2009_Book_FoodChemistry

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 reductase which reversibly
interconverts dehydroshikimate and shikimate is
a key enzyme in the biosynthesis of phenolic
compounds via the phenylalanine pathway.
Phenylalanine ammonia-lyase which catalyzes
the cleavage of phenylalanine into transcinnamate
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 a lipoxygenase and a hydroperoxide
lyase (cf. 3.7.2.3) and also occurs
by autoxidation. (Z)-3-Hexenal contributes to the
aroma of green tea.
Chlorophyllases participate in the degradation of
chlorophyll and transaminases in the production
of precursors for aroma constituents.
Demethylation of pectins by pectin methyl esterase
(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 fermentation.
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) 13.2 4.8
Consumer quality (Japan) 22.9 2.1
Consumer quality (China) 25.8 1.8
Black tea
Highlands (Sri Lanka) 28.0 1.6
Plains (Sri Lanka) 30.2 1.7
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 levels
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 compounds
involves methylation of hypoxanthine or
xanthine:
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 theanine
(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 phenolic
compounds between the two types of tea (Table
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-
21.2.5.5 Carbohydrates
(21.5)
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