Chlorogenic acid
also: CGA, chlorogenic acids
A family of antioxidant acids in green coffee that break down during roasting and shape acidity and bitterness.
Chlorogenic acids (often shortened to CGA) are a large group of plant compounds and one of the most abundant acid families in green-coffee. They are antioxidants, and coffee is one of the bigger sources of them in many people’s diets.
Roasting is where they matter most for flavor. As beans heat up, CGAs break down: some convert into quinic and caffeic acids, and at darker levels they form bitter compounds called chlorogenic acid lactones and, eventually, harsher phenols. This is a big part of why light roasts taste brighter and more acidic while dark roasts taste more bitter and less lively.
So CGA content drops steadily the longer and hotter you roast, trading acidity for body and bitterness. Understanding this helps explain why the same green coffee can read as crisp and fruity or flat and roasty depending on the roaster’s choices. For how that plays out across the spectrum, see roast-levels-explained and sour-vs-bitter.