Tea processing is the method in which the leaves and flushes from Camellia sinensis are transformed into the dried leaves for brewing tea. The types of tea are distinguished by the processing they undergo. In its most general form, tea processing involves oxidizing the leaves, stopping the oxidation, forming the tea and drying it. Of these steps, the degree of oxidation plays a significant role of determining the final flavour of the tea, with curing and leaf breakage contributing to flavour by a lesser amount.
General
Although each type of tea has different taste, smell, and visual appearance, tea processing for all tea types consists of a very similar set of methods with only minor variations:
- Picking: Tea leaves and flushes, which includes a terminal bud and 2 young leaves, are plucked from Camellia sinensis bushes typically twice a year during early spring and early summer or late spring. Autumn or winter pickings of tea flushes are much less common, though they occur when climate permits. Picking is done by hand when a higher quality tea is needed, or where labour costs are not prohibitive. Hand-picking is done by pulling the flush with a snap of the wrist and does not involve twisting or pinching the flush, since doing the latter reduces the quality of the leaves. Tea flushes and leaves can also be picked by machine, though there will be more broken leaves and partial flushes. It is also more difficult to harvest by machine on mountain slopes where tea is often grown.
- Wilting: The tea leaves will begin to wilt soon after picking, with a gradual onset of enzymatic oxidation. Wilting is used to remove excess water from the leaves and allows a very slight amount of oxidation. The leaves can be either put under the sun or left in a cool breezy room to pull moisture out from the leaves. The leaves sometimes lose more than a quarter of their weight in water during wilting.
- Bruising: In order to promote and quicken oxidation, the leaves may be bruised by tumbling in baskets or by being kneaded or rolled-over by heavy wheels. This also releases some of the leaf juices, which may aid in oxidation and change the taste profile of the tea.
- Oxidation /Fermentation: For teas that require oxidation, the leaves are left on their own in a climate-controlled room where they turn progressively darker. In this process the chlorophyll in the leaves is enzymatically broken down, and its tannins are released or transformed. This process is sometimes referred to as "fermentation" in the tea industry, although no true fermentation happens since this oxidative process does not generate energy (this step is also not driven by microorganisms; in other steps of tea processing--aging for example--microorganisms might be used that actually do carry out fermentation). The tea producer may choose when the oxidation should be stopped, which depends on the desire qualities in the final tea as well as the weather conditions (heat and humidity). For light oolong teas this may be anywhere from 5-40% oxidation, in darker oolong teas 60-70%, and in black teas 100% oxidation.
- Kill-green / Firing: Kill-green or shāqīng is done to stop the tea leaf oxidation at a desired level. This process is accomplished by moderately heating tea leaves, thus deactivating their oxidative enzymes, without destroying the flavour of the tea. Traditionally, the tea leaves are panned in a wok or steamed, but with advancements in technology, kill-green is sometimes done by baking or "panning" in a rolling drum. In some white teas and some black teas such as CTC blacks, kill-green is done simultaneously with drying.
- Yellowing: Unique to yellow teas, warm and damp tea leaves from after kill-green are allowed to be lightly heated in a closed container, which causes the previously green leaves to yellow. The resulting leaves produce a beverage that has a distinctive yellowish-green hue.
- Shaping: The damp tea leaves are then rolled to be formed into wrinkled strips, using a rolling machine which causes the tea to wrap around itself. This rolling action also causes some of the sap and juices inside the leaves to ooze out, which further enhances the taste of the tea. In many type of oolong, the rolled strips of tea leaf are then rolled to spheres or half spheres and is typically done by placing the damp leaves in large cloth bags, which are then kneaded by hand or machine in a specific manner. The strips of tea can then be formed into other shapes, such as being rolled into spirals, kneaded and rolled into pellets, or tied into balls, cones and other elaborate shapes.
- Drying: Drying is done to "finish" the tea for sale. This can be done in a myriad of ways including panning, sunning, air drying, or baking. However, baking is usually the most common. Great care must be taken to not over-cook the leaves.
- Curing: While not always required, some teas required additional aging, secondary-fermentation, or baking to reach their drinking potential. For instance, a green tea puerh, prior to curing into a post-fermented tea, is often bitter and harsh in taste, but becomes sweet and mellow through fermentation by age or dampness. As well, oolong can benefit from aging if fired over charcoal. Flavored teas are manufactured in this stage by spraying the tea with aromas and flavors or by storing them with their flavorants.
Type specific processing
Tea is traditionally classified based on the degree or period of "fermentation" the leaves have undergone".
White tea
Young leaves or new growth buds that have undergone minimal oxidation through a slight amount of wilting before halting the oxidation with heat. Though the young leaves may be shaped before drying, leaf buds processed into white tea are usually dried immediately after wilting. The buds may be shielded from sunlight to prevent formation of chlorophyll. White tea is produced in lesser quantities than most other styles, and can be correspondingly more expensive than tea from the same plant processed by other methods. It is less well known in countries outside of China, though this is changing with increased western interest in organic or premium teas.
Green tea
This tea has undergone the least amount of oxidation. The oxidation process is halted through quick application of heat after tea picking, either with steam, or by dry cooking in hot pans, the traditional Chinese method. Tea leaves may be left to dry as separate leaves or they may be rolled into small pellets to make Gunpowder tea. This process is time consuming and is typically done with pekoes of higher quality. The tea is processed within one to two days of harvesting.
Oolong (Wulong)
Oxidation is stopped somewhere between the standards for green tea and black tea. The oxidation process takes two to three days. In Chinese, semi-oxidized teas are collectively grouped as blue tea (literally: blue-green tea), while the term "oolong" is used specifically as a name for certain semi-oxidized teas.
Black tea/Red tea
The tea leaves are allowed to completely oxidize. Black tea is the most common form of tea in southern Asia (Sri Lanka, India, Pakistan, Bangladesh, etc.) and in the last century many African countries including Kenya, Burundi, Rwanda, Malawi and Zimbabwe. The literal translation of the Chinese word is red tea, which is used by some tea lovers. The Chinese call it red tea because the actual tea liquid is red. Westerners call it black tea because the tea leaves used to brew it are usually black. However, red tea may also refer to rooibos, an increasingly popular South African tisane. The oxidation process will take between two weeks and one month. Black tea is further classified as either orthodox or as CTC (Crush, Tear, Curl, a production method developed about 1932). Unblended black teas are also identified by the estate they come from, their year and the flush (first, second or autumn). Orthodox processed black teas are further graded according to the post-production leaf quality by the Orange Pekoe system, while CTC teas use a different grading system.
Post-fermented tea
Teas that undergo a second oxidation, such as Pu-erh, Liu'an, and Liubao, are collectively referred to as secondary or post-fermentation teas in English. In Chinese they are categorized as Dark tea or black tea. This is not to be confused with the English term Black tea, known in Chinese as red tea. Pu-erh, also known as Póu léi (Polee) in Cantonese is the most common type of post-fermetation tea in the market.
Yellow tea
Either used as a name of special tea processed similarly to green tea, or high-quality tea served at the Imperial court.
Kukicha
Also called winter tea, kukicha is made from twigs and old leaves pruned from the tea plant during its dormant season and dry-roasted over a fire. It is popular as a health food in Japan and in macrobiotic diets.
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