Maturation fixed acidity
The acidity of the spirit increase rapidly at first, and then levels off. For this plot, the acidity is measured in grams of acetic acid per 100 liters. As a comparision, white vinegar, which is 5% acetic acid would have an acidity rating of around 4000 on this chart. Or stated another way, an ounce of vinegar added to a four gallon bucket of water would have about the same acidity as whisky.
From Changes in Whisky While Maturing by A.J. Liebmann and Maurice Rosenblatt in the September 1943 issue of Industrial and Engineering Chemistry.