Tea chemistry

09/04/2023 Manoj Solanki

Tea Chemistry


IntroductionTea 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 teaThe enzymatic oxidation of

tea leaves is referred to as fermentation

If the enzymes are allowed to act, they turn green leaf black (black tea)

If the enzymes are inactivated by heat, as in blanching, then the leaf remains green (green tea)

Black Tea

Green Tea

Yellow Tea

Red Tea (Oolong)


Manufacture of black tea

Withering •Reduces moisture from about 75-80% to 55-65%•Moisture reduction converts the turgid leaf to a

flaccid material that is easily handled

Rolling •It establishes proper conditions for enzymatic

oxidation of the flavanols by atmospheric oxygen

Fermentation •Conversion of colourless catechins to a complex

mixture of yellow-orange to red-brown substances

FiringEnds fermentation process and reduce moisture

content to 3%


Manufacture of green tea In contrast to black tea manufacture,

withering and fermentation stages are omitted in green tea processing

Picking

Steaming (95°C)

Rolling (75ºC)

Drying (90°C)

Grading

Green Tea


Composition of Tea (%, dry weight basis)

Constituent Fresh Tea Black Tea Black Tea Brew *

Phenolic compounds 30 5 4.5Oxidized phenolic compounds

0 25 15

Protein 15 15 TracesAmino acids 4 4 3.5Caffeine 4 4 3.2Crude fiber 26 26 0Other carbohydrates 7 7 4Lipids 7 7 TracesPigments (chlorophyll and caroteniods)

2 2 Traces

Volatile compounds 0.1 0.1 0.1Minerals 5 5 4.5

* Brewing for 3 minutes


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).

http://en.wikipedia.org/wiki/Plant


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.

Chemical structure of Flavan-3-ol

Epigallocatechin (EGC)

Epicatechin (EC)

http://en.wikipedia.org/wiki/Flavan


Catechins are the main phenolic compounds present in fresh tea leaves:

Catechin EpicatechinEpicatechin gallateEpicatechin digallate

Flavanol• Flavanols (with an "a") are not to be confused

with flavonol (with an "o"), another class of flavonoids containing a ketone group.

Gallocatechin Epigallocatechin Epigallocatechin gallate Epigallocatechin digallate


Flavonols

• Flavonols (with an "o") are a class of flavonoids that have the 3-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.

Backbone of a flavanol


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 & rolling2) 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

• 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

5-N-ethyl-glutamine


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

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 TeaWithering• 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.


Reactions Involved in the Processing of TeaRolling • 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


Reactions Involved in the Processing of TeaRolling

• 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.


Reactions Involved in the Processing of Tea

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

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Transcript of Tea chemistry