Some modern distorting machines burst the leaf cells so thoroughly that the withering stage is no longer necessary. These machines, however, do not produce the larger leafy grades of teas. This type of fermentation is a series of chemical reactions where the enzymes act on the tannin and natural oils in the leaves. During fermentation, the leaf color deepens and flavor develops. In traditional processing, optimum fermentation is reached after two to four hours.
Fermentation is stopped by heating or firing.
This firing also preserves the leaves. Traditional firing was done on large pans or screens over fire, but now a drier that blows heated air into a chamber is used. In the drier, the leaves are cooled quickly to prevent overdrying and loss of quality. Black tea is the most popular type in the United States. Black teas are processed the longest and have a rich, reddish brown color and full aroma and flavor.