• Gather all the ingredients. Most Japanese recipes say to gently clean the kombu with a damp cloth. However, these days, kombu is pretty clean so just make sure it doesn‘t have any mold spots and it‘s ready to use. Do not wash or wipe off the white powdery substance as it has lots of umami.
  
• Make a couple of slits on 1 piece kombu (dried kelp) to release more flavor.
  

Method 1: Cold Brew Kombu Dashi (Mizudashi)

• Put 4 cups water and the 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 wintertime. You can also cold brew the Kombu Dashi overnight in the refrigerator.
  
• Remove the kombu from the bottle and reserve the spent kombu (see below). The 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 on the Stovetop (Nidashi)

• Put the kombu and water in a medium pot. If you have time, soak for 3 hours or up to a half day. The kombu’s flavor comes out naturally from soaking in water.
  
• Turn on the heat to medium low and slowly bring to a bare simmer, about 10 minutes.
  
• Meanwhile, clean the dashi by skimming the foam from the surface with a fine-mesh skimmer.
  
• Just before the dashi starts boiling, remove the kombu from the pot (see below for what to do with it). If you leave the kombu in the pot, the dashi will become slimy and bitter. 
  
• Now the Kombu Dashi is ready to use.
  

To Store

• 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 the spent kombu?

• Save the spent kombu in an airtight container and store it in the refrigerator for a week or in the freezer for up to a month.
  
• With the spent kombu, you can make Simmered Kombu (Kombu Tsukudani).
  
• You can also make Homemade Furikake (Rice Seasoning).