Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World

by Mark Miodownik

The reason why any sugar molecule—whether in a cocoa bean or a pan or anywhere else—turns brown when heated is to do with the presence of carbon. Sugars are carbohydrates, which is to say that they are made of carbon (“carbo-”), hydrogen (“hydr-”), and oxygen (“-ate”) atoms. When heated, these long molecules disintegrate into smaller units, some of which are so small that they evaporate (which accounts for the lovely smell). On the whole, it is the carbon-rich molecules that are larger, so these get left behind, and within these there is a structure called a carbon–carbon double bond. This chemical structure absorbs light. In small amounts it gives the caramelizing sugar a yellow-brown color. Further roasting will turn some of the sugar into pure carbon (double bonds all round), which creates a burnt flavor and a dark-brown color. Complete roasting results in charcoal: all of the sugar has become carbon, which is black.