Tea chemistry


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Tea chemistry

  1. 1. Tea Chemistry7/20/2012 Manoj Solanki
  2. 2. 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 Vietnam7/20/2012 Manoj Solanki
  3. 3. 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) plant7/20/2012 Manoj Solanki
  4. 4. 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)7/20/2012 Manoj Solanki
  5. 5. 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)7/20/2012 Manoj Solanki
  6. 6. Manufacture of black teaWithering 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 handled7/20/2012 Manoj Solanki
  7. 7. 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) Grading7/20/2012 Manoj Solanki Green Tea
  8. 8. Composition of Tea (%, dry weight basis) Black Tea Constituent Fresh Tea Black Tea Brew *Phenolic compounds 30 5 4.5Oxidized phenolic compounds 0 25 15Protein 15 15 TracesAmino acids 4 4 3.5Caffeine 4 4 3.2Crude fiber 26 26 0Other carbohydrates 7 7 4Lipids 7 7 TracesPigments 2 2 Traces(chlorophyll and caroteniods)Volatile compounds 0.1 0.1 0.1Minerals 5 5 4.57/20/2012 Brewing * for 3 minutes Manoj Solanki
  9. 9. 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 7/20/2012 Manoj Solanki
  10. 10. 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).7/20/2012 Manoj Solanki
  11. 11. 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)
  12. 12. 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 7/20/2012 Manoj Solanki
  13. 13. 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.7/20/2012 Manoj Solanki
  14. 14. FlavonolsFlavone backbone Quercetin Epicatechin7/20/2012 Manoj Solanki
  15. 15. GREEN TEA7/20/2012 Manoj Solanki
  16. 16. 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 thearubigins7/20/2012 Manoj Solanki
  17. 17. Black Tea Thearubigins are polymeric polyphenols that are formed during the enzymatic oxidation TheaflavinThere are chiefly 3 types of theaflavins inblack tea, namely Theaflavin (TF-1), Theaflavin-3-gallate (TF-2), Theaflavin-3,3-digallate (TF-3).A number of studies have been done ontheir possible health effects with positive7/20/2012 Manoj Solankiresults
  18. 18. Enzymes1) Polyphenol oxidases- located within cell of leaf epidermis & activity rises during withering & rolling2) Shikimate dehydrogenase- reversibly interconverts dehydroshikimase & shikimate via phenylalanine pathway3) Phenylalanine ammonia lyase- catalyse cleavage of phnylalanine into ammonia & cinnamate4) Proteinases – cause protein hydrolysis during withering resulting in rise of peptides & free amino acids5) Chlorophyllases participate in the degradation of chlorophyll and transaminases in the production of precursors for aroma constituents. 7/20/2012 Manoj Solanki
  19. 19. 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 tea7/20/2012 Manoj Solanki
  20. 20. 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 present7/20/2012 Manoj Solanki Caffeine
  21. 21. Carbohydrates• Sugars in tea leaves are:Glucose (0.72%)Fructose (0.4%)Sucrose (0.09%)ArabinoseRibose Rhamnose & galactose are bound to glycosides7/20/2012 Manoj Solanki
  22. 22. Lipids• Level is around 7%• Polar fraction (glycerophospholipids) in young leaves are predominant• Glycolipid predominate in older leaves7/20/2012 Manoj Solanki
  23. 23. 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. 7/20/2012 Manoj Solanki
  24. 24. 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 fermentation7/20/2012 Manoj Solanki
  25. 25. Volatile compounds• Constitute 0.01-0.02% of tea on dry basis• Black tea contain more volatile compound than green tea7/20/2012 Manoj Solanki
  26. 26. 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 step7/20/2012 Manoj Solanki of processing.
  27. 27. 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 7/20/2012 Manoj Solanki
  28. 28. 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.7/20/2012 Manoj Solanki
  29. 29. Reactions Involved in the Processing of TeaFiring• 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 proteins7/20/2012 Manoj Solanki
  30. 30. Withering7/20/2012 Manoj Solanki
  31. 31. Rolling7/20/2012 Manoj Solanki
  32. 32. Firing7/20/2012 Manoj Solanki