Tea is traditionally classified based on the degree or period of "fermentation" the leaves have undergone:
Young leaves or new growth buds that have undergone minimal oxidation through a slight amount of withering before halting the oxidative processes by being baked dry, with the optimal withering conditions at 30 degrees Celsius (65% relative humidity) for 26 hours. Withering of the leaves can last from around one to three days depending on the season and temperature of the processing environment. The buds may be shielded from sunlight to prevent the 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 the tea. There is an international disagreement on definition of white tea between China and other producing countries. In China, White tea is fully oxidized by letting the tea naturally dry out in sunlight. It is different from traditional black teas because it does not undergo any manmade processing such as rolling or curling.
This tea has undergone the least amount of oxidation. The oxidation process is halted by the quick application of heat after tea picking, either with steam, the 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 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, and if done correctly retains most of the chemical composition of the fresh leaves from which it was produced. Variation in steaming time for fixation or processing from additional stages of rolling and drying are sometimes used to improve or altering the flavour for types of green tea.
This tea is processed in a similar manner to green tea, but instead of immediate drying after fixation, it is stacked, covered, and gently heated in a humid environment. This initiates oxidation in the chlorophyll of the leaves through non-enzymatic and non-microbial means, which results in a yellowish or greenish-yellow colour.
This tea's oxidation is stopped somewhere between the standards for green tea and black tea. The processing typically takes two to three days from withering to drying with a relatively short oxidation period of several hours. In Chinese, semi-oxidized teas are collectively grouped as blue tea (青茶, literally: blue-green tea / "celadon tea"), while the term "oolong" is used specifically as a name for certain semi-oxidized teas. Common wisdom about lightly oxidized teas in Taiwan (a large producer of Oolong) is that too little oxidation upsets the stomach of some consumers. Even so, some producers attempt to minimize oxidation in order to produce a specific taste or allow the tea leaves to be easily rolled into the spherical or half-sphere form demanded by buyers in the market.
The tea leaves are allowed to completely oxidize. Black tea is first withered to induce protein breakdown and reduce water content (68-77% of original). The leaves then undergo a process known in the industry as "disruption" or "leaf maceration", which through bruising or cutting disrupts leaf cell structures, releasing the leaf juices and enzymes that activate oxidation. The oxidation process takes between 45–90 minutes to 3 hours and is done at high humidity between 20-30 degrees Celsius, transforming much of the catechins of the leaves into complex tannin. Orthodox processed black teas are further graded according to the post-production leaf quality by the Orange Pekoe system, while Crush, Tear, Curl (CTC; or "Cut, tear, curl") teas use a different grading system. Orthodox tea leaves are heavily rolled either by hand or mechanically on a cylindrical rolling table or a rotorvane. The rolling table consists of a ridged table-top moving in an eccentric manner to a large hopper of tea leaves, of which the leaves are pressed down onto the table-top. The process produces a mixture of whole and broken leaves, and particles which are then sorted, oxidized, and dried. The rotovane consisted of an auger pushing withered tea leaves through a vane cylinder which crushes and evenly cuts the leaves. CTC black teas is a production method developed by William McKercher in 1930 and consist of machines with contra-rotation rotors with surfaces patterning that cut and tear the leaves producing a product popular for use in tea bags. The rotovane to often use to precut the withered tea prior to the CTC and to create broken orthodox processed black tea.
Teas that are allowed to undergo a second oxidation after the fixation of the tea leaves, 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.
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