Black
tea leaf is a fully fermented tea. The leaves are harvested and left to
wither. The withered leaves are then rolled and left to oxidize. The leaves
are oxidized under controlled temperature and humidity. This process is
also called fermentation, though no fermentation takes place. The level
of oxidation determines the quality of the tea. Since oxidation begins at
the rolling stage itself, the time between these stages is also a crucial
factor in the quality of the tea. Then the leaves are dried to arrest the
oxidation process. Finally, the leaves are sorted into grades according
to their sizes (whole leaf, brokens, fannings and dust), usually with the
use of sieves. Black tea, as a property of its processing, has caffeine.