Wipe the surface of the kombu lightly with a damp cloth. The purpose of this is to remove any impurities, such as dirt or sand that may collect on the surface while the kombu is being dried. Take extra care not to wipe the white powder from the surface: that is mannitol which gives the stock its sweetness and umami.
2
Cut the kombu into 3" long pieces, about an inch in width. Place them into a container and pour the water over it to cover. Place the container into the refrigerator to steep overnight (24 hours).
3
After 24 hours, take the container from the refrigerator, and use a set of chopsticks to remove the kombu. Set the kombu aside for another use. Reserve.
Note
STORAGE. The kombu dashi will hold in the refrigerator for 2-3 days maximum.