Verjus, the acidic sour juice of green grapes, was extremely popular in sauces and dishes in the Middle Ages, but is now replaced in most recipes with wine or vinegar.
You can buy it in small bottles in gourmet shops (it costs more than many wines). French restaurants sometimes offer chicken au verjus, and some chefs prefer it to vinegar or lemon in a salad dressing because it’s more compatible with wine.
Images by BorgQueen at English Wikipedia., Dana Facaros