Frozen tuna is as cold as dry ice — never touch it with wet hands (fingers stick). Use rubber gloves.video 8-2
Prepare 40–50 °C salt water, 3–4 % salt (a little saltier than the sea).video 8-2
Rinse the block under running water to wash off chips, shavings and any marker writing on the skin.video 8-2
Soak in the salt water: about 1–2 min for a block, about 20 s for a saku — time proportional to size, temperature always the same. Without this soak you don't get the bright red colour.video 8-2
Sandwich between dry cloths in a container, refrigerate, defrost slowly a whole day. Start the day before.video 8-2
Breaking down
Use a long knife for tuna.
The kama (collar) can be removed for negitoro.
Remove the chiai (dark blood meat).
Slice holding the flesh, push and pull. One thumb width.
Akami (lean red meat): once the hard part is removed, lay the flat side down and cut 4 fingers' width.
Cutting for nigiri
Fatty side (skin side) away from you; akami side close to you.video 6-4
Grain: set the block so the grain runs toward you, curling toward you. Cut perpendicular to the sinews (nerves/tendons). For pure akami the tendons don't matter much.video 6-4
Tezaku (hand-size block, 8 cm already set): cut sideways in 3–3.5 cm widths for 8 × 3–3.5 cm rectangles. Too tall → take a layer off the top first. Aim the blade 3 mm in from the previous edge, then upright the knife to leave a koba (ridge). Last piece: work from the lower side on the left.video 6-4
Long saku: fatty side away, grain toward you. If the block is a full 8 cm, cut straight 8 × 3. If narrower (~5 cm), read which way the grain runs and cut against it, then koba.video 6-4
Koba faces up: grain shows less on the top, more on the rice side.video 6-4
Nigiri: more wasabi on fatty tuna.
Zuke (soy-marinated)
Old preservation method from before refrigeration, mostly for akami; today a cooking method.video 8-1
Make the marinade — ratio and alcohol burn-off → zuke.
Blanch the tuna 5 s until the surface turns white, then ice water. Stops the soy going too deep and too salty.
Wipe dry. In a bowl, a ladle of zuke, paper towel over. Marinate 1–2 h.
Cutting for sashimi — kaku-zukuri
Cubes, or rather rectangles (easier to plate). Halve the saku lengthwise; trim the sides so the four corners are clean.video 7-4
Cut bite-size, equal pieces. Knife: slide to the far end, tip touches the board, left index holds the fish, slide back — the piece doesn't move.video 7-4
Plating: no tsuma. Cut surface up, two side by side and one on top, or odd numbers (5), leaned or standing. Wasabi in a cone shape, front right.video 7-4
Storage
Frozen tuna must be kept below −40 °C. Never defrost and refreeze — more drip, less flavour.video 8-1
Refrigerate wrapped in paper towel + film, lidded.video 8-1