How to store a tin once it is open

Once a tin is open, the way you keep it changes, and most packaging only covers the time before that point. Under the Japan Tea Central Association's labeling standard, the storage line on a Japanese tea pack describes storage before opening. Its sample wording simply says to avoid high temperature and humidity and to watch out for transferred smells. For how long it lasts after the seal breaks, look to the makers, and the tea sellers do not fully agree.

You have more time than many people expect. The tea maker Ippodo says matcha does not spoil after opening. It does slowly lose its freshness, so Ippodo recommends finishing within a month for the best taste. Its FAQ adds that there is no hard expiration date. The best-before date on an unopened pack falls around six months after packaging.

What wears it down

Ippodo names four things that do the damage, which are open air, strong odors, direct sunlight and heat. Heat is the one researchers have measured most closely. A 2002 study by researchers at Ito En tracked chlorophyll, surface color and sensory scores over 24 weeks. Matcha held at minus 70 and at 4 degrees C stayed in good condition the whole time. At 25 degrees C it stayed in moderate condition. At 37 degrees C it became fairly bad in three or four weeks.

Color is the change you are most likely to notice in your own tin. In a 2020 study in Food Science and Biotechnology, Kim and colleagues followed matcha for two months at different temperatures. Its brightness and green values fell as temperature and time rose, and its catechins fell too. A review by the science team at Yunomi, a Japanese tea seller, says chlorophyll breakdown is why the color fades. It also says moisture can get in through condensation on a cold container, a poor seal or a wet spoon.

Keep in mind what those lab tests leave out when you read their numbers. They looked at temperature and time. Neither study's abstract describes a tin being opened and closed each day, which also lets in air. So use the figures as a guide to how fast heat works, and treat your open tin as more exposed than those samples.

The everyday routine

Nearly all of the good habits take a few seconds and cost nothing. These come from Ippodo, Yunomi and the matcha seller rocky's matcha.

• Leave the matcha in its foil bag. Ippodo says to roll the bag up, clip it shut and keep it inside the can between uses.
• Pour the powder straight into the can only if you make matcha about once a day. That is the point where Ippodo says the convenience makes sense.
• Pick a dark cupboard away from spices, coffee and other strong smells, as Ippodo suggests.
• Use a dry spoon every time and reseal right after scooping, as rocky's matcha advises.
• If your caddy has no bag inside, Yunomi's team says an aluminum-lined bag can cut air exposure further.

Fridge or cupboard

This is where the sources split, so it helps to see their positions side by side. Ippodo says room temperature is fine, as long as the spot avoids sun, heat, humidity and strong smells. Nara Tea Company, a retailer, says to keep an opened tin in the fridge at around 5 degrees C. It says to let the tin return to room temperature before opening, and to finish it in 4 to 6 weeks. Rocky's matcha says opened matcha is best within 1 to 3 months, and it treats the fridge or freezer as an option for longer storage. Yunomi's team says a daily drinker can use an airtight caddy in a cool, dry place. It saves the fridge for storage of 2 to 6 months, in several airtight layers to keep out fridge smells.

Those positions fit together better than they first appear to, and they point to a simple way of choosing. If you will finish the tin in about a month and you have a cool cupboard, use the cupboard. That is what Ippodo and Yunomi's team both describe. If your kitchen runs hot in summer, the Ito En result at 37 degrees C is a good reason to consider the fridge. A cold tin has its own hazard, because opening it too soon invites condensation. Yunomi's team suggests 2 to 4 hours of warming when matcha comes out of long-term cold storage, so a daily scoop from the fridge takes some planning.

Whichever you pick, try to settle on one place and leave the tin there. A tin that moves back and forth between cold and warm gets more chances to collect condensation. That is the problem all of these cold-storage instructions are trying to avoid.

Spare tins and the freezer

Sealed spares are a different case, and the freezer has a real role with them. Ippodo suggests opening and finishing one package at a time. If you want to keep tea past its best-before date, it says to freeze the unopened package. When you are ready, let it sit at room temperature for around five hours so the condensation evaporates and the tea adjusts. After you open it, Ippodo says not to put it back in the freezer. Yunomi's team adds that matcha in its original sealed packaging can spend a year in the freezer and keep much of its flavor.

Holding most of the tea back and exposing only a little is an old habit. Yunomi's team describes tencha for tea ceremonies being kept in ceramic jars in a cool, dry place. Only the amount needed was ground, a couple of days before the event, and it was held in a lacquered or ceramic caddy that closed tightly.