yuzu
A Japanese citrus fruit about the size of a mandarin, with thick, bumpy rind. Its juice and zest are sour and intensely fragrant, somewhere between lemon, grapefruit and mandarin.
Yuzu is an East Asian citrus fruit with a thick, bumpy yellow or green rind, little flesh and many seeds. It is used mainly for its juice and zest rather than the flesh. The juice is sharply sour, but what defines yuzu is the floral, bitter-fresh fragrance of the rind, which marks a dish even in small amounts and fades quickly with heat, so it is usually added at the end or used raw. Fresh fruit is rare and expensive in Europe and North America, turning up occasionally in winter at specialty grocers or by mail order. Bottled yuzu juice is easy to find in Asian grocery stores, often with added salt, which should be taken into account when seasoning; there is also yuzu kosho, a paste of rind, chili and salt.
Substitute:
A mix of lemon or lime juice with a little grapefruit juice and grated mandarin or orange zest comes close on acidity. The floral fragrance that makes yuzu what it is will be missing; the dish tastes bright and sour but more ordinary. Where yuzu is the star, as in ponzu or a dessert, bottled yuzu juice is worth seeking out over any substitute.