Ponzu

In a clean glass jar or non-reactive bowl, combine the yuzu juice, rice vinegar, soy sauce, and mirin. Do not heat the citrus — raw juice keeps its high, perfumed top note, and cooking would dull the sourness into something flat and stewed. Stir gently to blend. The liquid should be a deep amber that smells equally of sharp citrus and salty soy, neither one drowning the other.

Wipe the kombu lightly with a damp cloth (never rinse off the white powdery mannitol — that is pure umami) and submerge it in the base along with the katsuobushi. Cover and refrigerate for at least 24 hours, ideally 2 to 3 days. This cold infusion draws glutamate from the kelp and inosinate from the bonito into the acidic liquid, building the deep savory undertone that defines true ponzu. The flakes will swell and slowly sink as they saturate.

Set a fine-mesh sieve lined with a paper towel or cheesecloth over a clean jar and pour the ponzu through, letting it drip without pressing — squeezing the solids forces through bitter, cloudy fines. Discard the spent kombu and bonito. Bottle the strained ponzu and let it rest in the refrigerator at least a few hours so the flavors settle and marry. It keeps refrigerated for several weeks and only improves over the first week.