Whether tempering chocolate, making caramel, or churning ice cream, cooks need to be masters of crystals. When tempering chocolate, you need to get the cocoa butter to set in just the right crystalline form to give chocolates a firm, shiny surface. When freezing ice cream, you need baby-fine crystals for that velvety- smooth, melt-in-your- mouth sensation.
For candymakers especially, crystallization—or the lack of it—can be the undoing of a hard day’s work. Sometimes you need to prevent crystallization (when making brittle or caramel, for example), and other times you need to encourage the right kind of crystals to form. For candies like fudge, divinity, taffy, and pralines, you want a lot of tiny crystals so the candies will be smooth and creamy, yet still firm. For rock candy, however, a few very large crystals are what you want.
What are crystals, and how do they form? Crystals are the solid form of many food substances. Salt and sugar are two examples. Another familiar crystal is ice, the solid form of water. When these substances are heated or dissolved in a mixture, their molecules move about randomly. But as the molecules of a substance cool and slow down, they join together in a precise formal pattern unique to that substance. For the crystal to form, each molecule must be in exactly the right place.
To encourage crystal formation, you need to create the right environment. First, you need a very concentrated mixture of the substance. Second, the temperature must be low enough so the molecules are moving slowly. With very closely packed, slow-moving molecules, you can sometimes trigger crystal formation by just stirring the mixture, causing the molecules to bang into each other. By manipulating concentration and temperature, you can control the size and number of crystals to get better results in your caramel, pralines, brittles, and fudge.
Sugar concentration affects candy’s firmness
A high concentration means a lot of molecules squeezed very close together. In candymaking, you can increase the concentration of sugar molecules by boiling the sugar syrup. The longer you boil, the more water evaporates and the higher the sugar concentration becomes.
If you’ve made candies before, you know that you start out by boiling a sugar mixture until it reaches a certain temperature, which the recipe specifies. The recipe might also give a descriptive term, such as soft-ball or hard-crack stage, which refers to how the syrup will behave when dropped in very cold water. These temperatures and terms are actually just another way to express sugar concentration. Pure water boils at 212°F at sea level (at higher altitudes, water boils at lower temperatures). When a mixture has less water and more of another substance, such as sugar, the mixture boils at a higher temperature. Thus, a sugar mixture’s boiling temperature tells us how concentrated it has become.
For firmer candies, the concentration, and therefore the temperature, must be higher. A higher concentration means there will be more sugar molecules to go around once crystallization starts, giving you a harder or firmer candy. If you make a candy and it doesn’t firm up (crystals do not form), it probably means the sugar concentration was too low. The solution is to boil the sugar mixture to a higher temperature, which means you’ll eliminate even more water and thus increase concentration. Usually if you boil the mixture 3-or 4-degrees higher than the first attempt, the candy will work.
A cooler temperature produces smaller, more plentiful crystals
When a mixture is hot, its molecules are moving very fast; as the mixture cools, the molecules slow down and it’s easier for them to join. Cooling plays an important role in determining the number and size of crystals that will ultimately form, and that affects the texture of the final candy.
As I stated earlier, when you make candy, you first have to increase the concentration and the temperature of the sugar syrup so the molecules are packed close enough together. If you agitate the mixture slightly at this high temperature, whether by shaking the pan or even by just removing the thermometer, any undissolved sugar crystals on the side of the pan or on the thermometer could drop into the mixture. These few crystals (called “seed” crystals) would quickly attract more molecules and grow into big crystals, and the candy would be grainy. On the other hand, if you let the mixture cool undisturbed, the molecules will have slowed down considerably. If you stir vigorously at this point, you’ll get millions of baby crystals all at once. The more crystals that form, the smaller they will be (because there are fewer remaining free molecules to go around), and the smoother and creamier your candy will be.
So, the key to smooth yet firm fudge, pralines, and fondant is to first bring the mixture to a high enough concentration and then let it cool off somewhat before starting to stir. And once you do start to stir, stir fanatically and without stopping for the finest, creamiest texture.