Freshly opened matcha is bright green and fragrant. Stored badly, it turns yellowish or brown within a few weeks, loses its aroma and tastes flat, especially in a hot, humid climate like Thailand's.

The five enemies of green tea

Tokichi, a tea house in Kyoto, sums up what degrades tea into five things:

  1. Air Leaves exposed to air slowly lose their taste.
  2. Moisture Damp tea deteriorates faster.
  3. Heat Turns the green of the leaves and the brewed tea brown.
  4. Light Lowers quality.
  5. Surrounding smells Tea leaves absorb odours easily.

Matcha is a fine powder, so it has far more surface exposed to air than leaf tea and deteriorates faster.

Unopened bags

  • The fridge or freezer is fine if you won't use the bag for a while.
  • Always let the bag return to room temperature before opening. Opened cold, condensation forms on the powder immediately. Ippodo suggests resting a bag from the freezer for around 5 hours before opening.
  • Don't move the bag in and out of the fridge repeatedly, even unopened.
  • Check the best-before date on the label. Tea doesn't spoil the day after, but aroma and colour gradually fade.

Opened bags

  • Use it up quickly. Ippodo suggests within about a month; Tokichi suggests about two weeks in summer and a month in winter. Thai weather is close to a Japanese summer, so aim for 2–4 weeks.
  • Buy small. A 20–40 g bag you finish in a few weeks beats a large bag left open for months.
  • Press the air out and seal tightly, then put the bag inside an opaque, airtight tin or container.
  • Keep it in the darkest, coolest spot in the house, away from the stove, windows and strong-smelling food.
  • Always use a dry spoon. A wet spoon carries moisture into the bag.

Can an opened bag go in the fridge?

Most Japanese tea houses advise against it, because tea readily absorbs fridge moisture and odours. But if your home is very hot, or you drink tea only occasionally, you can use the fridge this way:

  1. Keep what you'll use in the next 1–2 weeks outside the fridge.
  2. Put the rest in a zip bag with the air pressed out, then inside a sealed box to block food smells.
  3. Store it in the vegetable drawer, not the door, where the temperature swings most.
  4. Let it come to room temperature before opening, every time.

How to tell matcha has gone stale

The colour shifts from bright green to yellowish or brown, the fresh aroma disappears, and the taste turns flat and more bitter. If it smells musty, has hardened into lumps from moisture, or shows any mould, don't consume it.

Matcha that has simply lost freshness but shows no such signs is still fine for baking or lattes, where other ingredients cover what's missing.

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Sources: Ippodo Tea — How long will my tea be fresh after opening · Tokichi — How to keep your tea fresh · Yamamotoyama — The correct way to store tea leaves