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.
For smaller crystals and a smoother texture, allow sugar syrup to cool slightly before stirring it.
To hinder crystallization, add another substance
In addition to concentration and temperature, there’s another factor that can influence crystallization: the purity of the mixture. Impurities in the mixture can inhibit crystal formation. One method to slow down or even completely prevent crystallization in candies is to compromise the mixture’s purity by adding a similar but slightly different sugar to the mix. When the mixture is very concentrated and slightly cooled (that is, ready to form crystals), the similar sugar can move into the place where the crystal is about to form. But since it’s not an exact fit, crystallization will not occur. For example, when you make caramel, you can add a little corn syrup, which is primarily glucose, and it will interfere somewhat with the crystallization of a sugar syrup composed of table sugar, which is sucrose. Another option is to add a mild acid, such as a few drops of lemon juice or vinegar or a pinch of cream of tartar. Acids will break down some of the sucrose in table sugar into glucose and fructose, so there will be three different sugars present, and crystals will not
You can use this method of adding similar but different sugars to have more control over candymaking. With small amounts of another sugar, you can slow down (but not completely prevent) the formation of crystals, helping to make many candies smoother (by giving you more time to stir and create more crystals in the mixture). Every year around the holidays, a friend of mine cooks up a double batch of my pralines. A single batch of this recipe produces wonderfully smooth pralines without any corn syrup, but once the recipe is doubled, crystallization occurs faster than my friend can spoon up all the candy. By adding a tablespoon of corn syrup to the ingredients, she can slow down crystallization long enough to let her spoon up all the candies before they get grainy. Keep in mind, though, that small amounts of different sugars make it more difficult to get crystals, so if you do decide to take this approach, it may be necessary to take the mixture to a higher temperature (and higher concentration) for the candy to set up.
How to use crystals to control candy texture
|Type of crystal||Result||How to accomplish|
|numerous, small crystals||creamy, smooth candy for fudge, praline, fondant||boil the mixture to high temperature for high concentration, let cool slightly, and then stir constantly and vigorously|
|fewer, large crystals||chunky texture for rock candy; can also lead to grainy candies (usually not desirable)||boil the mixture to high temperature for high concentration, agitate the mixture while still hot|
|no crystals, or slower crystal formation||caramel sauce or brittles, or for slowing crystallization for smoother candies||add corn syrup or a mild acid like lemon juice, vinegar, or cream of tartar to the mixture to interfere with crystallization|