Tea processing is the method in which the leaves from the tea plant Camellia sinensis are transformed into the dried leaves for brewing tea.
The categories of tea are distinguished by the processing they undergo. In its most general form, tea processing involves different manners and degree of oxidation of the leaves, stopping the oxidation, forming the tea and drying it.
The innate flavour of the dried tea leaves is determined by the type of cultivar of the tea bush, the quality of the plucked tea leaves, and the manner and quality of the production processing they undergo. After processing, a tea may be blended with other teas or mixed with flavourants to alter the flavour of the final tea.
Plucking: Tea leaves and flushes, which includes a terminal bud and two young leaves, are picked 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 and normally more for Oolong and Puer tea production. Picking is done by hand when a higher quality tea is needed, or where labour costs are not prohibitive. Depending on the skill of the picker, hand-picking is performed by pulling the flush with a snap of the forearm, arm, or even the shoulders, with the picker grasping the tea shoot using the thumb and forefinger, with the middle finger sometimes used in combination. Tea flushes and leaves can also be picked by machine, though there will be more broken leaves and partial flushes reducing the quality of the tea. However, it has also been shown that machine plucking in correctly timed harvesting periods can produce good leaves for the production of high quality teas.
Withering: The tea leaves will begin to wilt soon after picking, with a gradual onset of enzymatic oxidation. Withering 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 withering. The process is also important in promoting the breakdown of leaf proteins into free amino acids and increases the availability of freed caffeine, both of which change the taste of the tea.
Tossing/Bruising (Turning Over): Known in the Western tea industry as “disruption” or “leaf maceration”, the teas are bruised or torn in order to promote and quicken oxidation.The leaves may be lightly bruised on their edges by shaking and tossing in a bamboo tray or tumbling in baskets. More extensive leaf disruption can be done by kneading, rolling, tearing, and crushing, usually by machinery. The bruising breaks down the structures inside and outside of the leaf cells and allows from the co-mingling of oxidative enzymes with various substrates, which allows for the beginning of oxidation. 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. This is accompanied by agitation in some cases. 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. The tea producer may choose when the oxidation should be stopped, which depends on the desired 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. Oxidation is highly important in the formation of many taste and aroma compounds, which give a tea its liquor colour, strength, and briskness. Depending on the type of tea desired, under or over-oxidation/fermentation can result in grassy flavours, or overly thick winey flavours.
Fixation / 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 and removing unwanted scents in the leaves, without damaging 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.
Sweltering / 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 turn yellow. The resulting leaves produce a beverage that has a distinctive yellowish-green hue due to transformations of the leaf chlorophyll. Through being sweltered for 6–8 hours at close to human body temperatures, the amino acids and polyphenols in the processed tea leaves undergo chemical changes to give this tea its distinct briskness and mellow taste.
Rolling / Shaping:The damp tea leaves are then rolled to be formed into wrinkled strips, by hand or using a rolling machine which causes the tea to wrap around itself. This rolling action also causes some of the sap, essential oils, and juices inside the leaves to ooze out, which further enhances the taste of the tea. 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. In many types 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.
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. Baking is usually the most common. Great care must be taken to not over-cook the leaves. The drying of the produced tea is responsible for many new flavour compounds particularly important in green teas.
Aging / 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. Additionally, oolong can benefit from aging if fired over charcoal. Flavoured teas are manufactured in this stage by spraying the tea with aromas and flavours or by storing them with their flavorants.