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Based on the oxidation (also called fermentation) methods, tea is divided into- white or green tea (non-oxidized/non-fermented), oolong (semi- oxidized/semi-fermented) and black tea (fully fermented). Alternatively, based on the characteristic quality of the tea and the method of processing, teas can be classified into 6 types- white tea, green tea, yellow tea, oolong tea, black tea and dark tea (pu-erh). It is during the oxidation process, that primary polyphenols or catechins in fresh tea leaves are oxidized by endogenous enzymes such as polyphenol oxidase or peroxidase into secondary polyphenols such as theaflavins and thearubigins that contribute to the color, aroma, mouthfeel and flavors of black tea.
Approximately 70 types of tea polyphenols have been isolated from the leaves of the Camellia sinensis plant and constitute 18-36% of their dry weight. Polyphenols are absolutely amazing and have many preventative and restorative properties. They are antioxidants, anti-aging, anti-inflammatory and immune boosting. Through multiple mechanisms they are able to help with weight loss, lowering blood glucose, reducing cholesterol levels and promoting oral, musculoskeletal, skin, cardiovascular, and neurologic wellness. There are some studies showing that polyphenols in tea might be anti-carcinogenic but more research is needed.
Read our blog on the health benefits of tea to learn more about the wonderful potential health benefits of tea.
Tea polyphenols are roughly classified into the primary polyphenols that are contained in tea leaves and secondary polyphenols, which are formed during the oxidation/fermentation process with the aid of enzymes such as polyphenol oxidase and peroxidase. The most abundant primary polyphenols are flavan-3-ols, and catechins belong to this group. Catechins are the most abundant polyphenols in white and green teas as these are non-oxidized teas. The principal catechins are epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), epigallocatechin gallate (EGCG), catechin and gallocatechin. EGCG is the most abundant catechin, also most bioactive and so has the most health benefits. Catechins are responsible for the astringent taste, bitterness and flavors of green tea.
Secondary polyphenols such as theaflavins (theaflavin and its galloyl esters), thearubigins and theasinensins are formed during the oxidation/fermentation process through enzymatic conversion of catechins. Theaflavins content increases during oxidation and starts declining after reaching a peak while thearubigin content continues to increase during oxidation, and it is likely that theaflavins are a precursor to thearubigins. Theaflavins and thearubigins are the major polyphenolic compounds in black tea, while theasinensins are characteristic for oolong tea . Theaflavins and thearubigins are responsible for organoleptic properties of black teas such as the amber-red color, aroma, briskness, astringency, mouthfeel and aftertaste of the black teas. Interestingly the astringent properties of the secondary polyphenols is much lower than that of the primary catechin polyphenols. Therefore, theaflavins and thearubigins are associated with the quality of black tea.
For many years secondary polyphenols theaflavins and thearubigins that are predominant in black teas were thought to be weaker antioxidants than the primary polyphenol catechins that are predominant in white tea and green tea. Recent studies have shown that theaflavins are also excellent and comparable if not stronger antioxidants to catechins and have excellent free radical scavenging properties. Thearubigins, although considered as strong antioxidant agents, are less active compared to catechins or theaflavins. Theasinensins, characteristic of oolong tea, do have antioxidant properties but more studies are needed for further determination.
Tea contains oxidative enzymes such as polyphenol peroxidase and polyphenol oxidase which help in the oxidation process and conversion of catechins to theaflavins, thearubigins and theasinensins as explained above. As soon as the leaves are harvested there is some cell damage, the polyphenols are exposed to oxygen and these enzymes start the oxidative process which leads to browning of the leaves, thus giving tea its brownish color.
This process can be enhanced by maceration of the leaves which damages the cell wall and further increases oxidation. Maceration is the step used in production of oolongs and black teas. Polyphenol peroxidase and polyphenol oxidase can be inactivated with heat at temperatures of 150 F and above. So through use of steam or pan frying the oxidation process can be stopped. This is the first step in green tea production and explains why the tea leaves remain green. In oolongs, this process of deactivating the enzymes is done at different times in the production, after maceration and oxidation, thereby leading to different levels of oxidation in oolongs whereas black teas are oxidized completely. Polyphenol peroxidase and polyphenol oxidase can be inactivated by depriving them of moisture, and so this is done in case of the long withering time used in white teas which are minimally processed.
Tea contains many amino acids, with the most abundant and important being theanine. Amino acids contribute to 6% of the dry weight in tea leaves. Theanine is only found naturally in tea plants, a single species of mushroom called Basidiomycete Xerocomus badius and a plant called guatusa. Theanine is made by the roots of the tea plant in winter, and travels to the young shoots in spring. In the tea garden, with sunlight theanine is used in the production of polyphenols, but the plants are shaded either naturally by cloud cover or by a mechanical barrier, the theanine is conserved and not converted into polyphenols. Green tea bushes for gyokoro, matcha and our own Emerald Green tea from Darjeeling are grown under shade so that they have a very high theanine content. Theanine content in teas are variable and difficult to categorize based on the type of tea and its level of oxidation. In general concentration of theanine is the most in green tea (6.56 mg/g - milligram of theanine per gram of dry tea leaf ), white tea (6.26 mg/g) and oolong tea (6.09 mg/g) in the middle, black tea might be slightly lower (5.13 mg/g) and practically absent in pu’erh. Theanine is responsible for the umami taste in tea. It is also an amazing natural restorative compound which increases alpha waves in the brain, and so helps with relaxation and reducing acute stress. It balances the excitatory effects of caffeine in tea and creates a state of “focussed calm” or “mindful alertness.”
There are two major tea pigments chlorophyll and carotenoids which give leaves their color and also absorb light for photosynthesis. During oxidation, the green chlorophyll pigment changes into a black pigment called pheophytin which is responsible for the darker shades of oxidized tea leaves. Carotenoids on oxidation degrade into a variety of compounds of which the most studied is damascenone which is responsible for the sweetness of black tea.
Carbohydrates constitute 11% of the weight of the dry leaf and give sweetness to the tea. Carbohydrates are formed during photosynthesis and in turn provide energy to the tea plant and for various enzymatic reactions.
Methylxanthines are a family of alkaloids and on an average they constitute 2-5% of the weight of the dry leaf. The amount of methylxanthine is variable and is dependent on the cultivar, propagation method (cutting vs seed), climate and age of the leaves when plucked. The tea plant forms methylxanthines to protect itself against attack from insects. There are three types of methylxanthines that are found, of which caffeine is the most abundant.
Trace amounts of vitamins (vitamin B1, B2, B12, C and E) and minerals (fluorine, manganese, calcium, potassium, copper, zinc, nickel, phosphoric acid) are also present in dry tea leaves. The amount of minerals is dependent on the season (flush) and the processing methods. Fluorine in tea has been studied and thought to prevent and reduce tooth decay, though it can cause fluorosis if high levels are consumed.
These contribute to a miniscule 0.01% of the dry weight of the tea leaves, but are over hundreds-thousands of compounds and are solely responsible for the aroma and flavor of the tea! We find that absolutely remarkable! Some of the compounds are not present in tea leaves but are formed during processing. These volatiles enter the air from the tea leaves or tea liquor and then stimulate our olfactory system to give rise to aroma. Interestingly the flavors that we experience in tea are mostly about 80% as a result of aroma and the remaining through taste.
Now that you know the chemical composition of tea, why don’t you try out some teas from the wide selection that Herbs and Kettles have to offer to discern subtle differences and how the chemicals in tea affect the flavors and the feeling that the tea gives when you drink it. Try our wellness collection teas and experience the positive effects on your mind, body, and soul.
