2009_Book_FoodChemistry

956 21 Coffee, Tea, Cocoa
the peroxidation of unsaturated fatty acids, play
a role in black tea and are even more important in
green tea (Table 21.19). Thus, (Z)-l,5-octadien-3-
one, (Z)-3-hexenal and 3-methyl-2,4-nonandione
(MND) are responsible for the green and hay-like
notes in the aroma profile of this tea. Linolenic
acid is the precursor of the first two carbonyl compounds.
MND is a degradation product of furan
fatty acids (cf. 3.7.2.1.5) and is present in tea in
the concentrations shown in Table 21.19. A comparison
of the values for tea and for the beverage
made from it (Table 21.19) shows that the extraction
yield for most of the aroma substances
is >50%. β-Damascenone is an exception with
a yield of 11%.
21.2.5.9 Minerals
Tea contains about 5% minerals. The major element
is potassium, which is half the total mineral
content. Some tea varieties contain fluorine in
higher amounts (0.015–0.03%).
21.2.6 Reactions Involved in the Processing
of Tea
Changes in tea constituents begin during the
withering step of processing. Enzymatic protein
hydrolysis yields amino acids of which a part is
transaminated to the corresponding keto acids.
Both types of acids provide a precursor pool
for aroma substances. The induced chlorophyll
degradation has significance for the appearance
of the end-product. A more extensive
conversion of chlorophyll into chlorophyllide,
a reaction catalyzed by the enzyme chlorophyllase
(cf. 17.1.2.9.1) is undesirable since it
gives rise to pheophorbides (brown) and not to
the desired oliveblack pheophytins. Increased
cell permeability during withering favors the
fermentation procedure. As already mentioned,
a uniform distribution of polyphenol oxidases in
tea leaves is achieved during the conditioning
step of processing.
During rolling, the tea leaf is macerated and
the substrate and enzymes are brought together;
a prerequisite for fermentation. The subsequent
enzymatic oxidative reactions are designated as
“fermentation”. This term is a misnomer and
originates from the time when the participation of
microorganisms was assumed. In this processing
step, the pigments are formed primarily as
a result of phenolic oxidation by the polyphenol
oxidases. In addition, oxidation of amino acids,
carotenoids and unsaturated fatty acids, preferentially
by oxidized phenols, is of importance for
the formation of odorants.
Harler (1963) described tea aroma development
during processing: “The aroma of the leaf
changes as fermentation proceeds. Withered leaf
has the smell of apples. When rolling (or leaf
maceration) begins, this changes to one of pears,
which then fades and the acrid smell of the green
leaf returns. Later, a nutty aroma develops and,
finally, a sweet smell, together with a flowery
smell if flavor is present.”
The enzymatic oxidation of flavanols via the
corresponding o-quinones gives theaflavins
(Formula 21.6, IX–XII: bright red color, good
solubility), bisflavanols (XIII–XV: colorless),
and epitheaflavic acids (XVI, XVII: bright red
color, excellent solubility). The theaflavins and
epitheaflavic acids are important ben-zotropolone
derivatives that impart color to black tea.
A second, obviously heterogenous group of compounds,
found in tea after the enzymatic oxidation
of flavanols, are the thearubigins (XVIII,
XIX), a group of compounds responsible for the
characteristic reddish-yellow color of black tea
extracts (cf. 18.1.2.5.2, Formula 18.21). On the
whole, the phenol fraction of black tea consists of
the following main components (g/kg): thearubigens
(59.5), epigallocatechingallates (16.5), epigallocatechin
(10.5), epicatechingallate (8.0) and
theaflavin gallate (6.6).
Aroma development during fermentation is accompanied
by an increase in the volatile compounds
typical of black tea. They are produced
by Strecker degradation reactions of amino acids
with oxidized flavanols (Formula 21.7) and by oxidation
of unsaturated fatty acids and the carotinoid
neoxanthin.
During the firing step of tea processing, there is
an initial rise in enzyme activity (10–15% of the
theaflavins are formed during the first 10 min),
then all the enzymes are inactivated. Conversion
of chlorophyll into pheophytin is involved in reactions
leading to the black color of tea. A prerequisite
for these reactions is high temperature