Caramelization is a chemical process that takes place when sugar is heated to a high temperature. In this process, sugar is broken down in a complex series of chemical reactions. This causes the sugar to gradually pick up a brown color and makes its flavor much more complex. This process is often used in cooking in order to evoke richer and more complicated flavors from food. It is also critical in the making of candy and is responsible for the flavor, texture, and appearance of caramel.
Heat has a variety of different effects on food. If used correctly in the cooking process, heat does more than just break down indigestible elements and kill hostile microbes. The addition of heat to food triggers many complex chemical reactions. These reactions, including both caramelization and the Maillard reaction, produce many new organic compounds by breaking down and modifying the molecular structure of food. Most people find these compounds to have very pleasant flavors.
Sugar is the specific target of caramelization. Most foods contain some amount of one sugar or another. Fructose, sucrose, glucose, and maltose are all common in many different foods as are other sugars. As these sugars are heated, their chemical structure begins to break down. The specific temperature at which this process occurs varies from sugar to sugar, but most begin to caramelize at 310 degrees Fahrenheit (154 degrees Celsius), although fructose has a much lower caramelization point, and will begin to break down at 230 degrees Fahrenheit (110 degrees Celsius).
The low temperature at which fructose begins to break down means that it is the easiest sugar to caramelize, but also that care must be taken when working with foods that are rich in several different types of sugar. The flavor of onions famously benefits from caramelization. The sugar in onions comes in several types, however, including fructose. In order to successfully caramelize onions a cook must apply enough heat to begin breaking down the fructose but not so much as to break down the other sugars, as the fructose would overheat and begin to burn.
Careful temperature management is important in one other common application of caramelization. Sugar-based candy depends on the very diligent management of the temperature of sugar syrup. Caramels, unlike some other candies, are made by boiling off all of the water in sugar syrup. This leaves sugar that is just barely at the caramelization point. Some additional heating, up to about 350 degrees Fahrenheit (176.7 degrees Celsius), allows the reaction to proceed.
A careful candymaker can then keep the temperature relatively constant until the reaction has run its course and produced a rich, sweet, nutty flavor. Too much heat at this point will simply scorch the sugar. This produces smoke and ruins a batch of candy.