Wikipedia Article About Verjus on Wikipedia
Verjuice (verjus in French, "green juice") is a very acidic juice made by pressing unripe grapes or crab apples. Sometimes lemon or sorrel juice, herbs or spices are added to change the flavour. In the Middle Ages, it was widely used in French cuisine as an ingredient in sauces, as a condiment, or to deglaze preparations.
It was once used in many contexts where modern cooks would use either wine or some variety of vinegar, but has become much less widely used as wines and variously flavoured vinegars are more accessible nowadays. Nonetheless, it is still used in a number of French dishes as well as recipes from other European and Middle Eastern cuisines, and can be purchased at some gourmet grocery stores.
In The Medieval Kitchen: Recipes from France and Italy, the authors claim that the grape seeds preserved in salts were also called verjus during the Middle Ages.
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.