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Tea Knowledge

The Chemical Composition of Tea-Part 2: Alkaloids and Pigments

The Chemical Composition of Tea-Part 2: Alkaloids and Pigments

  • Thursday, 29 December 2022
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Chemical Components Related to the Quality of Tea Leaves Part 2


Alkaloids in tea

At present, the alkaloids found in tea are mainly caffeine, theobromine, theophylline and adenine as well as guanine and so on. The most abundant is caffeine, accounting for 2%-4% of the dry weight of tea leaves, followed by theobromine, accounting for 0.05% of the total, followed by theophylline, accounting for about 0.002%.

Caffeine is widely distributed in the tea tree, except for the seeds, all other parts contain caffeine; the content of caffeine in various parts of the tea tree varies greatly, concentrated in the new parts, with the most content in the tender buds and leaves; summer tea has higher caffeine content than spring and autumn tea, and the tea content of moderate shade and fertilization is often higher than that of open air and unfertilized.

Caffeine is an important taste substance of tea leaves with a bitter taste, and more than 80% of caffeine can be dissolved in water when brewing tea, which is one of the main bitter components of tea broth. Caffeine has a low bitterness threshold, and temperature and pH have an effect on its bitterness sensitivity; as pH increases and temperature increases, the threshold decreases and sensitivity increases. The increase of amino acid content in tea broth has an abating effect on the bitterness of caffeine, while the increase of tea polyphenol content enhances the bitterness of caffeine. In tea broth, caffeine can form complexes with catechins through hydrogen bonding, which can change its taste characteristics. In black tea broth, the complex formed by the hydrogen bonding of caffeine and theaflavin has a fresh taste, which can improve the freshness of tea broth, therefore, the caffeine content of tea is an important factor affecting the quality of tea.

Pigments in tea leaves

Pigment is a type of colored substance present in fresh tea leaves and finished tea, which is a component of the color of tea leaves,soup and leaf base. The pigments in tea can be classified according to their origin and solubility.

Chlorophyll is a pyrrole type green pigment. Tea chlorophyll is composed of blue-green chlorophyll a and yellow-green chlorophyll b, which mainly exists in the leaves of tea tree. Chlorophyll in tea leaves accounts for 0-3% -0.8% of the dry weight of tea leaves, and chlorophyll a content is 2~3 times of chlorophyll b. The total chlorophyll content varies greatly depending on the variety of tea plant, season and leaf maturity, and is lower in Assam Camellia sinensis(large leaf species) with yellow-green leaves and higher in China Camellia sinensis (small leaf species) with dark green leaves.

The composition and content of chlorophyll have a certain influence on the quality of tea leaves. Generally speaking, it is appropriate to process green tea with varieties with high chlorophyll content, and a large proportion of chlorophyll b in the composition is good because chlorophyll is the main substance that forms the color of the appearance of green tea and the color of the leaf base. Processing black tea, oolong tea, white tea, yellow tea, etc. has lower requirements for chlorophyll content than green tea.

Carotenoids

Carotenoids are a group of various colored compounds with yellow to orange-red color. The carotenoids in tea are mainly carotenes and lutein. Carotenes are insoluble in water, mostly orange-red, and are important fat-soluble pigments in tea, divided into α-carotene, β-carotene, y-carotene, δ-carotene and so on. The content of carotene in tea is about 0.06%, of which the main component is β-carotene, accounting for about 80% of the total carotene in tea. Carotene is more abundant in high mountain tea, and more in mature leaves than in young leaves. During the processing of tea, especially the processing of black tea, carotenoids will be degraded by oxidation to form α-ionone, β-ionone(violet aroma), dihydroactinidiolide(mild and light aroma, which can bring out other aromas), theaspirone and other aroma substances, which play a very important role in the formation of black tea aroma.

Xanthophylls is a class of yellow pigment, insoluble in water. Luteolin compounds in tea mainly include luteolin, zeaxanthin, cryptoxanthin,neoxanthin,5,6-diepoxide cryptoxanthin, etc. The content in tea leaves is generally 0.01% ~ 0. 07%, and the total content increases with the maturity of tea leaves. Luteolin compounds are related to the formation of black tea aroma, shape and color and leaf color.

Tea polyphenol oxidation products

There is another category of pigments in tea, which are not original in the fresh leaves, but formed during the processing, namely, the oxidation products of tea polyphenols formed by the oxidation and polymerization of tea polyphenols - such as theaflavin (TF), thearubin (TR) and theabrownin (TB).

Theaflavins (TFs) are a class of water-soluble pigments with orange color and astringency in black tea, which is a general term for compounds formed by the oxidation of polyphenols, including theaflavins, theaflavin-3-gallate (TF-3-G), theaflavin-3'-gallate (TF-3'-G), theaflavin-3, 3'-digallate (TF-3 , 3'-DG, TFDG) and nine others ,etc. Theaflavin content in black tea accounts for 1% -5% of solids, is the main quality component of black tea taste and soup color, plays an important role in the color, taste and quality of black tea, is the main component of black tea soup color "bright", is an important component of black tea taste strength and freshness, is also the formation of tea soup "golden circle". It is also the main substance to form the "golden circle" of tea soup.

Thearubigins (TRs) are a complex class of reddish-brown inhomogeneous phenolic compounds, which are the further oxidation products of theaflavins, etc. They are the most abundant substances in the oxidation products of black tea, accounting for 6%~15% of the dry weight of black tea. Thearubigins is brownish red in color, soluble in water, acidic in aqueous solution, dark red in color, weak in irritation, which is the main substance that constitutes the soup color of black tea, and plays an extremely important role in the taste and soup color concentration of tea soup. Thearubigins is involved in the formation of "cold and muddy" black tea, and can also combine with alkaline proteins to produce precipitates stored in the bottom of the leaves, thus affecting the bottom color of black tea.

Theabrownine (TBs) is a water-soluble, non-dialysis highly polymerized brown substance, whose main components are polysaccharides, proteins, nucleic acids and polyphenols, and is formed by further oxidation and polymerization of theaflavin and theaflavin. Theaflavins are dark brown in color, soluble in water, and the content accounts for 4% ~ 9% of the dry weight of black tea.Theabrownines are an important factor in the darkness and lack of astringency of black tea broth; their content is highly negatively correlated with the quality of black tea (r= -0.979). Theabrownines are an important factor in the darkness and lack of astringency of black tea broth; their content is highly negatively correlated with the quality of black tea (r= -0.979). Long time heavy withering and long time high temperature anoxic fermentation during the processing of black tea are important reasons for the accumulation of theabrownines; thearubigins and theaflavin will also be further oxidized and polymerized to form theabrownines during the storage of black tea.


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