Sour salt is not salt at all but it is citric acid. It is used to add an extra tart flavor to sour dough and rye breads. It may be used in canning to prevent fruit from turning dark. To prevent mold in sprouting, use 1/4 tsp Sour Salt per quart of soak or rinse water. In tomato canning, use 1/2 tsp Sour Salt per quart.