Tea processing

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 degrees of oxidation of the leaves, stopping the oxidation, forming the tea and drying it.
The innate flavor 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 flavor of the final tea. 
History.
The history of tea processing corresponds intimately with the role that tea played in Chinese society and the preferred methods of its consumption in ancient Chinese society. The domestication of tea and the development of its processing method processing likely began in the area around what is now Southwest China, Indo-Burma, and Tibet. Variations of these processing techniques are still used in modern tea processing albeit being far more mechanized.
Green.
Ancient Chinese society first encountered the tea plant in what is now south-eastern China and processed it as an additional medicinal herb for use in Chinese herbology. The processing technique used to process fresh tea leaves was to immediately steam the fresh tea leaves and dry them for preservation, which is the most ancient Chinese form of tea leaf processing known to date. This processing method was perfected near the end of the Han Dynasty (206BCE – 220CE) and produced a dried tea that would be classified today as "green tea" and quite similar to modern Japanese sencha. For consumption, dried tea leaves were either decocted with water and other herbs, or ground into a powder to be taken straight, or suspended in a liquid in the manner of matcha.
With the increase of tea's use in Chinese herbology, production methods changed, where the processed green tea leaves were not immediately dried after steaming. Rather the steamed tea leaves were first pulverized into a paste form, with the paste then formed in molds and slowly dried into brick tea, a technique well described by Lu Yu in his work "The Classic of Tea". Tender leaves and leaf buds were generally not used, as older mature tea leaves were preferred for tea production. Some tea bricks were also produced from mature whole leaves, which typically required the use of a cooked rice slurry (米湯) to bind the tea brick together. The preference of producing tea in brick form possibly stems from the fact that it can be more easily transported and stored.
Yellow and fermented.
The use of steam in fixation (殺青) of tea leaf enzymes is an important step in processing tea, with the leaves quickly cooled down and then undergoing further processing. The less tightly controlled methods of it in the past resulted in the creation of "yellow tea" when the tea leaves were over-steamed for fixation or were not quickly spread out, doused with water and cooled. Although green tea was the most popular in Lu Yu's time, he personally considered yellow tea to be superior to green.
Even when the leaves were quickly cooled, if they are left in piles (渥堆) for too long before processing, the leaves will begin to undergo microbial fermentation to produce "post-fermented tea". This technique is somewhat similar to composting, albeit tightly controlled, and still used in the production of Liu'an tea (安徽六安籃茶) and was more recently introduced for the production of the "ripe" type Pu-erh tea. The production of tea in brick forms and their storage also resulted in another type of post-fermented tea, which was produced by aging. The long transport and storage times of the day unwittingly allowed the tea bricks to undergo prolonged exposure to the elements and to various microflora, which resulted in the aging, oxidation, and fermentation of green brick teas. A brick of green tea that had been stored and aged into post-fermented tea was charred over charcoal to rid it of the layer of detritus, dust, and shiny multicoloured growths before being broken down into a powder, cooked, and then consumed. By the end of the Tang Dynasty (618–907CE) green, yellow, and post-fermented tea was commonly used in China and moved from purely being used in herbology to becoming a beverage drunk for pleasure.
Oolong and white.
The Tang Dynasty was also the period when oolong tea was first developed in the Fujian province. It was originally produced in thin brick form, known then under the name "Beiyuan" tea (北苑茶). The importance of the withering process for producing oolong tea was described by poet Huang Furen (皇甫冉) in his poem "送陸鴻漸棲霞寺采茶", which indicated that the processing of tea leaves is not a simple task, requiring the scaling of steep cliffs to pick the choicest leaves and the withering of the leaves under the sun and warm winds ("采茶非采菉，遠遠上層崖。布葉春風暖，盈筐白日斜...").
White tea (白茶) was also developed in the Fujian province with its first mentions in the Song Dynasty document "Treatise on Tea", where the delicate buds used for producing white tea, the difficulty in producing it, its taste, and its rarity were lauded. The production method of white tea was described by Ming Dynasty author Tian Yiheng (田艺蘅) in "Zhuquan Xiaopin" (煮泉小品) (produced in the 33rd year of the Jiajing Emperor) regarding Fuding white tea (福鼎白茶). In this work, he stated that tea buds that have undergone fixation by panning over flames (as with green tea) are second to white tea that was simply allowed to dry under the sun, since it's more natural in taste and lacks flavours imparted by the smoke and flames. ("芽茶以火作者为次，生晒者为上, 亦更近自然，且断烟火气耳")
Black.
The technique for producing black tea was first developed during the late Ming Dynasty in Wuyishan, Fujian, either resulting from the over-oxidation of tea-leaves during the manufacture of oolong tea or indirectly from the methods of manufacturing green and white teas. In the early 1600s, tea producers in the Wuyi Mountains began kneading the sun-withered tea leaves to macerate them, then allowed them to dry under the sun, thus reaching full oxidation and producing "Gongfu" black tea (工夫紅茶). When there was insufficient sun and temperatures were low, the withered leaves would be processed indoors in warmed rooms and allowed to fully oxidize, then smoked dry over pine fires thus producing lapsang souchong. According to oral traditions of the region, the discovery of Lapsang Souchong processing was due to military troops passing through a Wuyishan tea factory during the last years of the Ming Dynasty, causing delays to tea leaf processing thus resulting in a completely oxidized leaf that the producer salvaged by drying over a fire built from pine branches. By the Qing Dynasty, both Lapsang Souchong and Gongfu black tea were well recognized in China and noted in "Records on Yiwu mountain" (武夷山志) by the scholar Dong Tiangong (董天工).
Procedure.
General.
Although each type of tea has a different taste, smell, and visual appearance, tea processing for all tea types consists of a very similar set of methods with only minor variations. 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 and may render the tea unfit for consumption.
Type-specific.
Tea is traditionally classified based on the degree or period of oxidation or fermentation the leaves have undergone: