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Black teas are fully-oxidized teas. Legend recounts that in the seventeenth century, a cargo of tea that arrived in London from China, at the end of a particularly long voyage, had gone moldy in its boxes. Originally green, it had turned black but in spite of this the English found it very much to their taste and immediately re-ordered it. Though improbable, this anecdote does point the way to China, where this method of manufacturing black tea originated.
When the leaves are withered in the first part of the processing, they lose 50 to 60% of their moisture and are soft enough to be handled without tearing. The crop is spread in thin, even layers on racks made from cloth, bamboo, or wire, or in tanks with a perforated or grille base, ventilated by a light current of air that prevents the level of humidity from rising too high. The withering hall is always well ventilated and kept at a temperature between 68 and 75°F (20 to 24°C).
Rolling is intended to break down the cells to release the enzymes they contain, and encourage the reactions produced in them by oxidation. The more the leaves are rolled, the more rapidly oxidation takes place. It is the length and force of the rolling that decides the final result in the cup: lightly rolled leaves – light liquor with only slight astringency, more energetically rolled leaves – more full-bodied result. Rolling may be done by hand or mechanically.
Oxidation is a crucial operation that is primarily responsible for governing the quality of the tea. In oxidation, the humidity level needs to be maintained in order to facilitate the process. The leaves are spread on tables in layers1½ to 2½ inches (4 to 6 cm) thick, in a well-ventilated but draft-free hall, where the atmosphere is very damp (90 - 95%) and the ambient temperature is kept constant at between 68 and 71°F (20 to 22°C).
To stop the oxidation process once the desired level has been reached, the leaves must be heated immediately to a temperature which will destroy the heat-sensitive enzymes that cause it. This will have the further effect of reducing the humidity contained in the leaves. This roasting or drying-out process, is carried out in machines that expose the leaves to an average temperature of 194°F (90°C) for 15 to 20 minutes.
The next stage involves sorting the tea into different grades.
The tea is immediately sorted into two grades:
The broken leaves may have been damaged during the production process, or they can be produced artificially by being chopped by machine.
The whole leaves are then graded according to how fine they are. This is done by machine or, for the best qualities, with a handsieve.