• Gather all the ingredients.

  Wipe any dirt spots from the kombu with a damp cloth. Do not wipe off the white powdery compound called mannitol, as it's the key contributor to umami.

  Nami's Tip: The surface of kombu is pretty clean these days, so you may not need to wipe it.
  
• 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.
  
• Cover and steep at room temperature for 2–3 hours in the summer or 4–5 hours in the winter. You can also steep overnight in the refrigerator.
  
• Remove the kombu (see below for how to use it in other recipes). The dashi is ready to use.
  

Method 2: Kombu Dashi on the Stovetop (Nidashi)

• Put the kombu and water in a medium pot.

  Nami's Tip: If you have time, soak the kombu for 3 hours or up to a half day to release more flavor naturally.
  
• Set the heat to medium low and bring it to a bare simmer over about 10 minutes.
  
• Meanwhile, skim any foam from the surface with a fine-mesh skimmer.
  
• Just before the dashi starts boiling, remove the kombu (see below for how to use it in other recipes).

  Nami's Tip: If you leave the kombu in the pot, the dashi will become slimy and bitter. 
  
• The 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 up to 4–5 days or in the freezer for up to 2 weeks. I recommend using it sooner for the best flavor.

To Use the Spent Kombu

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