The most common form of dashi is a simple broth or stock made by heating kombu ( edible kelp ) and katsuobushi ( flakes of dried skipjack tuna ) in water and then straining the resultant liquid . The most common form of dashi is a simple broth or stock made by heating water containing kombu ( edible kelp ) and kezurikatsuo ( shavings of katsuobushi - preserved , fermented tuna ) to near-boiling , then straining the resultant liquid .