The term “tea flush” typically refers to the apical shoots, which include the terminal bud and two neighboring leaves.
Polyphenols in tea flush and green tea
Tea flush and green tea contain polyphenols, which are a group of compounds that determine the color and taste of tea. The main type of polyphenols in tea are flavonols, specifically catechins, which make up 60-80% of the total polyphenol content. Catechins are responsible for the bitterness and astringency of green tea and also play a role in forming theaflavins in black tea. The largest part of catechins in tea flushes are esterified with gallic acid in the 3-position. The content of catechins is higher in the summer season and in macrophyll cultivars compared to spring and microphyll cultivars. The catechin-fiber ratio determines the quality of tea.
Polyphenols in black tea
Black tea undergoes fermentation which results in the oxidation of catechins to form brown colored pigments known as theaflavin and thearubigin. Theaflavin contributes to the bright color and brisk taste of tea infusions. It is formed by the enzymatic oxidation and condensation of catechins with di- and tri-hydroxylated B rings. The preparation of theaflavins involves the extraction of black tea with hot water, further extraction with methyl isoburyl ketone, washing with 2.5% aqueous sodium bicarbonate solution, concentration, dissolving in water, washing with chloroform to remove caffeine, concentration until dry, redissolving in methanol-water (43:57 v/v), and eluting through Sephadex LH-20 and cellulose in ethyl acetate. Thearubigin is a group of orange-brown pigments formed by the enzymatic oxidative transformation of flavanols during the fermentation process of black tea. The chemical structure of thearubigins is not yet known. Theaflavins and thearubigins play a role in determining the quality of black tea infusion.
2. Flavonols and Flavonol Glycosides
Tea contains flavonols and flavonol glycosides, but only in small quantities. The three main flavonol aglycones found in fresh tea flush are kaempherol, quercetin, and myricetin, which have different degrees of hydroxylation on the B ring (mono-, di-, and tri-hydroxy substitutions, respectively). These flavonols can exist as free flavonols or glycosides, where the glycosidic group may be glucose, rhamnose, or galactose. In macrophyll cultivars, the most significant glycoside is 3-glucosides, while rhamnodiglucosides predominate in microphyll cultivars.
Tea contains flavones, which are water-soluble compounds that give green and black tea its yellow color. Researchers have identified 18 different flavones in green tea, including C-glycosyl flavones like vitexin, isovitexin, and three different C-glycosyl apigenin isomers, as well as saponarin, vicemin-2, theiferin A and theiferin B.
4. Phenolic Acids and Depsides
Tea contains phenolic acids and depsides such as gallic acid, chlorogenic acid, coumaryl quinic acid, and 3-galloyl quinic acid. Theogallin is a major depside found in tea, which is closely related to the quality of black tea. Green tea contains 0.4-1.6 g/kg dry weight of gallic acid, and the amount of free gallic acid increases during fermentation.
5. Amino Acids
Tea flush contains 2-4% of amino acids that contribute to the taste and aroma of tea. The most abundant amino acid is theanine, unique to tea, and found in levels of 50% of the free amino acid fraction. Theanine has two enantiomers, L- and D-theanine, and its relative ratio may indicate the quality of tea. Other amino acids found in tea flush are glutamic acid, arginine, and aspartic acid, among others. High levels of free amino acids, especially leucine and isoleucine, are related to the quality of green tea, but not black tea. Theanine has shown to have pharmacological and physiological effects, including a relaxation-causing effect in humans.
6. Chlorophyll, Carotenoids and Other Pigments
Tea leaves contain chlorophylls and carotenoids as the main pigments. Chlorophyll content decreases during black tea manufacturing, and degradation products such as pheophytin A, pheophytin B, and pheophorbide cause the brown and black color of black tea infusion. There are 15 reported carotenoid compounds in tea flush, with ß-carotene, lutein, and zeaxanthin being the most important components. Carotenes play an important role in the formation of black tea quality and aroma, and undergo appreciable degradation during the manufacturing process of black tea. Several aromatic compounds are formed from carotenoids via thermal degradation during tea manufacturing.
Tea flush contains carbohydrates such as free sugars (glucose, fructose, sucrose, raffinose, and stachyose) and polysaccharides (hemicellulose, cellulose, and other extractable polysaccharide fraction). Sucrose is the main primary product of photosynthesis and increases with the growth of tea shoots. The monosaccharides and disaccharides contribute to the sweet taste of tea infusion. Cellulose and hemicellulose contents negatively affect the tenderness of the tea shoots, with less cellulose and hemicellulose resulting in a higher quality of made tea product. Tea polysaccharides have been shown to decrease blood-glucose levels and are recommended for the treatment of diabetes. The composition of tea polysaccharides differs with the tenderness of the raw material and season.
8. Organic Acids
Tea flush contains several organic acids including succinic, oxalic, quinic, malic, and citric acids. Some of these acids contribute to the tea’s aroma while others can become aromatic through reactions. The total amount of organic acids in tea flush ranges from 0.5% to 2.0% of its dry weight. The highest amount of organic acid is quinic acid, followed by oxalic acid, which forms crystals in the vacuoles of leaf cells.
9. Caffeine and Other Alkaloids
Tea contains caffeine, which is a stimulant found in coffee and other sources. The average daily caffeine intake from all sources in the US is estimated to be 200mg, while in the UK it is 240mg. In some countries, the daily caffeine intake is much higher. Tea is one source of dietary caffeine and can account for up to 44% of caffeine intake in the UK. The type of tea and method of preparation affect caffeine content. Caffeine is not significantly reduced during tea processing, and may even react with other compounds to enhance the tea’s flavor. Caffeine is rapidly absorbed after oral intake and has a half-life of 3-7 hours. It is pharmacologically classified as a central nervous system stimulant and a diuretic. The effects of caffeine have been disputed, but moderate consumption is unlikely to have significant negative effects. Tea also contains small amounts of other methyl-xanthines.
Tea contains various minerals in varying concentrations, and their extraction rates during the infusion process differ. The tea plant is known to concentrate fluorine, aluminum, copper, and manganese. Fluorine in tea is beneficial for human health, particularly in the prevention of dental caries, but excessive intake of fluorine may lead to dental fluorosis. Aluminum is also concentrated in tea leaves, but only some of it is extracted into the infusion due to its low extraction rate. Copper is important in tea biochemistry because it is contained in the polyphenol oxidase enzyme, while manganese is a critical element for human health as it participates and catalyzes the activity of many enzymes.
Tea is a good source of vitamins, especially vitamin C. Green tea has the highest vitamin C content, with 150-300 mg in 100 g. Oolong and black tea have lower vitamin C content due to the fermentation process. Green and black tea both contain around 1-2 mg of vitamin B per 100 g of tea, which is extracted into the infusion during brewing. Vitamin E, which has anti-aging and anti-cancer properties, is found in the lipid fraction of tea and ranges from 24-80 mg per 100 g of tea. Tea also contains vitamin K, with 300-500 i.u. per gram of tea, so drinking 5 cups of tea per day can meet the human requirements for vitamin K.
Tea contains important enzymes, including polyphenol oxidase (PPO), which is crucial for making black tea. PPO is a Cu-containing protein and attacks only certain parts of tea polyphenols. PPO activity is highest in tea flush and is found in various parts of tea leaves. The activity of PPO changes during tea manufacturing, increasing during withering and fermentation and decreasing after drying. Other enzymes in tea and their functions are listed in a table.