Bittersweet is from the language of chocolate, and describes the co-presence of positive bittering compounds balanced by sweetness. It is directly related to caramelization, but has inputs from other roast reactions, as well as bittering flavors such as trigonelline. Bittersweet is usually a roast flavor term, but is always specific to the green coffee too (good bittersweetness would not develop at any roast level in a coffee without the native compounds to engender it). Usually, bittersweetness of a coffee develops as the roast gets darker and eventually overpowers other flavors. It dark roasts, acidity is reduced, while the caramely taste of sugars form the stimulating bittersweetness.