There are several ways of crushing herbs and spices, and they have different effects on flavor. Grinders, choppers, and mortars all generate heat. The hotter the aroma molecules get, the more volatile they become and the more readily they escape, and the more reactive and changeable they become. The original flavors are best preserved by prechilling both spice and grinder to keep the aromatics as cool as possible. Food processors slice into herbs and introduce a lot of air and therefore aroma-altering oxygen, while a pestle pounding in a mortar crushes herbs and minimizes aeration. Careful chopping with a sharp knife leaves much of the herb structure intact to provide fresh flavor while minimizing cell damage to the cut edges; by contrast, a dull knife crushes rather than cuts, bruises a wide swath of cells, and can result in rapid brown-black discoloration.