Did you know that kombu seaweed that is used for dashi stock doesn’t have a real expiration date? At the grocery store, any packaged product, including kombu seaweed, is required to print expiration date on their package. As long as kombu seaweed is kept in dark cold place, it can be used for almost forever unless molded. In fact, some people let the kombu seaweed stored for the better taste. Kombu seaweed is already dried and stored for a year before ship out, but aging kombu will reduce sliminess off the surface and tangle smell of the sea, enhancing umami
Back in the days, Kombu seaweed that was harvested at Hokkaido, Northern Japan, was transported to Osaka where kombu seaweed consumption was high. However, kombu did not always delivered to Osaka due to lack of mass transportation. It was not rare to store over winter for heavy snow and transported after winter. When they find out that aged kombu that was stored over winter tastes better, aged kombu were considered luxury kombu seaweed. Nowadays, mass transportation system have improved and made kombu to be transported even in the harsh weather. Storing expensive kombu seaweed in a storage to age was too risky and costly due to maintenance fee and possible mold occurring. Consequently less and less manufacture still store kombu to be aged long term anymore.
In our company, we harvest kombu during summer and stored in storage made particular for kombu seaweed for the maximum taste. Not only we store kombu at kombu storage, premium quality kombu seaweeds are stored for years in the ripening chamber just like vintage wine.
Dashi kombu can be enjoyed even after ages as long as stored correctly. Even if you only use once in a while, just stock them in your pantry and use them for your convenience.
Just make sure before using, Kombu seaweed have white powder on a surface called mannit. Make sure you smell kombu before using it to figure out if its mold or not.