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Verjuice, literally from the French vert (green) and jus (juice) is the juice of unripe fruit and was once a common souring substance, though little used today.
What is Verjuice and How Do You Use It? Photo Gallery
In England, it was made from sour green apples, preferably crab apples. The small amount of sugar present in such juice was allowed to ferment by standing a day or so, after which scum was removed and salt added. Sometimes it was concentrated by boiling. Such verjuice would keep well, due to its high acidity and salt content. Very sour fresh cider which has not yet vinegared may be substituted for it.
In France, verjuice was made by a similar process from the juice of sour green grapes, and there were special varieties for the purpose. Today, verjuice is mainly used in the preparation of mixed mustards, but is almost always a made-up and not a natural product. (See Mustards.) It should not be confused with must which is the unfermented juice of ripe grapes, though sometimes verjuice means the juice of ripe white grapes (Perdreaux au verjus). (In many grape-growing areas, green grapes are occasionally used in meat dishes. I have come on them sometimes and they are excellent.)