The simplest browning reaction is the caramelization of sugar, and it’s not simple at all. When we heat plain table sugar, essentially just molecules of sucrose, it first melts into a thick syrup, then slowly changes color, becoming light yellow and progressively deepening to a dark brown. At the same time, its flavor, initially sweet and odorless, develops acidity, some bitterness, and a rich aroma. The chemical reactions involved in this transformation are many, and they result in the formation of hundreds of different reaction products, among them sour organic acids, sweet and bitter derivatives, many fragrant volatile molecules, and brown-colored polymers. It’s a remarkable change, and a fortunate one: it contributes to the pleasures of many candies and other sweets.