To prepare tea leaves for black tea, whole leaves are cut at peak freshness from the tea plant, Camellia sinensis, and allowed to wither. Withering naturally reduces the amount of water in the plant, then the leaves are set aside to dry and oxidize in a process called fermentation. The chemical composition of the tea leaves changes through fermentation: the longer the leaves are allowed to oxidize, the darker the leaves will be. A lengthy oxidation process is what gives black tea its distinct, bold flavor.
While the preparation process for green tea begins in the same way, the amount of fermentation is different, yielding drastically different results. Green tea leaves are sometimes allowed to wither slightly—but are then either pan-fired, oven-dried or steamed to prevent oxidation from taking place. Since the leaves don’t undergo fermentation, they retain their green color and produce a lighter, more grassy taste.