Generally, Kakushi boucho is used as part of preparing the ingredients and cut into them so that they absorb the flavor better, or to make it easier for heat to pass through them so they cook better.
Also, it refers to adding cuts with a thin knife to cut the lines in sushi toppings that are tough and harder to chew. For example, very fresh squid has an elasticity as if it’s about to burst. When squeezed in the hand, the shari (vinegared rice) doesn’t stick to the squid and the sushi falls apart easily. Putting these two incompatible pieces together is one way the sushi chef’s skill is showcased. This requires a “Kakushi Boucho.” Squid meat is especially tough and if slits are not made in it, the topping will remain in your mouth long after the rice is gone.
Naturally this is not unique to squid. When eating tuna, have you ever noticed that the white lines in the meat are sometimes left in your mouth? That’s why fine Kakushi boucho is used to cut those white lines in advance. This does not just improve the texture, adding Kakushi Boucho also increases the surface area of the topping, which also increases the sensation of the umami and sweetness of the fat. In addition, with fish that contain a lot of fat, such as tuna, make it keep the Nikiri shoyu better.
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