Food Chemistry

954 21 Coffee, Tea, Cocoa

of great importance for tea fermentation. Their
activity rises during the leaf withering and rolling
process and then drops during the fermentation
stage, probably as a consequence of reactions
of some products (e. g., o-quinones) with the
enzyme proteins.
5-Dehydroshikimate reductasewhich reversibly
interconverts dehydroshikimate and shikimate is
a key enzyme in the biosynthesis of phenolic
compounds via the phenylalanine pathway.
Phenylalanine ammonia-lyasewhich catalyzes
the cleavage of phenylalanine into transcin-
namate and NH 3 , is equally important for
the biosynthesis of phenols. Its activity in tea
leaves parallels the content of catechins and
epicatechins.
Proteinases cause protein hydrolysis during
withering, resulting in a rise in peptides and free
amino acids.
The observed oxidation of linolenic acid to (Z)-
3-hexenal, which then partly isomerizes to (E)-2-
hexenal, is catalyzed by alipoxygenaseand ahy-
droperoxide lyase(cf. 3.7.2.3) and also occurs
by autoxidation. (Z)-3-Hexenal contributes to the
aroma of green tea.
Chlorophyllasesparticipate in the degradation of
chlorophyll andtransaminasesin the production
of precursors for aroma constituents.
Demethylation of pectins bypectin methyl es-
terase(cf. 4.4.5.2) results in the formation of
a pectic acid gel, which affects cell membrane
permeability, thus resulting in a drop in the rate
of oxygen diffusion into leaves during fermenta-
tion.

21.2.5.3 Amino Acids

Free amino acids constitute about 1–3% of the
dry matter of the tea leaf. Of this, 50% is thea-
nine (5-N-ethylglutamine) and the rest consists
of protein-forming amino acids;β-alanine is also
present.
Green tea contains more theanine than black tea.
Generally, there is a characteristic difference in
amino acid content as well as difference in pheno-
lic compounds between the two types of tea (Ta-
ble 21.17).
The contribution of theanine to the taste of green
tea is discussed. Theanine biosynthesis occurs
in the plant roots from glutamic acid and ethy-

Table 21.17.Amino acids and phenolic compounds in

green and black tea (% dry matter)

Tea Phenolic Amino

compounds acids

Green tea

Prime quality (Japan) 13. 24. 8

Consumer quality (Japan) 22. 92. 1

Consumer quality (China) 25. 81. 8

Black tea

Highlands (Sri Lanka) 28. 01. 6

Plains (Sri Lanka) 30. 21. 7

lamine, the latter being derived from alanine. The

compound is then transported into the leaves. The

analogous compounds, 4-N-ethylasparagine and

5-N-methylglutamine, are present at very low lev-

els in tea leaves.

21.2.5.4 Caffeine

Caffeine constitutes 2.5–5.5% of the dry matter

of tea leaves. It is of importance for the taste of

tea. Theobromine (0.07–0.17%) and theophylline

(0.002–0.013%) are also preset but in very low

amounts. The biosynthesis of these two com-

pounds involves methylation of hypoxanthine or

xanthine:

(21.5)

21.2.5.5 Carbohydrates

Glucose (0.72%), fructose, sucrose, arabinose

and ribose are among sugars present in tea

leaves. Rhamnose and galactose are bound

to glycosides. Polysaccharides found include