Katsuobushi is simmered, smoked and fermented skipjack tuna . It is also known as bonito flakes. Katsuobushi or similarly prepared fish is also known as okaka.
Shaved katsuobushi and dried kelp—kombu—are the main ingredients of dashi, a broth that forms the basis of many soups (such as miso) and sauces (e.g., soba no tsukejiru) in Japanese cuisine.
Katsuobushi's distinct umami taste comes from its high inosinic acid content. Traditionally made katsuobushi, known as karebushi, is deliberately fermented with Aspergillus glaucus fungus in order to reduce moisture. Katsuobushi has also been shown to impart "kokumi" (i.e. enhances flavor).