The Breakdown of Nonvolatiles
One sensory indicator of coffee’s flavour degradation due to storage is an increase in its harsh, sour, and bitter qualities. Ted Lingle attributes this flavour change to the breakdown of chlorogenic acids in the brew as it is stored. Chlorogenic acids (CGAs) are the most abundant type of acid found in brewed coffee. Lingle tells us the content of CGAs in the finished brew reach as high as 15% of the total dissolved solids (The Coffee Brewing Handbook, 1985).
The molecular structure of Chlorogenic Acid and its component parts, Quinic and Caffeic Acid (Santos, et al, 2006)