Oxidation is a process through which tea leaves are exposed to the air in order to dry and darken, contributing to the flavor, aroma, and strength of different teas. Just as other fruits and plants, like apples or avocados, brown when exposed to oxygen, tea leaves go through a similar process after they are harvested.
As tea leaves are oxidized, they undergo unique changes that influence their chemical composition. This is a key step in processing tea, with different levels of oxidation results in different varieties, including black, green, white, and oolong.
Oxidation vs. Fermentation
While it’s common for people to use the terms oxidation and fermentation interchangeably, they’re actually different processes. Fermentation involves microbial activity and refers to the process undergone by aged teas such as pu-erh.
Oxidation and Tea
Different kinds of tea have different levels of oxidation, resulting in a wide variety of appearances and flavor profiles. Tea leaves begin to oxidize as soon as they are plucked, and the level of oxidation is a key factor in tea processing that results in different categories of tea.
Black teas are fully oxidized, resulting in a dark, rich cup of tea that is high in caffeine. Oolong teas are partially oxidized, and their oxidation level can vary widely between that of black and green teas. Green teas are largely unoxidized, and undergo a firing or steaming process, sometimes known as “killing the green,” soon after harvesting in order to halt oxidation, resulting in a lighter, more mellow cup of tea. White teas, because of their minimal processing, undergo a small amount of oxidization as they dry.
While all teas are made from the same plant, camellia sinensis, oxidation is largely responsible for the differences between different types of tea. In general, the longer tea leaves are allowed to oxidize, the darker and stronger the tea made from those leaves will be!