Nankotsu

Nankotsu literally means soft bone, the Japanese word for cartilage. The dish belongs to the whole-chicken, nose-to-tail eating of izakaya and yakitori cooking, in which every part of the bird is used, and it spread with the postwar growth of yakitori dining. The breastbone cut is called yagen nankotsu because its shape resembles the yagen, a boat-shaped grinding mortar long used to crush herbs in traditional medicine; the knee cut is called hiza nankotsu or genkotsu. The precise origin of cartilage as a prepared dish is unrecorded. Izakaya cartilage karaage most often uses the firmer knee cartilage, while the softer breastbone cut is common on grilled skewers.

The defining quality is crunch. Cartilage is firm and crisp with a clean, almost crackly snap and very little meat to chew. The fried version is crunchy on the surface and crunchy through, salty, and often touched with garlic; the breastbone cut eats softer than the harder, gravelly knee cut. The grilled version is charred and smoky, chewy-crunchy at the edges. Done well it gives an audible crunch with no toughness; done badly it is rubbery and limp when under-fried, or hard and tiring to chew when overcooked.

Cartilage is connective tissue, built from a dense extracellular matrix of type II collagen reinforced by proteoglycans such as aggrecan, which bind large amounts of water. That stiff, highly hydrated matrix is what makes it firm and crisp. Unlike muscle, it has almost no contractile fiber, so the sensation is crunch rather than the fibrous pull of chewing meat. Deep-frying at around 170 to 180°C crisps the surface through Maillard browning and dehydration while the cartilage itself stays crunchy: at these short frying times its collagen has no chance to gelatinize, which is precisely why long, moist braising is needed to soften the same tissue into tenderness. A light starch or flour coating sets into the crisp shell. On the grill the surface chars through Maillard reactions as surface moisture is driven off.