Verjuice

Verjuice (verjus) is the juice of underripe grapes. It was widely used as a sauce flavoring and condiment in medieval and Renaissance French and Italian cooking. Very acidic, its function in a sauce is somewhat like that of vinegar or lemon juice but not exactly like either one. It is the latest darling of French chefs and is beginning to show up in sauces, usually for fish. It seems to go particularly well with fennel and coriander seeds. Try experimenting with verjuice as a replacement for lemon juice in fish sauces.