Maturation proof
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Changed in proof during maturation
During maturation, the proof of the spirit initially drops as some of moisture from the barrel enters the spirit. After that initial drop in proof, the ethanol concentration begins to rise as water passes out of the barrel at a faster rate than the ethanol.
From Changes in Whisky While Maturing by A.J. Liebmann and Maurice Rosenblatt in the September 1943 issue of Industrial and Engineering Chemistry.