How to make Kombu Dashi (Japanese cold-water method)


Kelp (or “kombu” in Japanese) is really good for both you and the planet. It’s packed full of minerals (calcium, copper, iodine and iron), polyunsaturated amino acid. The World Health Organisation (WHO) recommends a ratio of omega 6 : omega 3 of less than 10, but this ratio for kombu is 1, which is fantastic!

They grow and are harvested all year around without the need for chemicals, water or forest clearing. Most importantly they are excellent (and much better than plants growing on land) at absorbing carbon dioxide both from atmosphere and ocean, which can help towards combatting climate change.


Japanese people use kombu all the time for their cooking. We mainly use it to obtain stock (dashi) and cook various things with it. The left-over kombu is then used to make other dishes such as tsukudani, which goes so well with rice. There is absolutely nothing to be wasted with this amazing algae!

Traditionally kombu dashi can be taken in two ways: cold method or hot method.

1) Cold method

Kombu is simply placed in water and soaked over 12 hours in a refrigerator.

Pro’s: It’s easy! You get a clear, refined flavoured stock. No energy used.

Con’s: Time-consuming to make it.

2) Hot method

Kombu is soaked in water for 10 min then heat is applied just before it reaches boiling point. Kombu is then removed.

Pro’s: Quick.

Con’s: You can over-boil the kombu which makes the final stock product a strong sea-weedy taste which is also slightly slimy.

In this post I will introduce you to the cold method. I like this because I don’t need to use a hob, which means no energy being consumed! I tend to start making it (usually about 1-2L at a time) a night before I need some stock, then I freeze whatever is left over.

The proportion of kombu you need is 1-2% w/v which means you need 10-20g per 1000ml of water. Use filtered water if you can.


Kombu from Cornwall

I live in the U.K., which is a great excuse to try out British kombu. In this recipe, I use organic kombu (kelp in their product description) from Cornish Seaweed Company. I love their sustainability pledge and this is exactly the kind of seaweed that we should be encouraging more people to be eating; they are good for both you and the planet.

The Cornish kombu seem thinner than some of the Japanese ones I have seen. For this recipe I have used 2% w/v (i.e. 20g in 2L water). It managed to make such a beautifully refined kombu stock and I will soon be blogging about how to use this stock to make a scrumptious vegan ramen stock – possibly the world’s most climate-smart and planetary healthy recipe ever!


20g Cornish Organic Kelp

1L filtered water


  1. In a container with lid, pour the filtered water.
  2. You can dust kombu away with a clean dry kitchen roll, but I didn’t bother as mine were very clean.
  3. Add the kombu.
  4. Put a lid on and place in refrigerator for at least 12 hours.
  5. Remove kombu – I don’t throw them away. I freeze mine and use them for other cooking.
  6. Dashi complete. Use the stock within 2 days. I freeze the left-over dashi until the next use.