
Why Is Katsuobushi Called “the Hardest Fish in the World”?
Many people have probably seen those thin, pinkish flakes of dried fish sprinkled over takoyaki and okonomiyaki, or used to make Japanese soup stock.
But did you know that… before it becomes those delicate flakes, katsuobushi is so hard that it requires a special shaving tool because it cannot be sliced with an ordinary knife?
This incredible hardness comes from a traditional Japanese production process that takes several months.
It begins with:

Boiling the bonito → repeatedly smoking it → aging it with beneficial mold → drying it several times
This process removes almost all of the moisture from the fish, leaving only around 10–15% moisture. As a result, the fish becomes extremely hard and can be stored for a long time.
In addition to extending its shelf life, the aging process also enhances its umami flavor, particularly through inosinate (inosine monophosphate).
That is why katsuobushi is a key ingredient in dashi, the essential Japanese soup stock, and one of the distinctive flavors of Japanese cuisine.