Last week I had an opportunity to attend a matcha tea tasting with the Aoi Tea company.
It was educational to see how they taste and grade matcha. To begin with, we looked at the tencha leaves. The tea plants for matcha are grown under shade, and the tender leaves are harvested, immediately steamed and dried. Then the leaf part from in between the veins are used.This is tencha, before the leaves are ground.
The tea is stored as tencha and when an order comes in, the tea will be freshly ground. Matcha comes in 30 or 40 gram cans. It can take an hour or more to grind enough tea for a can. Please store your matcha in the freezer or refrigerator. Unopened the matcha will most likely be good up to a year. Once you open the can, it will last up to 30 days if stored in the refrigerator. Opened cans of matcha will last maybe a week or so before they become oxidized. If you are buying expensive matcha, store it in the refrigerator or freezer otherwise you are wasting your money. Oxidized matcha that you don't want to drink can be used for cooking or baking.
Fortunately, I don't have to worry about storage. At my house, a can of matcha once opened rarely lasts more than two weeks.