Verjus is a vinegar made from the must of unripe grapes with a mild acidity of about 4.5% and is widely used in classic French cuisine. The percentage of acetic acid is very low, making the verjus pleasantly fresh. Use it especially to deglaze the fond in which you have cooked light poultry, fish, or shellfish; this way you get a nice base to start a sauce. Also very tasty to deglaze, for example, veal.