Yuzu is a Japanese citrus fruit, which has a unique flavour and is hard to substitute. It is available in Australia in bottles from good Japanese food stores. The flavour is similar to that of grapefruit, with the acidity of a lemon. Fresh yuzu is almost impossible to find. It is usually sold salted, which is fine for this recipe, but if it is not available, use the combined juice of one lemon, one grapefruit and one lime.