Sweetness is one of four basic sapid (in the mouth) tastes: Sour, Sweet, Salty, Bitter and Umami (savory flavors). While most would say bitterness is undesirable, coffee has essential bitterness to it. Most undesirable bitterness is formed by roasting defects (flash roasting, or slow baking of the coffee), too-light roast (astringent, trigonelline bitterness) or dark roasting (burned roast taste, no remaining sucrose). Bitterness is experienced from the rancid oils and residues of dirty brewing equipment. There are many types of bitterness, hence not one avenue to tracking down its source. Bitterness as a positive quality is balanced with residual sweetness, and we use the term bittersweet or bittersweetness to describe this, as in darker chocolate flavors.