Traditional matcha is prepared from leaf tea quite immediately before the tea ceremony. Nowadays packaging and desiccating agents are good enough to allow for pre-packing the grounded tea as a product. As you can imagine, quality varies dramatically and I believe, according to previous experience, certain additives are used in some products to make it easy to dissolve in colder water. However, there are others that are purely tea. Usually they have shorter shelf life of one year.
Some higher end products I had been given stated that usage within 6 months from production. This is with the understanding that the product is finished within a couple of weeks after opening. What happens after that? The tea is just less fresh and the taste starts to change. As to how long it takes for it to turn bad, I need an answer too.
