Producing tea is all about oxidation. White tea has little or no oxidation and Black tea has a lot of oxidation of the tea leaves. In the middle we have yellow and green tea. The provenance of the tea will effect flavour but also the processing....
Hardly any processing and no heat. White tea differs principally from green teas in that the white tea process does not incorporate any steaming or pan-firing. The name comes from the silver needle white teas that are used to produce the leaves. The tea plant is called "chaicha". The leaves are thinner and smaller and have a silvery-white down.
The flavour is described, typically as delicate with a lingering floral fragrance and a fresh, mellow, sweet taste with no astringency and grassy flavours.
Yellow tea is partially oxidised tea leaves with some mild steaming involved. During the processing the chlorophyll is broken down and partly oxidized. The taste of yellow tea is somewhat milder, and described as being somewhere between white tea and green tea. The aroma is described as flowery and fresh.
Tea is first plucked from the stem, two leaves and a bud. the leaves are then cleaned and dried and often steamed for less than one minute in large vats. This is kills enzymes that may initiate or cause oxidation. The leaves are then kneaded by hand, piled and dried for about ten hours during which time it is constantly turned. The leaves are then pan fired and this stops the oxidation process for good. Typical flavour descriptors include: sweet, bittersweet, nutty, vegetal, buttery, floral and fruity.
The final black tea that we buy is a highly oxidised product. The polyphenolics that were green have effectively been oxidised (like Iron and rust!) and are now darker and often a black/brown colour.
There is a critical withering part of the process where the leaves withered and rolled this process bruises the leaves and lets the chemicals in the leaves mix which initiates the oxidation process . The leaves are then separated by a roll breaker to allow for the full oxidation of the tea leaves. This process goes on for several hours while the leaves are spread out in a cool place. Drying / Pan Firing then stops the process. The leaves can then be sized, sorted and blended accordingly. Taste profiles of black tea are varied but really are an extension of the green tea flavour with more astringent and cooked notes reflecting the extra oxidation.