Candy comes in all shapes, sizes, flavors, colors, and textures. Whether it's cotton candy, saltwater taffy, or fudge, the ingredients shared by all candies are sweeteners and water. Commonly used sweeteners include white table sugar, molasses, corn syrup, and the artificial alternatives saccharin and aspartame. Other ingredients include butter and fats, flavoring, acids, and gelatin. Whether a candy's texture is gooey, brittle, or something in between depends on which ingredients are used and how the application of heat changes them at the molecular level. That's right—candy making is a highly scientific process.

Table sugar is a crystalline and cube-like form of sucrose, a molecule composed of 12 atoms of carbon, 22 atoms of hydrogen, and 11 atoms of oxygen. When you add sucrose to water and heat the mixture, the sugar crystals dissolve and go into solution. As the temperature increases, more and more water boils off, leaving a higher concentration of sugar in solution.

A candy's sweetness and texture are the direct product of the cooking temperature and how much water remains once the sugary solution, or syrup, is removed from heat to cool. For example, the syrup used to make marshmallow reaches the "firm ball" stage when the concentration of sugar in solution is 87 percent and the syrup cooked between 245° and 250° Fahrenheit (118°–121° Celsius). The syrup used for butterscotch reaches the "soft crack" stage with a sugar concentration of 95 percent and a temperature between 270° and 290°F (132°–143°C).

Cooking at high temperatures allows more sugar to remain in solution than is normally possible. From this unstable state, sugar molecules naturally begin to crystallize back into a solid if allowed to cool without interruption. If you're making rock candy, this is fine. The sugary solution will gradually cool and its signature large crystals will form. For many other types of candy, however, crystallization is not the desired outcome. To prevent crystallization, candy makers add "interfering agents." These agents introduce differently sized and shaped molecules that make it difficult for crystals to form.

Cream of tartar and gelatin are examples of interfering agents. Cream of tartar is an acid; acids act on sucrose to break it into its two simpler components—the smaller molecules glucose and fructose. Because crystals form when similarly sized and shaped molecules interlock, the presence of smaller molecules among larger ones prevents crystallization. Adding gelatin to water as sugar begins to crystallize causes swelling, which forms a stress on sugar crystals. This stress changes the molecular structure of the sugar. The crystalline structure disappears and a jelly or gum forms.

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