Learn to Make a Knife Out of Tuna Fish

S35VN, M390, and 154CM are known to be some of today’s most sought after blade materials. But, a YouTuber recently overlooked these options and instead made a knife out of katsuobushi, a variety of dried Japanese tuna. The Kiwami Japan YouTube channel uploaded a 10-minute video to teach you the process of turning a hunk of katsuobushi into a working, kiridashi-style knife.

Katsuobushi shavings are used as a topping in soups and on Okonomiyaki (Japanese pancakes) and it holds the record for the world’s hardest food according to the Guinness Book of World Records. Katsuobushi may look like a piece of driftwood, but it starts life as a skipjack tuna. The fish is deboned, cut into fillets, trimmed of fat, and smoked for 30 days. After that, the smoked fillets are spray-covered in a food-safe mold culture and rotated between fermenting and drying stages for half a year. An adult skipjack tuna weighs around six pounds, but at the end of this procedure, each fillet has retained only a quarter of its original mass.

How do you turn this rock-hard fish product into a usable knife? The YouTuber starts by working down the katsuobushi on a kezuriki, a hollow box with a blade on its lid used to shave off bits of the durable fish product for cooking. Once the katsuobushi has been shaved into a narrower shape, it’s time to break out the files and whetstones. Then, much like someone touching up an edge on their knife, the YouTuber works the katsuobushi back and forth to “sharpen” it. He ends up with a sizable, semi-transparent cutting tool in the style of a kiridashi. You can see the full and fascinating process below.

A kiridashi is a traditional Japanese utility knife that dates back thousands of years in various forms. Often they are on the small side, with an angled, chisel-ground edge. Kiridashi may look aggressive to some, but the knife’s typical use encompasses mundane chores like sharpening pencils, pruning plants, and wood marking.

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It’s hard to say whether or not the katsuobushi kiridashi can handle those jobs, but in the video, it manages to slice through paper and stab through a soda can. Edge retention probably won’t be on par with Maxamet blade steel, but it will definitely taste better.

Knife featured in image: Kiwami Japan Katsuobushi Kiridashi