Black tea is made by allowing a profound enzymatic transformation of the leaf fluids. The leaves are withered for hours, rolled repeatedly for as much as an hour, then are allowed to rest for between one and four hours, during which enzyme action turns them a coppery brown and causes them to emit the aroma of apples. Finally, the leaves are air-dried at temperatures around 100°C, and become quite dark.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee.
By permission of Scribner, a division of Simon & Schuster, Inc.