>Katsuobushi is made of katsuo or bonito, skipjack tuna, a saltwater fish. Bonito is rich in protein. If unprocessed, it has a 25% protein content, and if used to make katsuobushi, its protein content increases to 77%. It is also rich in inosinate, an important umami substance; the umami is multiplied many times over when combined with glutamate. This is the mechanism behind ichiban dashi (“first soup stock”) in Japanese cuisine.

Source: https://www.umamiinfo.com/richfood/foodstuff/katsuobushi.htm...