Why is coffee bitter?

Caffeine is bitter, but research suggests it accounts for only around 10–15 percent of coffee’s bitterness. Much of the rest comes from chlorogenic acids in the green bean, which break down during roasting into bitter lactones and, in darker roasts, into harsher phenylindanes. That is why dark roasts often taste more bitter even though they can contain slightly less caffeine by weight. Brewing also matters: over-extraction, from water that is too hot, grinding too fine or brewing too long, pulls out more bitter compounds. Humans may be sensitive to bitterness because in nature it often signals toxins.