These reactions happen when sugar is heated at or above its melting point. Like the Maillard reaction, caramelization requires heat to drive the complex, multistep reaction. The first part of the reaction is to melt the solid sugar crystals into liquid form. Then the sugars are further heated where they begin to lose OH groups and C atoms as smaller, often volatile compounds. Further heating will induce the collisions necessary to join many of the degraded sugars into large, viscous, dark‐colored polymers. It is possible to heat the sugar too much; extended heating will create a burned, carbon (charcoal-like) residue. Complete oxidation (burning) of sugars will convert all the carbon, hydrogen, and oxygen atoms to CO2 and H2O leaving nothing behind. For solid, crystalline sugars in contrast to liquid sugars like honey or syrups there are two common techniques to make caramel, the wet and the dry methods. The dry method is simple and begins with a slow heating of solid sugar, such as sucrose (i.e., table sugar), until the solid is turned to a liquid.
As a solid is heated, eventually the noncovalent intermolecular interactions holding the molecules in the crystal lattice are defeated, and the sugar molecules melt and begin to move more freely. This melting occurs as the sugar molecules acquire enough energy to overcome the intermolecular forces binding them into an orderly crystalline lattice. Melting does not occur instantaneously, because molecules bound to each other in a lattice must absorb the energy and then physically break the intermolecular forces.
Typically, the outside of a crystal will melt faster than the inside, as it takes time for the heat to penetrate. For many years, the conventional thinking was that addition of heat first melted sucrose into a liquid, and then the liquefied sugar broke down via chemical changes to a caramel product. However, Dr. Shelly Schmidt, a food chemist at the University of Illinois, published interesting research findings in a series of papers. This work demonstrates that when heated slowly, some of the sugar molecules begin to chemically change into caramel products while still in a solid, crystal lattice form.