Diane McKay, G89, N97, N00, assistant professor at the Friedman School who studies antioxidant foods, responds
Black, oolong, green and white teas all come from the same plant, Camellia sinensis. They differ in how the leaves are processed after harvesting and before drying. All will eventually lose their flavor, and the phytochemicals (primarily flavonoids) they contain will degrade.
However, dried tea leaves that are kept dry will not spoil, and as long as they are stored away from heat, water, light and air, the flavor and phytochemical content can be maintained for up to two years.
The more fermented and intact the dried leaves are, the longer they will last. Black tea leaves are more fermented than green or white, and oolong is somewhere in between. Measures of intactness vary from leaf, to broken leaf, to fannings and dust. Fannings are small pieces of leaves, while dust is the tiny particles left at the bottom of the barrel.
Tea bags most often contain fannings and dust because they brew quickly. Some tea bags do contain whole leaves, but they tend to be larger, to allow the leaves room to expand.
To keep your tea flavorful and flavonoid-rich for one to two years, transfer your tea bags or leaves to an airtight container as soon as possible after purchase, and store it away from the stove and sink.