By Harold McGee
The aroma chemicals of herbs and spices are volatile: that is, they’re small and light enough to evaporate from their source and fly through the air, which allows them to rise with our breath into the nose, where we can detect them. High temperatures make volatile chemicals more volatile, so heating herbs and spices liberates more of their aroma molecules and fills the air with their odor. Unlike most of the objects that we sense around us, which we see or touch or hear, aromas are an invisible, intangible presence. To cultures that knew nothing of molecules and odor receptors, this ethereal, penetrating quality suggested a realm of invisible beings and powers. So herbs and spices became important in the sacrificial fires and incense of religious ceremonies; they were offerings to the gods, a way of evoking their presence and imagining their heaven. Perfumes—a word that comes from the Latin for “through fire”— have long been the source of a similarly mysterious appeal.