Bring the saké to a boil and simmer on low heat for about one minute so the alcohol evaporates.
Add mirin, soy sauce and a piece of kombu (3cm x 3cm), bring to the boil and simmer for 5 minutes. The kombu will bring the fishy flavour.
Leave to cool down and add a few sprays of liquide smoke, then store into fridge.
To make the zaru soba bouillon, you’ll need 50ml of this concentrate per person. The ratio is 1/3 sauce + 2/3 ice-cold water.
Your home-made mentsuyu does not contain any fish so you can keep it refrigerated for months!