In the past few posts, we focused on sugar’s roles in baked goods and its interactions with other ingredients. But sugar is also crucial to candies like caramel, fudge, and fondant, for which careful control of sugar crystallization is paramount for texture. In this post, we’ll explore the chemistry of crystal formation in the candy making process.
Four factors influence sugar crystallization.
As we discussed in the introduction to sugar, sugar crystals are highly organized structures of sugar molecules. They dissolve when water molecules pull them apart into individual molecules, and they recrystallize if they return to that organized structure. In baked goods, crystallization is controlled by the grind of the sugar and the ratio of sugar to water to determine only whether the sugar crystallizes, but in candies, control over crystal size is also important. For example, a smooth fudge has tiny sugar crystals. If the crystals get too big, the fudge becomes grainy.
Before we discuss how to control sugar crystals, let’s consider how they form in the first place. In the introduction to sugar, we highlighted a few factors that help sugar dissolve in water. Now, imagine that we already have a solution of sugar in water. How can we encourage the sugar molecules to pack together and crystallize?
Concentration of sugar
In our earlier discussion about dissolving sugar, we said that lower concentrations of sugar dissolve more easily. The fewer the sugar molecules, the more likely they are to be separated by water molecules, the more easily they dissolve. Conversely, the more sugar there is, the greater the likelihood the sugar molecules stick to each other and crystallize. We can remove water and concentrate the sugar by heating the syrup until the water boils off.
Temperature of solution
Another strategy we discussed to dissolve sugar was to heat the solution. The hotter the water molecules, the more energy they have to hold the sugar molecules apart from each other. To crystallize the sugar, then, we can cool the solution. As it loses heat, its saturation point decreases. It becomes more difficult for the water molecules to keep hold of the sugar molecules, and the sugar crystallizes out of solution.
Stirring the solution
The third strategy we discussed to dissolve sugar was to stir the solution. Stirring encourages more water molecules to come into contact with the sugar crystals and pull them apart. But the opposite of that, letting a sugar solution sit undisturbed, does not promote crystallization.
In order for sugar to crystallize, the solution must first be concentrated and cool enough for the sugar molecules to overcome the water molecules keeping them apart. To ensure this, we can dissolve a lot of sugar in a minimal volume of hot water, then let the solution cool. Such a solution is supersaturated, which means that it contains more sugar than the water should be able to dissolve. But if we just let the supersaturated syrup sit and cool, it doesn’t crystallize. The sugar molecules bump into each other as they float around, but they rarely align in the correct orientation to form a crystal.
However, if we stir this supersaturated solution or shake it, we force all of the sugar molecules into different positions and increase the likelihood that some of them line up. And as soon as a few sugar molecules align correctly, the other molecules use them as a guide, snap into place, and crystallize in a flash. The resulting solution is saturated, which means that it dissolves only as much sugar as it can hold, and the additional sugar is in its crystalline form. If we don’t stir the supersaturated solution, but we drop a sugar crystal in, the solution also crystallizes. The sugar acts as a seed that guides the dissolved sugar molecules into the correct orientation for a crystal. Other objects, such as a speck of dust or a string dangled in the solution, can also provide a place for crystals to start growing.
Purity of sucrose
Sugar molecules can only crystallize if they find each other and fit together in just the right way. Crucially, the crystal pattern can only accommodate one type of molecule: just sucrose, for example. If there are other molecules around, sucrose is less likely to crystallize because these impurities get in the way. One way to prevent crystallization, then, is to add other sugars. Corn syrup, for example, contains the sugar glucose, and it’s often used in candy recipes (and pecan pie) to prevent crystallization while still adding sweetness.
Acids also prevent crystallization. As we’ve discussed in the context of proteins, acids mediate the electrical interactions between atoms and molecules. In the case of disaccharides like sucrose, acids tweak the balance right where the two monosaccharide links join together. With the help of heat, acid separates sucrose into glucose and fructose. Once formed, both of these sugars interfere with the crystallization of sucrose.
Working with sugar in candies
Different candies have different textures, which are determined by the concentration, size, and number of sugar crystals they contain. To control texture, then, we have to control the crystals. We first dissolve sugar in water to “erase” their current formations, then carefully monitor the factors we discussed so that the sugar recrystallizes in the form we want. Let’s discuss some ways we monitor these variables and how they affect the resulting candies.
Syrup concentration determines hardness.
Fudge is soft, nougats are chewy, and brittles are hard and crunchy. These candies vary in their hardness because they contain increasing amounts of sugar. Since sucrose is hard and crunchy, the more there is, the harder the candy. Think about it: as we boil syrup, it gets thicker and thicker. The thicker the syrup, the less likely it is to move out of the way when our teeth bite down on it, the harder we perceive it to be. The syrup thickens because water boils off, leaving the remaining solution with less water and a greater concentration of sugar. The longer the syrup is cooked, the less water remains, the harder the candy.
But how long do we cook the syrup? One way to determine the final texture of a cooking sugar solution is to drop a bit into ice water. The cold water cools the syrup, and the consistency of the cooled sugar determines the stage of the syrup. If the sugar forms threads in the water, the solution is still fairly dilute. If the solution is more concentrated, it might form a hard ball, which is good for nougat. Lollipops and brittles are made from syrups at the hard crack stage, which contain a minimal amount of water.
Another way to gauge the concentration of the sugar solution is temperature. As a solution of water and sugar heats up, the water begins to boil off. Why? Heat gives molecules energy to move faster. When water molecules gain enough energy to break free of the hydrogen bonds that hold them to their neighbors, they escape the solution as steam. The hotter the solution, then, the faster the water molecules move, the more water breaks away from the solution, the fewer water molecules are left in the syrup. Solutions that form a thread in ice water are around 230°F (110°C). As the temperature increases, less water remains, and the candy becomes firmer and harder. If the syrup reaches around 320°F (160°C), the water has completely boiled off and the sugar begins to melt.
Caramel contains no water.
A syrup that has all the water boiled off becomes caramel, which contains only sugar and compounds derived from it. (This is the caramel of flan and crème brûlée, not chewy caramel candies, which are formed from firm ball syrups at around 245°F [118°C].) When sugar reaches 320°F (160°C), it begins to melt, which means that the hydrogen bonds holding the sugar molecules together in a crystal break and the sugar liquefies. As the temperature increases to over 338°F (170°C), sucrose caramelizes, and the molecules quickly break down into smaller fragments that may react further with each other. Ultimately, sucrose can be converted into an amalgam of over 100 different compounds, most of them brown, with different flavors that contribute to the taste of caramelized sugar.
Since caramel is just broken-down sugar, it can be made without water. For this method, heat is applied directly to sugar until it caramelizes, like in the topping for crème brûlée. However, because it’s difficult to control the distribution of heat, this method only works for dark caramels, which are heated to higher temperatures for longer periods of time. The extended exposure to heat breaks down most of the sucrose. As a result, dark caramels have more flavor and color than lighter caramels, and they’re also softer because there isn’t much sucrose left.
Lighter caramels are paler in color and have less caramel flavor. And because they contain more sucrose, they are harder. To better control the temperature for these caramels, sugar and water are heated together until the sugar dissolves, all the water boils off, and the sugar caramelizes. Because the water molecules absorb a lot of the heat, this method provides a more controlled, even heating of the sugar.
Cooking syrups are at risk of crystallizing.
However, as the solution loses more and more water, the sugar concentration increases and the solution is more likely to crystallize before we want it to. (This is a problem for all candy recipes with dissolved sugar.) This is why candy recipes don’t agitate the solution after the sugar dissolves—it could trigger crystallization. The sugar could also crystallize if a sugar crystal falls into the pot. These crystals often come from the side of the pot, where boiling syrup has splashed and the water has vaporized. You can brush down the sides of the pot with cold water to dissolve these crystals and wash them back into the syrup. You can also oil the pot to prevent sugar from sticking to the sides. If the sugar does crystallize, though, you’re merely back where you started: sugar crystals in a pot. You can just add water to dissolve the sugar again and start over.
Crystal size
In addition to the amount of sugar in our candies, the size of the crystals is also important. Fondant and fudge need tiny crystals, while rock candy is a big crystal. Crystal size determines mouthfeel and appearance. Smaller crystals make for a smooth texture because they melt as soon as they hit our tongues. And the smaller the crystals, the shinier the candy looks because the crystals create a smooth surface. Finally, because tiny crystals provide more spaces to fold and bend, candies with smaller crystals are less likely to crack.
Temperature while stirring
To get sugar to recrystallize, we need to stir a supersaturated solution. If we stir while the solution is still warm, some sugar will recrystallize, but a lot of the sugar molecules remain dissolved in solution because the warm water still has a high saturation point. As the syrup slowly cools, the dissolved sugar has a lot of time to migrate to the established sugar crystals and attach to them when they crystallize out of solution. For large crystals, then, seeds should form while the solution is still warm.
For fine crystals, though, the solution shouldn’t be stirred until it’s cool. Because the saturation point of the water at that point is low, a lot of sugar will crystallize. These sugar molecules instantly solidify right where they are in the solution, forming tons of tiny crystals.
Crystal inhibitors
As we discussed, impurities like the glucose in corn syrup prevent sucrose from crystallizing, and acid breaks sucrose down into glucose and fructose, which also serve as impurities that prevent crystallization. Other ingredients also keep crystals small. For example, fat and protein from dairy products like butter, milk, and egg whites coat sugar crystals, preventing other sugar molecules from bonding and enlarging the crystal.
Candy texture is a balance.
Candy recipes balance these factors to create the perfect texture. If a recipe contains a lot of fat, for example, stirring the solution while it’s hot can still produce tiny crystals. If it contains glucose and acid, it’s less likely to crystallize. Ultimately, an understanding about sugar crystallization can help us make better candies or tailor them to our own tastes.
References
Corriher, S. O. Bakewise; Scribner: New York, 2008.
Corriher, S. O. Cookwise; William Morrow and Company, Inc.: New York, 1997.
Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.
Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry, 6th ed.; Freeman, W. H. & Company: New York, 2012.