Gather all the ingredients. Most Japanese recipes would say gently clean the kombu with a damp cloth. However, kombu is pretty clean these days so just make sure it doesn't musty. DO NOT wash or wipe off the white powdery substance (Mannitol), which contributes to the umami flavor in dashi.
Make a couple of slits on the kombu, which will help to release more flavor.
Method 1: Cold Brew Kombu Dashi (Mizudashi)
Put water and kombu in a large bottle.
Put the cap on and let it steep on the counter for 2-3 hours in the summertime and 4-5 hours in the winter time. You can also cold brew kombu dashi overnight in the refrigerator.
Remove the kombu from the bottle and reserve the used kombu (see below). Kombu dashi is now ready to use. If you are not using the dashi right away, save it in a bottle and keep in the refrigerator for 4-5 days or in the freezer for 2 weeks. I recommend using it sooner for the best flavor.
Method 2: Kombu Dashi with Boiling Water (Nidashi)
In a medium pot, put the kombu and water. [optional] If you have time, soak for 3 hours or up to a half day ahead of time. Kombu’s flavor comes out naturally from soaking in water.
Turn on the heat to medium-low heat and slowly bring to a bare simmer, about 10 minutes.
Meanwhile, clean the dashi by skimming the surface.
Just before the dashi starts boiling, remove kombu from the pot (see below for what to do with it). If you leave the kombu inside, the dashi will become slimy and bitter.
Now kombu dashi is ready to use.
If you are not using the dashi right away, keep it in a bottle (or airtight container) and store in the refrigerator for 4-5 days or in the freezer for 2 weeks. I recommend using it sooner for the best flavor.
What to Do with Used Kombu
Save the used kombu in the airtight container for a few days in the refrigerator or for up to a month in the freezer. With the leftover kombu, you can make Kombu Tsukudani (simmered kombu) or Homemade Furikake (rice seasoning). I usually save the leftover kombu enough to make tsukudani.