Tea chemistry

Tea Chemistry

7/20/2012 Manoj Solanki

Introduction
Tea refers to:
– the plant Camellia sinensis (Thea sinensis)
– the dried, processed leaf manufactured from it
– extracts derived from the leaf, and
– the beverages prepared from the leaf or extract of
the species
Originated in Southeast Asia in an area that includes
China and India, and probably Myanmar, Laos and
Vietnam


Tea varieties
Two major varieties of Camellia sinensis are
recognized:
→Chinese variety (sinensis) a smaller-leaved (5-12 cm)
plant

→Assam variety (assamica) a large-leaved (15-20 cm)
plant


Cont…..
The first harvest is obtained after 4-5 years
The shrub can be used for 60-70 years

Harvesting season depends upon the region and climate
and lasts for 8-9 months per year or leaves can be plucked
at intervals of 6-9 days all year round

The younger the plucked leaves, the better the tea quality

The white-haired bud and the two adjacent youngest leaves
are plucked (famous “two-leaves and bud” formula)


Types of tea
 Black Tea The enzymatic oxidation
of tea leaves is referred to
 Green Tea as fermentation
If the enzymes are
 Yellow Tea allowed to act, they turn
green leaf black (black tea)
 Red Tea (Oolong)
If the enzymes are
inactivated by heat, as in
blanching, then the leaf
remains green (green tea)


Manufacture of black tea

Withering Rolling Fermentation Firing
• Reduces • It establishes • Conversion of Ends
moisture from proper colourless fermentation
about 75-80% conditions for catechins to a process and
to 55-65% enzymatic complex mixture reduce moisture
• Moisture oxidation of the of yellow-orange content to 3%
reduction flavanols by to red-brown
converts the atmospheric substances
turgid leaf to a oxygen
flaccid material
that is easily
handled


Manufacture of green tea Picking

 In contrast to black tea manufacture,
Steaming (95°C)
withering and fermentation stages
are omitted in green tea processing
Rolling (75ºC)

Drying (90°C)

Grading

7/20/2012 Manoj Solanki Green Tea

Composition of Tea (%, dry weight basis)
Black Tea
Constituent Fresh Tea Black Tea
Brew *
Phenolic compounds 30 5 4.5
Oxidized phenolic compounds 0 25 15
Protein 15 15 Traces
Amino acids 4 4 3.5
Caffeine 4 4 3.2
Crude fiber 26 26 0
Other carbohydrates 7 7 4
Lipids 7 7 Traces
Pigments 2 2 Traces
(chlorophyll and caroteniods)
Volatile compounds 0.1 0.1 0.1
Minerals 5 5 4.5
7/20/2012 Brewing
* for 3 minutes Manoj Solanki

Phenolic Compounds
• Phenolic compounds make up 25–35% of the dry
matter content of young, fresh tea leaves.
• Flavanol compounds are 80% of the phenols
• During fermentation the flavanols are oxidized
enzymatically to compounds which are responsible
for the color and flavor of black tea.
• The reddish-yellow color of black tea extract is
largely due to theaflavins and thearubigins


Flavonoid
• Flavonoids (or bioflavonoids) - are a class of plant secondary
metabolites.
• They can be classified into:

• flavonoids, derived from 2-phenylchromen-4-one (2-phenyl-1,4-
benzopyrone) structure (examples: quercetin, rutin).
• isoflavonoids, derived from 3-phenylchromen-4-one (3-phenyl-1,4-
benzopyrone) structure
• neoflavonoids, derived from 4-phenylcoumarine (4-phenyl-1,2-
benzopyrone) structure.

• The three flavonoid classes above are all ketone-containing
compounds, and as such, are flavonoids and flavonols. The terms
flavonoid and bioflavonoid have also been more loosely used to
describe non-ketone polyhydroxy polyphenol compounds which are
more specifically termed flavanoids, flavan-3-ols (or catechins).


Flavanol
• Flavan-3-ols (flavanols) are a class
of flavonoids – term is mainly used for non-
ketone polyhydroxy polyphenols – Flavanoids
• These compounds include the catechins and
the catechin gallates.

Epicatechin (EC)

Chemical structure of Flavan-3-ol

7/20/2012 Manoj Solanki Epigallocatechin (EGC)

Flavanol
• Flavanols (with an "a") are not to be confused
with flavonol (with an "o"), another class
of flavonoids containing a ketone group.

Catechins are the main phenolic compounds present
in fresh tea leaves:
Catechin  Gallocatechin
Epicatechin  Epigallocatechin
Epicatechin gallate  Epigallocatechin gallate
Epicatechin digallate  Epigallocatechin digallate


Flavonols
• Flavonols (with an "o") are a class
of flavonoids that have the 3- Backbone of a flavanol
hydroxyflavone backbone

• Their diversity stems from the different
positions the phenolic -OH groups. They are
distinct from flavanols (with an "a", like
catechin), another class of flavonoids.


Flavonols

Flavone backbone
Quercetin Epicatechin


GREEN TEA


Black Tea
• During fermentation the flavanols are oxidized
enzymatically to compounds which are responsible for
the color and flavor of black tea.
• The reddish-yellow color of black tea extract is largely
due to theaflavins and thearubigins.
• The astringent taste is caused primarily by flavonol-3-
glycosides.
• The catechins are turned from the monomer structure
to become the dimers that are the theaflavins and the
oligomers that are thearubigins

Black Tea

Thearubigins are polymeric
polyphenols that are formed
during the enzymatic
oxidation
Theaflavin
There are chiefly 3 types of theaflavins in
black tea, namely
 Theaflavin (TF-1),
 Theaflavin-3-gallate (TF-2),
 Theaflavin-3,3-digallate (TF-3).
A number of studies have been done on
their possible health effects with positive
results

Enzymes
1) Polyphenol oxidases- located within cell of leaf
epidermis & activity rises during withering & rolling
2) Shikimate dehydrogenase- reversibly interconverts
dehydroshikimase & shikimate via phenylalanine
pathway
3) Phenylalanine ammonia lyase- catalyse cleavage of
phnylalanine into ammonia & cinnamate
4) Proteinases – cause protein hydrolysis during withering
resulting in rise of peptides & free amino acids
5) Chlorophyllases participate in the degradation of
chlorophyll and transaminases in the production of
precursors for aroma constituents.

Amino acids
5-N-ethyl-glutamine

• Constitute about 1% of dry matter of tea
leaves
• Of this 50% is theanine & rest consists of
protein forming amino acids
• Β- alanine is also present
• Green tea contains more theanine than black
tea


Caffeine
• Constitutes 2.5-5.5% of dry matter of tea
leaves – importance in the taste of tea

• Theobromine ( 0.07-0.17%) & theophylline
( 0.002- 0.013%) are also present

7/20/2012 Manoj Solanki Caffeine

Carbohydrates
• Sugars in tea leaves are:
Glucose (0.72%)
Fructose (0.4%)
Sucrose (0.09%)
Arabinose
Ribose
 Rhamnose & galactose are bound to
glycosides


Lipids
• Level is around 7%
• Polar fraction (glycerophospholipids) in young
leaves are predominant
• Glycolipid predominate in older leaves


Pigments
 Chlorophyll is degraded during tea processing.
 Chlorophyllides and pheophorbides (brownish in
color) are present in fermented leaves, both being
converted to pheophytines (black) during the firing
step.
 Fourteen carotenoids have been identified in tea
leaves. The main carotenoids are xanthophylls,
neoxanthin, violaxanthin and β-carotene
 The content decreases during the processing of black
tea.


Minerals
• Contains 5% minerals
• Major element is potassium ( half of mineral
content )
• Copper is a constituent of tea catechol oxidase
• Approx 12-18 ppm of copper is necessary to
produce enough catechol oxidase for
fermentation


Volatile compounds
• Constitute 0.01-0.02% of tea on dry basis
• Black tea contain more volatile compound
than green tea


Reactions Involved in the Processing of Tea
Withering
• 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
• Chlorophyll degradation has significance for the appearance of
the end-product.
• Conversion of chlorophyll into chlorophyllide, a reaction
catalyzed by the enzyme chlorophyllase – more extensive is
undesirable as give rise to pheophorbides (brown) and not the
desired oliveblack pheophytins.
• Increased cell permeability during withering favors the
fermentation procedure. A uniform distribution of polyphenol
oxidases in tea leaves is achieved during the conditioning step
of processing.

Rolling
• Tea leaf is macerated and the substrate and enzymes are
brought together; - The subsequent enzymatic oxidative
reactions are designated as a prerequisite for fermentation
• In this processing step, the pigments are formed primarily as a
result of phenolic oxidation by the PPO. In addition, oxidation of
amino acids, carotenoids and unsaturated fatty acids,
preferentially by oxidized phenols, is of importance for the
formation of odorants
• The enzymatic oxidation of flavanols via the corresponding o-
quinones gives theaflavins - bright red color, good solubility
• A second, heterogenous group of compounds, found in tea
after the enzymatic oxidation of flavanols, are the thearubigins
- a group of compounds responsible for the characteristic
reddish-yellow color of black tea extracts

Rolling
• 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.


Firing
• During this step 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
and an acidic environment. The undesired brown color
is obtained at higher pH’s.
• The astringent character of teas is decreased by the
formation of complexes between phenolic compounds
and proteins

Withering


Rolling


Firing


Tea chemistry

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