Sugar: An Introduction

Sugar is a ubiquitous ingredient in baking, and we often think of it as a sweetener. But while sugar undoubtedly provides flavor, it also plays important roles in the texture and structure of baked goods and of candies such as caramel and fudge. In this post, we’ll introduce the chemistry of all sugars, which will help us understand their roles in the kitchen.

Sugar is a carbohydrate.

Sugar belongs to a class of large molecules called carbohydrates. Just as proteins are strings of amino acid beads, carbohydrates are chains of saccharide links, each of which contains only carbon, hydrogen, and oxygen atoms. Saccharides are distinguishable from other molecules by the ratio of these atoms (two hydrogens and one oxygen per carbon), and they differ from each other in their chemical structure. Sugars only contain one or two saccharides, called mono- or disaccharides, respectively, but carbohydrates can be linear or branched chains of tens of thousands of saccharides. Large carbohydrates, called polysaccharides, include starch and cellulose, which is a dietary fiber.

The sugars we use most often in baking, such as granulated sugar and brown sugar, consist mainly of the disaccharide sucrose. The two saccharide links that make it up are called fructose and glucose, which are also prevalent as monosaccharides. Fructose makes fruit sweet. Glucose is the sugar that we and most living things use for energy. Other sugars we come across in food include lactose, a disaccharide of glucose and galactose found in milk products, and maltose, a disaccharide of two glucose molecules. Maltose is more commonly used in East Asia, but it’s also the main sugar in malt syrup, which is often used in breads. Malt syrup and other syrups such as honey, molasses, maple syrup, and corn syrup are liquids that contain some combination of these sugars dissolved in water, sometimes with other flavor compounds.

Sugars commonly used in baking include the monosaccharides fructose and gluctose and the disaccharides sucrose, lactose, and maltose.

Note that these drawings illustrate only which atoms are connected, not their position in three-dimensional space. This makes the difference between glucose and galactose, the uncolored half of lactose in the figure above, difficult to demonstrate. Here is a more accurate representation of the structure of these sugars. Red represents oxygen (O), white represents hydrogen (H), and black represents the carbons at the corners in the drawings above. The only difference between these sugars is the position of the oxygen closest to the camera.

Ball and stick models of glucose (top) and galactose (bottom). Red balls represents oxygen atoms, white balls represent hydrogen atoms, and black balls represent carbon atoms.

Sugars are polar and form crystals.

As you can see in the drawings above, all the sugars have a lot of –OH sticking out. These represent an atom of hydrogen (H) bonded to an atom of oxygen (O) bonded to the rest of the ring. Chemists call them hydroxyl groups, and they are the key to many of sugar’s functions in baking. The hydrogen and oxygen in the hydroxyl group share electrons to form their bond. Think of each atom like a kid and of the electrons like toys that they’re fighting for. In this case, oxygen is much stronger than hydrogen, so he ends up with the toys (or electrons) for more of the time. A molecule (or a part of a molecule) like this with unevenly distributed electrons is polar. And because electrons are negatively charged particles, the oxygen atom acquires a partial negative charge and the hydrogen atom acquires a partial positive charge.

Positive and negative electric charges attract much as the north and south poles on a magnet do. The partially negative oxygen is attracted to positive atoms, and the partially positive hydrogen atom is attracted to negative atoms. They find these atoms on other molecules, and the attraction is called a hydrogen bond. When there’s only one type of sugar molecule around (just sucrose, for example), the molecules hydrogen bond to each other in a highly organized structure called a crystal. These are the crystals we scoop out of a bag.

In the drawing below, sucrose molecules are simplified into hexagons (glucose) connected to pentagons (fructose). These is not a physically accurate representation of how these molecules pack into a crystal, but it emphasizes that the crystal is an organized arrangement of sucrose molecules held together by hydrogen bonds.

Sugars like sucrose form hydrogen bonds to each other and organize into solid crystals.

Water and sugar attract because they’re both polar.

Water, which is an oxygen atom bonded to two hydrogen atoms, is also polar. The oxygen hogs the electrons from both hydrogen atoms and acquires a partial negative charge, leaving the hydrogens with partial positive charges. In liquid form, water molecules constantly form, break, and reform hydrogen bonds to each other as they slosh around.

When we add sugar crystals to water, we combine partially charged hydrogens and oxygens on two kinds of molecules, and opposite charges attract regardless of what the rest of the molecule looks like. Water molecules hydrogen bond all around the sugar molecules and pull them away from each other, ultimately dissolving the crystal. If we remove the water, perhaps by evaporating it, sugar molecules reform their hydrogen bonds to each other and recrystallize. Note that sugar attracts water just as much as water attracts it. We could rephrase this paragraph by saying that sugar molecules attract the water to surround them. Scientists refer to this ability to attract and absorb water as hygroscopicity.

Water molecules form hydrogen bonds around sugar molecules and pull them off the crystal. The separated sugar molecules are dissolved in the water.

Three factors influence the amount of sugar that dissolves.

If we’re baking a cake, we want the sugar to completely dissolve in the batter. But if we want a crisp cookie, the sugar needs to crystallize. If we’re making fudge, we want the sugar to dissolve then recrystallize, but if we’re making caramel, the sugar cannot crystallize. Evidently, we need to control the dissolution and crystallization of sugar. How do we do this? Think about adding sugar to coffee. How can we dissolve more sugar?

Of course, we can add more coffee. The more water molecules there are, the more sugar molecules they can separate. But if we continue to add sugar, we reach a point where every water molecule is bonded to sugar. This is the saturation point. If we add more sugar, there isn’t enough water to dissolve it and it stays a crystal. We take advantage of this when we sprinkle sugar on the tops of muffins before baking. There isn’t enough water on the surface of the batter to dissolve the crystals, so we get a crunchy crust.

Adding water dissolves more sugar because there are more water molecules to separate the sugar molecules.

If we stirred that sugar into the batter, though, it might dissolve. Stirring moves dissolved sugar molecules away from the crystal so that fresh water can surround the next sugar molecule. By replenishing the supply of water by a sugar crystal, we encourage the crystals to dissolve faster. Similarly, smaller particles dissolve faster because they have more surface area for the water to bond. This is the reason we sprinkle muffins with coarse sugar. If we tried to use powdered sugar, it would dissolve right into the muffin.

Smaller sugar crystals dissolve faster because there are more exposed surfaces for water to hydrogen bond.

But say that we’ve been stirring and stirring the coffee and there are still sugar crystals and we really want them to dissolve. (I don’t drink coffee, but I admit this seems like an excessive amount of sugar.) What now?

We can warm up the coffee. It’s much easier to dissolve sugar in hot liquids than in cold liquids—this is why lemonade and iced tea recipes add the sugar to hot water to make a concentrated syrup. Dissolving sugar requires energy. The water molecules have to work to pull the sugar molecules away from each other. They’re like dog owners who are trying to keep their sugar molecule dogs from jumping all over each other at the park. When we add heat, we add energy. The water molecules move faster to pull the sugar crystals apart, and they have more energy to hold onto the individual dissolved sugar molecules even if they try to squirm away. Hotter liquids actually have a higher saturation point. Many candy recipes take advantage of this and dissolve a lot of sugar in hot liquid. As the liquid cools, the saturation point decreases, sugar must recrystallize, and we control the sizes of the crystals that form.

Hotter water has more energy to hold sugar molecules in solution.

Sugar plays diverse roles in baking.

Many of sugar’s roles in baking can be ascribed to its polarity and consequent attraction to water. In the next post, we’ll explore its effect on the structure, texture, and shelf life of baked goods. We’ll also discuss how different sugars vary in their effects.

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.



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