Verjuice

An acidic, sour liquid made from unripe fruit, primarily grapes. Verjuice is used in preparations like sauces and mustards to heighten flavor, much as lemon juice or vinegar would be employed. Not widely used since medieval and renaissance times, verjuice is now enjoying a comeback in many dishes. Though it is occasionally available in specialty gourmet shops, verjuice is extremely difficult to find in the United States.