Eel Filleting — Osaka Style vs Edo Style

The two dominant schools of unagi butchery evolved along strict regional lines: Kansai cooks developed the Osaka (belly-open) cut, working from the ventral side, while Edo-period Tokyo cooks codified the dorsal cut partly out of cultural aversion to anything resembling a ritual throat-cut, a sensitivity rooted in samurai sensibility. Both styles predate the Meiji era and remain the technical benchmarks for unagi preparation across Japan and any serious kitchen working with freshwater eel.

You are working with a live or recently killed eel. The animal's musculature is longitudinal and dense with intramuscular fat — McGee notes in On Food and Cooking that eels carry unusually high lipid content distributed through the flesh, which is precisely why the butchery method matters so much. Damage the fat pockets during filleting and you lose the yield and the self-basting character that defines good kabayaki. Osaka style: pin the eel belly-up through the eye or head into a wooden board. Entry cut is ventral, running from just below the jaw down toward the tail. The spine is followed from the inside, the knife angled to ride the vertebrae cleanly. The fish stays in one piece — head on — until the spine is removed. The belly flap remains largely intact. This produces a single butterflied sheet. Because entry is ventral, the dorsal fat seam stays undisturbed. Osaka cooks grill the eel without prior steaming; the fat must survive butchery because it has to do work on the grill. Edo style: the eel is pinned dorsal-side up through the neck region. The cut enters from the back, follows the spine down, and the fish is opened like a book with the backbone removed from above. In Tokyo practice the fillet then goes to the steamer before the grill — mushimushi — which softens the connective tissue and renders some intramuscular fat out before the final baste-and-grill. Edo butchery produces a slightly thinner, more even fillet because the dorsal approach peels away from both sides of the spine with gravity helping the flesh fall open. Both styles use an unagisaki — a specialized spike-pointed knife with a short, stiff blade around 18–21 cm. Trying to work either method with a standard deba is not a substitute; you lose the fine spine-tracing control the unagisaki is built for. In service, the Osaka fillet holds together on high direct heat without steaming; the Edo fillet is engineered for the two-stage method. Matching butchery to cooking protocol is the whole point.

Eel fat is rich in long-chain omega-3 and omega-6 fatty acids with a relatively low melting point — McGee observes in On Food and Cooking that fish lipids are predominantly unsaturated and remain fluid at refrigerator temperatures, which is why eel fat renders so readily on the grill and carries fat-soluble flavor compounds from the tare deep into the flesh. The collagen in eel's connective tissue converts to gelatin at relatively moderate sustained temperatures, producing the characteristic soft-but-cohesive mouthfeel of properly cooked unagi. The Edo steaming step accelerates this collagen hydrolysis before the grill, which is why Tokyo-style kabayaki has a softer texture than Osaka-style, which relies entirely on dry grill heat to manage both fat rendering and collagen conversion simultaneously.