring during their processing varies depending on the desired final product. In green tea processing, oxidative reactions, discussed above, are completely discouraged. In black tea processing, there is more extensive oxidation of the catechins, other polyphenols, amino acids, and unsaturated fatty acids. Fewer oxidative reactions occur in the processing of the semifermented teas.
Black Tea Processing Withering 0032
Processing of tea beverages starts as soon as the leaf is detached (plucked or harvested) from the plant. The polyphenol oxidase activity decreases, while the catechins levels vary. ()-Epicatechin, ()-epigallocatechin gallate, and ()-epicatechin gallate levels decline. This decline is associated with oxidative transformations. Caffeine levels rise, while protein levels decline. This decline is caused by an increase in the activity of proteolytic enzyme activity, which hydrolyzes the proteins to amino acids, with a concomitant increase in the level of free amino acids. Carotenoid compounds degrade due to photoisomerization to volatile flavor compounds. Fatty acid esters are hydrolyzed to free fatty acids that oxidize during fermentation through a lipoxygenase-initiated reaction to volatile flavor compounds associated with the green notes in tea. Terpene and other alcohol glycosides hydrolyze to simple alcohols that contribute to tea aroma. These transformations, which continue up to the point at which the leaf is macerated, are collectively called ’chemical wither.’ Usually, the leaf is subjected to moisture loss to make it more flaccid, so that maceration is easy. Moisture loss, the most visible change in the leaf before maceration, is referred to as ’physical wither.’ Chemical wither benefits mostly flavory black teas, as it improves the black tea aroma and to some extent benefits plain
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black tea as it reduces the level of green taste. Physical wither benefits both flavory and plain black tea quality. Hard physically withered leaves are easier to macerate and make more aromatic black teas. Withering therefore plays an important role in black tea processing. Maceration
Maceration ruptures the leaf cell structure, exposing the chemical constituents of the cells, mainly polyphenols, oxidative and degradative enzymes, lipids, amino acids, etc., to oxygen. Most importantly, catechins come into contact with the polyphenol oxidase enzyme, initiating ’tea oxidation,’ which is erroneously referred to as ’fermentation.’ Several methods are used, but the most common are the orthodox rolling, crush, tear, and curl (CTC), and Laurie tea processor (LTP) methods. The method used has a significant effect on the resultant black tea. There is less cell matrix destruction with the use of orthodox rollers than the other two. The orthodox maceration therefore leads to fewer oxidative reactions, and fermentation is slow. Cell matrix destruction is greater in the CTC and LTP maceration methods, leading to more extensive oxidation, but these teas are less aromatic, with higher plain tea quality parameters. LTP manufacture requires a softer physical wither than CTC processing.
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Fermentation
Most chemical transformations occur during the fermentation phase of black tea processing. These transformations are responsible for the characteristic taste and aroma products of black tea. As illustrated in Figures 2 and 3, the catechins and polyphenol oxidase form the theaflavins and thearubigins. The amino acids and fatty acids also oxidize to various volatile flavor components. For LTP and CTC manufacture, the process is complete after 60–120 min.
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Firing (Drying)
Firing is necessary to terminate fermentation and to dry tea for storage and transport. As a result of the temperature rise, some reactions are accelerated until the rise is adequate to denature the enzymes or moisture has been adequately removed to prevent reactions occurring, but a lot of changes occur to give black tea its character. As a result of firing, the color changes as a result of the transformation of chlorophyls to pheophytins and pheophorbides. Some caffeine is lost while the amount of volatile flavor compounds is reduced. The volatile flavor compounds that result from various pyrolytic reactions are formed during firing.
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5752 TEA/Processing Sorting 0037
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The fired black tea is sorted first by removal of fiber then by separation into different particle sizes. The various particle sizes define the various grades. Generally, although the grades have different chemical compositions, quality is not solely dictated by grade.
its spectral relationship to some benzotropolone derivatives. Agriculture and Biological Chemistry 28: 64–71. Yamanishi T (1999) Tea flavour. In: Jain NK (ed.) Global Advances in Tea Science, pp. 707–722. New Delhi: Aravali Books.
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Processing
Green and Semifermented Teas
P O Owuor, Tea Research Foundation of Kenya, Kericho, Kenya
There are fewer chemical transformations in the processing of the other teas compared with black teas. In green tea processing, attempts are made to insure that there is no oxidation, especially that of the catechins. The process therefore starts with steaming or roasting to deactivate polyphenol oxidase activity. The green teas are made from tea plant varieties with a lower catechin content than those for black tea, but this level is sufficient to create astringency. The volatile components of green tea are basically those of the primary products. Partially fermented teas undergo incomplete fermentation. Several types exist. Some are processed by roasting, whereas others are subjected to hightemperature rolling. See also: Amino Acids: Properties and Occurrence; Caffeine; Carotenoids: Occurrence, Properties, and Determination; Chlorophyl; Sensory Evaluation: Aroma; Tannins and Polyphenols
Further Reading Owuor PO (1992) A comparison of gas chromatographic volatile profiling methods for assessing the flavour quality of Kenyan black teas. Journal of the Science of Food and Agriculture 59: 189–197. Owuor PO (1995) Results from factory processing and black tea quality research in Kenya (1982–1994). Tea 16: 62–69. Robertson A (1992) The chemistry and biochemistry of black tea production – the non-volatiles. In: Willson KC and Clifford MN (eds) Tea: Cultivation to Consumption, 1st edn, pp. 555–601. London: Chapman & Hall. Robinson JM and Owuor PO (1992) Tea aroma. In: Willson KC and Clifford MN (eds) Tea: Cultivation to Consumption, 1st edn, pp. 603–647. London: Chapman & Hall. Sakata K, Gou W and Moon JH (1999) Tea chemistry with special reference to tea aroma precursors. In: Jain NK (ed.) Global Advances in Tea Science, pp. 693–706. New Delhi: Aravali Books International. Takino Y, Imagawa H, Horikawa H and Tanaka A (1964) Studies on the mechanism of oxidation of tea leaf catechins – formation of a reddish/orange pigment and
Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Tea Beverages produced from tea leaves include black tea, green tea, and various partially fermented teas such as oolong and pouchong. All of these products are cultivated and harvested using similar procedures, but variations in manufacturing methods determine the final product. This article deals with tea processing from cultivation through to packaging.
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Cultivation Tea was introduced into many countries of the world from South-east Asia and grows in climates ranging from the Mediterranean to the hot humid tropics. Commercially viable plantations have been established between as far north as Turkey and Georgia (24 N), and as far south as Argentina (27 S), at altitudes ranging from sea level to 2700 m. Successful commercial cultivation of tea requires a minimum annual rainfall of about 1400 mm when irrigation is not carried out. Rainfall needs to be well distributed with at least 120 mm per month. Prolonged drought adversely affects tea growth, and in such conditions, irrigation is advocated. The interaction of soil texture and rainfall distribution is an important factor to be considered when assessing the suitability of an area for tea. Tea does not tolerate water-logged conditions. Ambient temperatures of 12–30 C are considered ideal for growing tea. Temperatures above 30 C, accompanied by low humidity, have been shown to inhibit active growth. The optimum soil temperature for active growth within the feeder root depth is 20–25 C. Using long-term average yield data from Kenyan tea estates, situated at altitudes between 1500 and 2250 m, it has been shown that annual tea production falls by 200 kg of black tea per hectare for every 100 m rise in altitude. This has been attributed to decreases in air and soil temperature. The tea plant cannot withstand frost conditions. Night
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