Black Teas are freshly-picked leaves which are withered until they are pliable enough to be rolled, releasing juices and enzymes that react with oxygen. The bruised and sticky leaves, whose juices are now exposed to the air, are spread out and allowed to reacte with oxygen. They begin to turn brown, as would a freshly sliced apple. After the leaves are oxidized, they are fired to “seal” and dry the leaf--preventing any further changes.
Oxidation changes the leaf's properties and accounts for the dark, rich colors and strong, brisk flavors characteristic of this tea type. Essentially, it is this oxidation process that makes black tea different from green tea.