Tea processing

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Tea processing is the method in which the leaves and flushes from Camellia sinensis is 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 oxidising 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 a lesser amount flavour.

[edit] General

Although each type of tea has different taste, smell, and visual appearent, tea processing for all tea types consist 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 twice a year during early spring and early summer or late spring. Autumn or winter pickings of tea flushes are much less common, though occurs 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 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 removed excess water from the leaves and allows a very light amount of oxidation. The leaves can be either put under then sun or left in a cool breezy room to pull moisture out from the leaves. The leaves sometimes lose more then a quarter of its weight in water during wilting.
Bruising: In order promoted and quicken oxidation, the leaves may be bruised by tumbling in baskets, kneading by , or being rolled-over by heavy wheels. This also releases some of the leaf juices, which may aid and oxidation and chance the taste profile of the tea.
Oxidation: For teas that require oxidation, the leaves are left on their own in a closed room where the turn progressively darker. This this process the chlorophyll in the leaves are enzymatically broken down, and its tannins are released or trasformed. This process is referred to as fermentation in the tea industry, however no true fermentation happens since the process is not driven by microorganisms. The tea producer may chose when the oxidation shout be stopped. For light oolong teas this by be anywhere from 5-40% oxidation, in darker oolong teas 60-70%, and in black teas 100% oxidation.
Kill-green: Kill-green or shāqīng (殺青) 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 teas leaves are panned in a wok or steamed, but with advancements in technology, kill-green is sometimes done by baking or "panned" in a rolling drum. In CTC black teas, kill-green is done simultaneously with drying.
Yellowing: Unique to yellow teas, warm and damp tea leaves from after kill-green is allowed to be light heated in a close container, which causes the previously green leaves to yellow.
Shaping:The damp tea leaves are then rolled to be formed into wrinkle strips. This is typically done by placing the damp leaves in large cloth bags, which are then kneaded by hand or machine to form the strips. This rolling action also causes some of the sap and juices inside the leaves to ooze out, which further enchances the taste of the tea. The strips of tea can then be formed into other shapes, such as being rolled into spirals, kneded and rolled into pellets, or tied into balls 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 addition aging, secondary-fermentation, or baking to reach their drinking potential. As well, flavoured teas are manufactured by spraying with aromas and flavours or stored with its flavorant.

Without careful moisture and temperature control during its manufacture and life thereafter, fungi will grow on tea. This form of fungus causes real fermentation that will contaminate the tea with toxic and sometimes carcinogenic substances and off-flavours, rendering the tea unfit.

[edit] Type specific processing

Tea is traditionally classified based on the degree or period of "fermentation" the leaves have undergone:^[1]

White tea: Young leaves (new growth buds) that have undergone almost no oxidation; 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: The oxidation process is stopped after a minimal amount of oxidation by application of heat, either with steam, a traditional Japanese method, 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 gun-powder 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: 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 (青茶, literally: blue-green tea), while the term "oolong" is used specifically as names for certain semi-oxidized teas.^[2]
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 high-quality tea served at the Imperial court, or of special tea processed similarly to green tea, but with a slower drying phase.
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.