When we think of food and oxidation, usually the idea conjures something undesirable: browning fruit or withered lettuce, for example. However, a process of controlled oxidation is crucial to unlocking the flavor, color, caffeine and nutritional content of most teas.
Oxidation is a process whereby tea leaves are dried and browned through exposure to the air, which opens their flavors and aromas by unlocking certain molecular compounds. Generally speaking, the longer a leaf is allowed to oxidize, the stronger the tea. The leaves of black teas are made using a relatively long process of oxidation, while green teas undergo a much shorter one.
Tea leaves begin to oxidize as soon as they are plucked, and some teas are processed simply by letting the leaves be exposed to air for a period of time. Other oxidizing methods entail rolling, tumbling or macerating the leaves first in order to intensify or accelerate the oxidation. These methods work by breaking the cell walls within the leaves, allowing oxygen to enter more freely.
Tea oxidation is typically performed at temperatures around 80 degrees Fahrenheit. While higher temperatures and a humid environment can increase oxidation, much higher temperatures (140 degrees and above) will actually halt the process by deactivating the enzymes that cause it.
After a batch of leaves are sufficiently oxidized, tea producers use a technique called “fixing” that involves steaming, baking or pan-firing the leaves to stop the oxidation process.
Fixing takes skill because the leaves need to be heated sufficiently but not overheated, which can curtail their flavor. In order to keep tea fresh, it should be stored in an airtight container, away from light, moisture, and heat.