In the introduction to sugar, we discussed the chemistry that makes sugar form crystals and attract water. That attraction gives sugar its roles in the texture and structure of baked goods. In this post, we’ll explore how sugar’s interactions with water make it such a versatile ingredient.
The journey of a sugar molecule.
Last time, we identified three factors that determine how much sugar dissolves: the amount of water, the temperature, and the surface area of the sugar particles. Using these concepts, let’s follow a sugar molecule through a typical recipe.
First, sugar is combined with other ingredients. Dry ingredients don’t have much of an effect, but as soon as sugar senses water, it starts to form hydrogen bonds and dissolve. Depending on the amount of water available, the sugar may dissolve completely, as in cakes, or only partially, as in cookies.
When the batter or dough enters the oven, the liquid starts to heat up. Its saturation point increases and more sugar dissolves, especially if it’s finely ground. The heat also gives sugar more energy to pull water away from other molecules like proteins and starch. And as temperatures rise further still, any remaining sugar crystals liquefy. All of these processes thin out a batter or dough, and it thickens and solidifies only when structure molecules like gluten, egg proteins, and starch set.
When the structure solidifies, the batter or dough is cooked, and we remove it from the oven. A significant amount of water has vaporized, and as the baked good cools, the saturation point of any remaining water decreases and liquid sugar solidifies. Depending on how much water is left, sugar may recrystallize. We see this as a crunchy crust on cookies and some muffins.
Throughout this journey of dissolution and crystallization, sugar has affected texture, shape, and moistness. Let’s take a closer look at what it did.
Sugar makes baked goods tender.
In its urgent quest for water, sugar takes water away from other molecules, including the ones that ultimately provide structure. Gluten, egg proteins, and starch all need water to set. The less water available, the more heat energy they need to coagulate and gelatinize, and the later the structure of the baked good develops. By keeping water away from these molecules, sugar delays the formation of structure and is thus considered a tenderizer. Sugar also interferes with the structure molecules themselves. For example, it binds to the proteins glutenin and gliadin and prevents gluten from forming.
Baked goods that don’t contain enough sugar are tough. The structure molecules have access to all the water they need to coagulate and gelatinize at a low temperature, so the batter solidifies before it’s risen. Because the baked good doesn’t contain enough air pockets, it’s dense like a squished piece of bread. On the other hand, if there’s too much sugar, the structure won’t set properly. Without sufficient scaffolding, the batter may not rise, or it might collapse when it comes out of the oven. Some chocolate cake recipes actually use this to their advantage to create a dense, fudgy center.
Sugar contributes to shape.
But before proteins and starch set, batters and doughs become more fluid. Remember from our sugar molecule’s journey that heat makes sugar dissolve, grab water, and liquefy, thinning batters and doughs.
Thin batter deforms easily. Cake batter, for example, is susceptible to tunneling as rising air bubbles push batter out of the way. It may also collapse if the oven door opens and the temperature drops dramatically. When the temperature drops, the air bubbles that filled the batter shrink, and the batter falls. Think about a cup of iced coffee. The ice cubes, like the expanding gases, take up a lot of the space inside your cup. Opening the oven door is like scooping out the ice cubes. The cup suddenly looks a lot emptier because there are no ice cubes to help take up space.
In cookies, thinned dough spreads along the baking sheet until the proteins set. Cookies with lots of sugar spread more. Powdered sugar, which is more finely ground than granulated sugar, dissolves faster and theoretically promotes more spread. However, powdered sugar also contains cornstarch, which thickens the dough and prevents it from thinning and spreading. Some sugars, like brown sugar, molasses, and honey, are also especially acidic, which helps cookies spread less by setting proteins faster.
Sugar keeps baked goods moist.
As sugar thins batters and doughs in the oven, hot, dry air vaporizes water into steam that rises out of the baked good. Steam helps to leaven, but if too much water escapes, the baked good becomes dry. Sugar prevents this by holding onto water and preventing it from escaping as a gas.
As we discussed in the introduction to sugar, liquid water molecules are connected by hydrogen bonds that form and reform as the molecules slosh around. For water to become steam, individual water molecules need to break all their hydrogen bonds to the rest of the liquid. Heat provides the energy for this. In baked goods, however, sugar keeps a strong grip on water molecules and maintains their hydrogen bonds to the rest of the molecules in the liquid. As a result, the water molecules don’t vaporize, and they stay in the batter or dough.
Different sugars hold water with different strengths. Brown sugar, molasses, and honey all contain fructose, which has a structure that makes it more hygroscopic than sucrose or glucose. Because fructose holds more water, baked goods containing these sugars are typically moister, softer, and chewier.
Sugar adds crunch.
When we pull baked goods out of the oven, they begin to cool. Depending on how much water is left inside, some sugar may recrystallize for a crunchy texture. Muffin tops are often sprinkled with sugar before baking for a crunchy crust. As long as the sugar is not too finely ground, the high concentration of sugar on the surface ensures that it will not dissolve. Cakes and brownies with high ratios of sugar may also develop a crust. Crumb crusts for pies are actually held together by crystallized sugar. In the oven, sugar (including the sugar from the crumbs) either liquefies or dissolves in the little liquid in the recipe (often from the butter). When we remove the crust from the oven, the sugar recrystallizes into a hard solid and glues the crumbs into place.
Crunchy cookie outsides
Cookie recipes especially utilize sugar for a range of different textures. For example, several strategies make cookie exteriors crisp. Balls of cookie dough might be rolled in sugar before baking. This has the same effect as sprinkling sugar on muffins. The sugar can’t find enough water to dissolve, so it crystallizes and forms a crisp exterior. Chocolate crinkle cookie dough is usually coated in powdered sugar for a dramatic contrast between the dark cookie dough and the white powdered sugar, but some recipes roll the dough in granulated sugar before the customary powdered sugar. The coarser granulated sugar prevents the powdered sugar from dissolving into the cookie’s surface, and it makes the outside of the cookie crunchier.
Chocolate crinkle cookies and other cookies such as gingersnaps get their cracked appearance because the sugar on the surface is crystallized before the cookie expands. As the cookie bakes, gases expand, the cookie becomes bigger, and the crystallized surface splits in a dramatic pattern. In chocolate crinkle cookies, the powdered sugar never dissolved. But cookies like gingersnaps aren’t coated with sugar crystals. How does sugar crystallize on the surface before the cookie even expands?
Gingersnaps and other crackly cookies have very little liquid and a lot of sugar. If they contain a coarser sugar (depending on the amount of liquid, granulated sugar is coarse enough), most of the sugar won’t dissolve. As the cookie dough heats up in the oven, a couple things happen. First, water on the surface of the cookie vaporizes, leaving sugar crystals behind, the same way that evaporated seawater leaves salt crystals behind. Second, sugar crystals liquefy and migrate to the surface of the cookie to replenish some of the lost liquid. Both of these processes create a crust of sugar that is later cracked by the expanding cookie dough.
Crunchy cookie insides
The high ratio of sugar in gingersnaps also means that a lot of the sugar recrystallizes when the cookie cools. This makes the cookie firm and crunchy. Cookies with a lot of fructose, however, tend to be softer. As we discussed above, fructose holds onto more water. But fructose also interferes with sugar crystallization.
Fructose is never the only sugar in a cookie. Sources of fructose such as brown sugar, molasses, and honey contain sucrose as well. And when there are two types of sugar, they both have a hard time crystallizing and adding crunch to a cookie. As we discussed in the introduction, sugar crystallizes when sugar molecules arrange themselves in a highly ordered pattern. These patterns form best when there is only one type of molecule—just sucrose or just fructose. When another type of molecule is present, it disrupts the pattern and prevents the crystal from forming. As you can imagine, the more fructose there is, the harder it is for sucrose molecules to organize into crystals. Thus, honey, which contains a lot of fructose, is unlikely to crunch. However, brown sugar and molasses, which contain very little fructose in comparison to sucrose, can still produce crunchy cookies like gingersnaps. I’ll be sharing a post soon that compares cookies made with honey to cookies made with maple syrup, which is also mostly sucrose.
Sugar crystallization is also important for the texture of candies. In fudge, for example, we want a creamy texture, so we want to keep the crystals as small as possible. For caramels, we don’t want the sugar to crystallize. We’ll discuss strategies for controlling sugar crystallization in candy making in a later post.
Sugar’s roles are not limited to its attraction to water.
Sugar’s attraction to water makes it an important ingredient for texture and structure in our baked goods. But of course, sugar is a sweetener, and it also plays important roles in browning and aeration. We’ll explore these ideas in the next post.
References
Corriher, S. O. Bakewise; Scribner: New York, 2008.
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.