ALT The Chemistry of Guinness. WHY DO THE BUBBLES FALL? At the center of the glass, the bubbles don't experience the drag they do at the sides, so they rise rapidly. As the bubbles at the center rise, they pull the liquid with them. When the liquid reaches the surface, it flows outward, toward the sides of the glass. When the current flows down the sides of the glass, it's strong enough to pull smaller nitrogen bubbles with it, creating the falling effect. WHAT CAUSES THE BITTERNESS OF GUINNESS? During the brewing process, hops release the a-acid humulone. Guinness uses hops with high levels of a-acids, which isomerize when boiled to form iso-a acids, the primary source of bitterness in beers. THE COLOR OF GUINNESS The dark color of Guinness has been attributed to melanoidins. These are polymeric structures created during the malting process that give beers their dark hue. They are created by the Maillard reaction, in which sugars and proteins react when heated.