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1
If using dried konbu, wipe the surface with a dry cloth to remove any surface salt without washing away the glutamate bloom. Soak in cold water for 20–30 minutes until pliable but not fully rehydrated. Drain and cut into pieces of 1–2 cm. If using post-dashi konbu, proceed directly to cutting — no soaking is required, and the subsequent reduction time should be shortened accordingly. The pieces should be uniform in size: surface area determines the rate of glaze penetration.
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2
Combine the soy sauce, mirin, sake, sugar, and rice vinegar in a wide, heavy-based saucepan. Add the cut konbu. Set over a medium-low heat and bring the liquid to a gentle simmer. The liquid will initially appear thin and pale; this is correct. Do not raise the heat to accelerate this stage — high heat at the outset produces surface reduction before the interior of each piece has softened, yielding a hard exterior with an underdone centre. The liquid should murmur, not boil.
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3
Maintain the gentle simmer for 45–60 minutes, turning the konbu pieces occasionally as the liquid reduces. This is where the dish lives or dies: the balance of soy, mirin, sake, sugar, and vinegar must be sustained throughout the reduction — insufficient sugar produces a hard, brittle result; excessive sugar produces sticky, tooth-coating sweetness without depth; insufficient vinegar produces grey-black, overly hard tsukudani. The liquid will thicken progressively into a lacquered glaze. The konbu will deepen from olive-green to a dark, glossy green-black. When only a few tablespoons of thick syrup remain in the pan and each piece is coated in a sticky, concentrated glaze, proceed to the test.
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4
Remove a single piece and allow it to cool for one minute on a clean surface. Press it gently between two fingers. It should hold its shape and spring back slightly — neither hard and brittle nor soft enough to collapse. If it is still yielding without resistance, return the pan to the heat and continue reducing. If it holds and springs, the tsukudani is done. The glaze on the cooled piece should be sticky and dense, not wet.
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5
Remove from the heat. Spread the pieces on a tray or transfer to a clean jar, ensuring no pieces are pressed tightly together while still hot, which would cause them to fuse. Allow to cool completely at room temperature before sealing. The finished tsukudani should be intensely savoury-sweet on the palate, with concentrated glutamate depth and a clean finish from the vinegar — not cloying, not sharp.