There are two methods; one is to soak kombu kelp in water on overnight to extract Dashi and the other is to extract by the heat. The glutamic acid contained in kombu kelp is weak against heat. To avoid the bitter taste, if you make Dashi by the heat, kombu kelp must be taken out before the water starts boiling.
Using the heat is quick and easy to extract and the Dashi has less kombu kelp bad smell. But it quickly goes bad. Soaking water makes the Dashi rich in flavor although it takes longer.
Head chef of Japanese cuisine of Tokyo Kaikan
makombu kelp: 8g (approximately 10cmx10cm)