There are many factors that determine the quality of different types of tea. Soil conditions, climate, and harvesting all play a role in the ultimate taste of your brew. However, it’s the manufacturing process — particularly how tea leaves are fermented — that defines whether a tea becomes green, white, oolong, or black.
Green tea leaves are spread out on trays to dry. When most of the water is evaporated, the leaves are heated to prevent any further fermentation from ruining the tealeaves. Some manufacturers then roll the tealeaves into shapes, which unfold during the brewing process. 175˚F (80˚C) is the ideal brewing temperature for green teas.
White tea leaves are stored at room temperature for several hours after they are harvested. During this time, water evaporates and the leaves can wither to half of their original weight. The leaves are then roasted until they dry up almost completely. Tea of this type is best brewed at 185˚F (85˚C).
After oolong tea leaves are plucked, they are left out in the sunlight to dry. The dry leaves are then shaken in baskets, causing the leaves to bruise. This process infuses oils and flavors into the leaves.
The leaves are then left to ferment for varying lengths of time, depending what characteristics the manufacturer wants the finished product to have. Longer fermentation times mean the brew will have a darker color and nutty flavor, while shorter fermentation times mean the brew will have lighter characteristics closer to green teas. Oolong teas should be brewed at 195˚F (91˚C).
Black tea is more oxidized than any of the other tea types. It has a stronger taste, and the longer fermentation process gives the tea leaves themselves a blackish color. Machines that shred slightly withered leaves process most tea of this type. The shredded leaves cut the fermentation time in half. Black teas are best brewed at 212˚F (100˚C).