The amount of acid in the must, liquor, or finished wine. Insufficient acidity in the must will result in a poor fermentation and a slightly medicinal and flat taste. Too much acid will give wine an unpleasant sourness or tartness. Some acid is necessary for fermentation, and up to one-fourth of the initial acid content will be consumed by the yeast during fermentation. Low-acid musts are usually corrected by adding tartaric acid (the principle acid in grapes), malic acid, citric acid, or acid blend. An acid testing kit is indispensable in measuring initial acidity. There are two measures of acidity used in winemaking; see also pH.