ponzu
A thin Japanese seasoning sauce of citrus juice, soy sauce, vinegar and dashi, tart and salty with a fresh citrus aroma.
Ponzu originally referred to a blend of citrus juice and vinegar; today it usually means ponzu shoyu, that blend combined with soy sauce, mirin and a stock of kombu and bonito flakes. The citrus comes from Japanese fruits such as yuzu, sudachi, kabosu or daidai, which have a more bitter, floral fragrance than lemon. The sauce is thin, distinctly sour, salty and rounded with umami, and its aroma fades with heat, so it is used cold as a dip, dressing or finishing drizzle. It is sold in Asian grocery shops, increasingly on the Asian shelf of larger supermarkets, and online. Once opened it belongs in the fridge, where it slowly loses its freshness.
Substitute:
Soy sauce with roughly the same amount of lemon or lime juice, a splash of rice vinegar and a little mirin comes close in taste; a touch of orange or grapefruit juice brings the aroma nearer to Japanese citrus. What is missing is the depth of dashi and the distinct fragrance of yuzu or sudachi, so the mix tastes flatter and more sharply sour.