Sugar in the Kitchen: Flying Solo

While the polarity of sugar and its attraction to water give it the crucial properties we discussed in the last post, sugar also functions independently of water. In the image above, sugars from milk give muffins more color. Sugar also provides volume, aeration, and, of course, flavor for our baked goods.

Sugar adds bulk.

When we pour sugar into our mixing bowls, we’re adding something that takes up space. Think about rocks in a tank of water. When we add the rocks, we add mass and volume: the water tank gets heavier, and the water level rises. This holds true even if we break the rocks into pebbles. In fact, no matter how many times we split the rocks, the weight of the tank will not change, and the water level will not go back down.

Sugar does the same thing in our batters and doughs. If we add 5 grams of sugar to 50 grams of water, the total mass will be 55 grams, even if the sugar dissolves. We can’t see it, but the sugar’s still there and it’s just as heavy as before—the crystals have just been separated into microscopic molecules, the way the rocks were crushed to sand. Sugar crystals also contribute volume. Many frostings, for example, are essentially spreadable sugar. Without the sugar, there’s really no frosting (not just in volume, but also in taste!).

However, sugar’s effect on water volume is slightly different from that of rocks because it dissolves. If the rocks take up a 10 cubic centimeters of space and we add it to 50 cubic centimeters of water, the total volume is 60 cubic centimeters and the water level rises accordingly. However, if we added 10 cubic centimeters of sugar to 50 cubic centimeters of water, the total volume would be somewhere between 50 and 60 cubic centimeters, and the water level would not increase as much as the tank with rocks. It’s as if the sugar absorbed some of the water (or vice versa). Actually, the sugar has dissolved.

Both rocks and sugar add mass and volume to water. Both conserve their mass; however, sugar dissolves in water and does not add as much volume as rocks do.

As we discussed in the introduction, sugar dissolves when water molecules pull sugar molecules away from each other. The result is a dense arrangement of sugar and water molecules, but if we look closely, we find that the sugar molecules try to fit into the existing spaces between the water molecules. They don’t entirely succeed—the water molecules still move apart a little bit to accommodate them—but they don’t make the water molecules move too much. Thus, the water volume increases, but only by a little. On the other hand, when we add rocks to the water, the rocks don’t dissolve and squeeze between the water molecules. Instead, the rocks displace the water, and the water level rises.

Sugar aerates.

Along with bulk, sugar adds another crucial component to our batters and doughs: air. Sugar crystals have rough surfaces that trap air and carry it into the mixing bowl with them. Creaming butter and sugar, for example, is an important step in many cakes. The sugar brings air into the butter, and these air pockets are later enlarged by chemical leaveners to make a light, fluffy cake.

Sugar crystals have rough edges that trap air and bring it into batters.

Unlike sugar crystals, syrups are liquid, and there are no rough edges to carry air. However, because syrups are thick, with the help egg whites they can hold air that is beaten into them.

Sugar contributes to browning.

When the aerated batter heats up in the oven, sugar undergoes complex chemical changes that develop the color and the flavors associated with browning. Browning results from two processes, caramelization and Maillard reactions. Caramelization is a chemical reaction of sugars alone, while Maillard reactions require both sugars and proteins. Both processes are still poorly understood, but we know that heat encourages the molecules to form new brown compounds that have complex flavors.

As we discussed in the context of proteins, all atoms and molecules are constantly moving, like kids in a room. The kids could be wriggling in place as they play in a group or running around. But if we give the kids candy, they become invigorated and start bouncing off the walls. To atoms and molecules, heat is a form of energy like sugar for the kids. As we increase the temperature, wiggling atoms start to run, skip, and jump, moving away from their original playmates to join other groups or to play by themselves. Ultimately, heat encourages bonds to break and form, rearranging the atoms from sugars and amino acids into new compounds. Some of these compounds are tiny, and they vaporize and bring the wonderful scent of cooking food to our noses. Other compounds become quite large and contribute the color and rich flavors of browned food.

In the oven, sucrose crystals break down into monosaccharides. The monosaccharides undergo caramelization, or they participate in Maillard reactions with proteins to produce brown flavor compounds and aromatic fragments that vaporize.

The more energy our sugar molecules have, the more browning reactions occur. Thus, higher temperatures and longer baking times produce more color and flavor. Different sugars also brown at different rates. Disaccharides like sucrose must first separate into monosaccharides before participating in browning reactions, so they contribute less color. Monosaccharides, on the other hand, can immediately utilize heat energy to brown, so they produce more color.

Of the monosaccharides common in baking, fructose browns more than glucose because it more readily participates in Maillard reactions. For a sugar to react with an amino acid, it must open its ring and adopt a linear shape, as shown below. Because of its structure, a higher percentage of fructose naturally exists as a ring, so it’s more likely to react.

Glucose can exist in a ring or a linear structure.

We also get more browning if there are more sugars and proteins available. For example, muffins made with milk brown more than muffins made with water because the milk contains additional sugars and proteins that brown. Similarly, milk and egg washes produce beautiful golden brown surfaces by providing extra proteins to participate in the Maillard reactions.

Milk contains sugars and proteins that can participate in Maillard browning, so muffins made with milk are darker than those made with water.

Acids prevent browning.

Acidity also influences the extent of browning, mostly through its effect on the Maillard reaction. If you remember our discussion of proteins, we described molecular interactions as kids fighting for toys, and we said that acids and bases mediate these interactions by changing the number of toys available to the kids. The role of acids and bases in the Maillard reaction is the same.

Picture two groups of atom kids playing with electron toys. We have the sugar kids (blue in the drawing below) in one corner of the room and the amino acid kids (orange) in another. With encouragement from heat energy, the amino acid kids start to migrate across the room, and they realize that one of the sugar kids isn’t getting enough toy time. So the two groups merge together and the amino acid kids share their toys with the sugar kids (shown by the arrow). At this point, a new bond forms and the Maillard reaction starts.

At the start of the Maillard reaction, an amino acid (orange) shares electrons with a sugar (blue). Acids discourage amino acids from sharing, so they prevent Maillard browning from occurring.

Acids are like the parents who take toys away to prevent fighting. In an acidic environment, the amino acid kids barely have enough toys to share among them. And if they don’t have enough toys, they’re unlikely to share and merge with the sugar kids. Thus, acids discourage Maillard reactions, and acidic batters are pale.

On the other hand, bases are like the parents who get extra toys so the kids don’t fight. In a basic environment, the amino acid kids have more than enough toys, so they’re happy to share with the sugar kids. The two groups merge and react, and we get browning. This is why pretzels, which are soaked in a basic lye or baking soda solution before baking, get so dark.

Sugar sweetens and enhances taste.

Although browning reactions bring complex flavors to our baked goods, sugar primarily contributes sweetness. If the sugar isn’t already dissolved, our saliva dissolves it and carries it to our taste buds where it’s sensed. Because sugars vary in their chemical structures, they’re perceived with different levels of sweetness. Fructose, for example, can be 1.5 times sweeter than sucrose. And because fructose binds to water more tightly than other sugars, it’s carried to the taste buds more quickly, and we perceive a stronger, sharper sweetness.

Some sources of sugar such as honey, molasses, and maple syrup contribute their own distinct flavors, but sugar also enhances the flavors of foods like fruits, vegetables, and meat, Brown sugar paired with chocolate creates a fudgy taste. And sugar can helps us register flavor molecules that linger in our noses, like peach and mint. If your mint gum has lost its flavor, try eating a little sugar. The mint flavor should come back.

Sugar is versatile.

Throughout the last couple posts, we’ve discussed sugar’s many roles in baking. Most of them broadly apply to baked goods like cakes, cookies, muffins, and quick breads, but sugar is also crucial to meringues, fruit preserves, pie fillings, and breads. In the next post, we’ll take a closer look at some of sugar’s more specific uses in the kitchen.



References

Corriher, S. O. Bakewise; Scribner: New York, 2008.

Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.

Goldfein, K. R.; Slavin, J. L. Why Sugar is Added to Food: Food Science 101. Comprehensive Reviews in Food Science and Food Safety 2015, 14, 644-656.

Lee, J. W.; Thomas, L. C.; Jerrell, J.; Feng, H.; Cadwallader, K. R.; Schmidt, S. J. Investigation of Thermal Decomposition as the Kinetic Process That Causes the Loss of Crystalline Structure in Scucrose Using a Chemical Analysis Approach (Part II). Journal of Agricultural and Food Chemistry 2011, 59, 702-712.

Lee, J. W.; Thomas, L. C.; Schmidt, S. J. Investigation of the Heating Rate Dependency Associated with the Loss of Crystalline Structure in Sucrose, Glucose, and Fructose Using a Thermal Analysis Approach (Part I). Journal of Agricultural and Food Chemistry 2011, 49, 684-701.

Provost, J. J.; Colabroy, K. L.; Kelly, B. S.; Wallert, M. A. The Science of Cooking: Understanding the Biology and Chemistry Behind Food; John Wiley & Sons, Inc.: Hoboken, 2016.

Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function, 7th ed.; Freeman, W. H. & Company: New York, 2014.

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