What degrades matcha
Matcha loses freshness through oxidation when exposed to air, fading from photo damage when exposed to light, and aroma loss when stored warm. Humidity is the fastest enemy because moisture causes clumping and accelerates flavor decay. The same tin can read fresh on day 30 and tired on day 90 from environment alone.
The 60 to 90 day window
Once a 30 to 40 gram tin is open, plan to finish it within 60 to 90 days. This is the window producers like Ippodo and Marukyu Koyamaen treat as the realistic shelf for opened matcha. Unopened tins sealed and stored cool can hold for one to two years, but the printed best by date assumes the seal stays intact.
Refrigerator and freezer
Refrigeration is acceptable for opened tins if the container is fully airtight, but always bring the tin to room temperature before opening so condensation does not form on the powder. Freezing unopened, sealed tins extends shelf life usefully, but freezing an opened tin invites moisture and is generally not worth it.
Signs a tin has turned
Color drifts from vibrant jade toward yellow or olive. Aroma flattens from fresh grass and seaweed toward dry hay. Bitterness rises and umami drops in plain usucha. A tin showing two of these signs is no longer worth a thin whisked preparation, but is often still usable in lattes, ice cream, or baking.