The Sweet Science of Candymaking                 

By Tom Husband  October 2014

Spanish version

Rock candy

To make most types of candies, you always start by dissolving sugar in boiling water. This forms a sugar syrup, which you can cool down by taking it off the burner. But howyou cool down the syrup can make all the difference.

For instance, if you want to make rock candy, you need to let the syrup slowly cool down over many days until big sugar crystals form. But if you want to produce fudge, you need to continuously stir the syrup after an initial cooling period, so when the sugar crystals form, they stay small and do not grow too much. If you want to make cotton candy and glass candy, you need to cool the syrup quickly to keep crystals from forming.

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The main difference between these different types of candies is whether sugar crystals form and, if so, what their size is. So how do sugar crystals form, and what causes them to have different sizes when the syrup is cooled down?

Let’s assume we can see sugar at the molecular level. Each grain of sugar consists of a small crystal made of an orderly arrangement of molecules called sucrose. Sucrose is an example of a carbohydrate. The basic unit of a carbohydrate is a monosaccharide or simple sugar—such as glucose or fructose (Fig. 1). These simple sugars can be linked together in infinite ways. Sucrose is a disaccharide made up of glucose and fructose (Fig. 1 and Fig. 2).

In a sugar crystal, the sucrose molecules are arranged in a repeating pattern that extends in all three dimensions, and all of these molecules are attracted to each other by intermolecular forces—a type of interaction that binds molecules together and is weaker than the bonds between atoms in a molecule. 

When you add granulated sugar to water, some of the sucrose molecules start separating from one another because they are attracted to the water molecules (Fig. 3). When water and sucrose molecules are close to each other, they interact through intermolecular forces that are similar to the intermolecular forces between sucrose molecules.