verjuice
Sour juice pressed from unripe green grapes, known in Persian cooking as ab ghooreh. Mildly tart and lightly fruity, gentler than vinegar.
Verjuice is the pressed juice of unripe grapes, harvested before they develop sugar; in Iran it is called ab ghooreh, in Turkey koruk suyu. It is clearly sour but softer and rounder than vinegar or lemon, with a fresh, green grape note and none of the bite of acetic acid. That is why it can be added generously to stews, soups, salads and dressings without making them harsh, and it sits well alongside wine because it does not mask its flavour. You will find it in Persian and Middle Eastern grocers, Turkish supermarkets, delicatessens and from online suppliers. Once opened, keep the bottle in the fridge.
Substitute:
Lemon juice diluted roughly half and half with water comes closest in acidity, but brings a distinct citrus aroma that verjuice lacks. A little mild white wine vinegar with a splash of grape juice gets nearer the grape note but tastes sharper; in either case start sparingly and adjust to taste.