Food Chemistry

21.2 Tea and Tea-Like Products 953

Table 21.15.Composition (%, dry weight basis) of
fresh and fermented tea leaves and of tea brew

Constituent Fresh Black tea Black tea
flush brewa

Phenolic
compoundsb 30 5 4. 5
Oxidized phenolic
compoundsc 025 15
Protein 15 15 +d
Amino acids 4 4 3. 5
Caffeine 4 4 3. 2
Crude fiber 26 26 0
Other carbohydrates 7 7 4
Lipids 7 7 +
Pigmentse 22 +
Volatile compounds 0. 10. 10. 1
Minerals 5 5 4. 5

aBrewing time 3 min.bMostly flavanols.cMostly

thearubigins.dTraces.eChlorophyll and carotenoids.

uble in hot water; this is significantly more than
for roasted coffee.

21.2.5.1 Phenolic Compounds (cf. 18.1.2.5)

Phenolic compounds make up 25–35% of the

dry matter content of young, fresh tea leaves.
Flavanol compounds (Table 21.16) are 80% of
the phenols, while the remainder is proantho-
cyanidins, phenolic acids, flavonols and flavones.
During fermentation the flavanols are oxidized
enzymatically to compounds which are respon-
sible for the color and flavor of black tea. The
reddish-yellow color of black tea extract is largely
due to theaflavins and thearubigins (cf. 21.2.6).
The astringent taste is caused primarily by fla-
vonol-3-glycosides. Quercetin-3-O-[α-L-rhamno-
pyranosyl-(1→6)-β-D-glucopyranside] is especi-
ally active with a threshold value of 0.001 μmol/l.
Also of importance are (threshold values):
kaempferol-3-O-[α-L-rhamnopyranosyl-(1→6)-
O-β-D-glucopyranoside] (0.25 μmol/l), querce-
tin-3-O-β-15-D-galactopyranoside (0.43 μmol/l),
quercetin-3-O-β-D-glucoside (0.65 μmol/l) and
kaempferol-3-O-β-D-glucopyranoside (0. 67
μmol/l). The enzymes are inactivated in green
tea, hence flavanol oxidation is prevented. The
greenish or yellowish color of green tea is due

Table 21.16.Phenolic compounds in fresh tea leaves

(% dry matter)

Compound Content

(−)-Epicatechin 1–3

(−)-Epicatechin gallate 3–6

(−)-Epicatechin digallate +a

(−)-Epigallocatechin 3–6

(−)-Epigallocatechin gallate 9–13

(−)-Epigallocatechin digallate +

(+)-Catechin 1–2

(+)-Gallocatechin 3–4

Flavonols and flavonolglycosides

(quercetin, kaempherol, etc.) +

Flavones

(vitexin, etc.) +

Leucoanthocyanins 2–3

Phenolic acids and esters

(gallic acid, chlorogenic acids)

p-Coumaroylquinic acid, theogallin ∼ 5

Phenols, grand total 25–35

aQuantitative data are not available.

to the presence of flavonols and flavones. Thus,

tea which is processed into green or black tea is

chemically readily distinguishable mainly by the

composition of phenolic compounds. Green tea

contains 17.5% and black tea 14.4% of polyphe-

nols (expressed in gallic acid equivalents). The

main components in green tea are the catechins

(90% of the polyphenol fraction), which account

for only 25% in black tea.

Changes in the content of the phenols occur dur-

ing tea leaf growth on the shrub: the concentra-

tion decreases and the composition of this frac-

tion is altered. Therefore, good quality tea is ob-

tained only from young leaves. Among the re-

maining phenolic compounds theogallin (XI in

Formula 18.14) plays a special role, since it is

found only in tea and is correlated with tea qual-

ity.

21.2.5.2 Enzymes

A substantial part of the protein fraction in tea

consists of enzymes.

The polyphenol oxidases, which are located

mainly within the cells of leaf epidermis, are