Sugar in the Kitchen: Special Applications

Over the last few posts, we discussed a lot of sugar’s roles in baked goods. It’s important for flavor, texture, structure, and color in cookies, cakes, and muffins. But sugar’s roles in baking extend further. Sugar is important in meringues as a stabilizer, in yeast breads as a source of energy for the microorganisms, and in fruit desserts to preserve the structure and texture of the fruit. In this post, we’ll explore sugar’s myriad roles in these sweets.

Sugar stabilizes meringues.

As we discussed in “Proteins in the Kitchen,” meringues are a system of air bubbles, proteins, and water. Proteins unfold, line air bubbles, and anchor them to water molecules. But sugar is also crucial to the stability of a meringue—the meringue will collapse into a puddle without it.

Meringues are networks of protein, air, and water reinforced by sugar.

Sugar serves two functions in meringues by attracting water. First, by keeping some water away from the proteins, sugar slows the formation of the protein network. Granted, this means that it takes longer to beat the egg whites to volume (which is why most recipes don’t add sugar until the egg whites foam), but it also prevents proteins from sticking together. If proteins clump, usually as a result of overwhipping, they become too stiff to stretch around air bubbles, the meringue loses air, and the meringue collapses. Sugar also maintains the proteins’ flexibility by keeping them wet. Like spaghetti, protein strands are stiff when they’re dry. When we add sugar to meringue, it dissolves in the water from the egg whites and forms a syrup, which coats the protein strands and protects them from the dry air. The syrup is also thick, so water doesn’t drip into a puddle at the base of the meringue.

Sugar feeds microorganisms.

Just as the cells in our bodies use glucose as fuel, microorganisms eat sugar for energy. The yeast in our bread, for example, can use all sugars except for lactose to grow and divide. As they flourish in our doughs, the byproducts of their metabolism—their waste, if you will—provide leavening and flavor to bread. In fact, all fermented foods, such as beer, yogurt, and wine, start off with some sugar to feed the microorganisms that will flavor it. In bread dough, yeast can eat any sucrose we add. They also eat sugars from starch. Remember that starch is a polysaccharide, a chain of thousands of glucose links. Yeast can’t access the glucose when it’s in a big chain, but enzymes called amylases break off individual links that the yeast can metabolize. Malted flour, which millers often add to white flour, is one source of amylases that helps feed yeast during fermentation.

Sugar increases shelf life.

Sugar powers the microorganisms that ferment our foods, but it also feeds undesirable microorganisms, like disease-causing bacteria and molds. Yet sugar is often used as a preservative for everything from egg yolks to fruits. How does sugar encourage such a long shelf life?

At high concentrations, sugar actually prevents microbial growth. By hanging onto water molecules, sugar keeps water away from microbes. Without water, they cannot survive. In fact, at high enough concentrations, sugar will pull water out of the microbes themselves, which kills them. The same thing happens to foods that are packed in sugar. They dry out as they lose water through diffusion, a process we’ll discuss more in the next section.

Sugar preserves the structure of fruits.

Sugar’s ability to limit microbial activity makes it a useful preservative. But with fruits and vegetables, sugar can also enhance flavor, color, structure, and texture, which makes it useful for desserts like fruit pies. In both cases, sugar triggers a process called diffusion, which moves water out of the microbe or the fruit.

Molecules move to create balance.

Diffusion is a form of movement. Let’s imagine a subway during a pandemic. When people get on, they follow social distancing guidelines and spread out through the subway as much as they can. If they find that the second car is much emptier than the first, for example, they will take seats there. The number of people per car, or the concentration of people throughout the subway, evens out as people find seats because the people diffuse across the cars.

The same thing happens when we dissolve sugar (or anything else) in water. If we add a spoonful of sugar to a glass of water, it eventually distributes itself throughout the water even if we don’t stir. In terms of the pandemic subway, when the sugar molecule people dissolve into the water, they walk away from each other and into the empty cars until they are spread as far apart as possible. Either way, the concentration of sugar is the same everywhere throughout the cup.

When sugar crystals are dissolved in water, the molecules diffuse evenly throughout the water.

Sugar and water diffuse into and out of fruits.

This movement can also occur across surfaces. Just as people move between subway cars, molecules move between objects. When we drop a strawberry slice into a glass of water, we see two distinct things: strawberry and water. But to a water molecule, which can enter the strawberry, it’s just a different subway car. The strawberry car is pretty cramped, but it’s mostly full of luggage—the proteins, fiber, and other molecules that make up the strawberry. But water molecules only care about avoiding other water molecules, so they diffuse into the strawberry. The strawberry plumps up, and its flavor and color are diluted. At the same time, the sugar molecules in the strawberry also diffuse. They see that the water outside the strawberry is empty of other sugar molecules, and they move out of the strawberry.

When a strawberry is placed in water, water diffuses in and sugar diffuses out.

But what if we drop the strawberry into a concentrated sugar solution? In this case, the concentration of water outside the strawberry is very low—the syrup is mostly sugar. To the water molecules inside the strawberry, the outside car looks much emptier, so they diffuse out of the strawberry and into the syrup. Meanwhile, sugar molecules from the syrup see that there is less sugar inside the strawberry, so they diffuse in. In this case, the strawberry loses weight and shrivels as water diffuses out of it, but its flavor and color are concentrated and enhanced.

When a strawberry is placed in a concentrated sugar solution, water diffuses out and sugar diffuses in.

In desserts, sugar and water molecules diffuse into or out of fruits when we macerate or poach them. The concentration of the syrup determines not only the fruit’s appearance and flavor, but also its texture and structure. For mushy fruit, like applesauce, we generally cook the fruit in lower concentrations of sugar, then add more sugar to sweeten the apples. For fruit that maintains texture and structure, like poached pears, we use higher concentrations of syrup.

Cell walls provide structure in plants.

To understand the role of sugar in preserving structure and texture, let’s discuss where they come from in the first place. Every cell in a plant is surrounded by a cell wall. These walls are the scaffolding of the fruit. If the walls are broken, the scaffolding collapses and the plant becomes mushy. We see this in thawed berries. When the berries were frozen, the water that was neatly packaged into each cell crystallized and expanded, breaking the cell wall around it. As the berries thaw, the ice melts and leaks out of the cells, leaving behind a watery mush of broken cell walls. Similarly, when we cook fruits in water, the water could diffuse into the cells so quickly they burst and the fruit loses structure.

When cell walls break (rectangles), water leaks out and the plant becomes mushy.

The cell wall scaffolding is glued together by a polysaccharide called pectin, and a similar change in texture happens if this glue weakens. In fresh apples, for example, cell walls are held firmly in place. When we bite into the apple, the force breaks the cell walls, releasing the water in the cells with a juicy crunch. But as time passes, enzymes break down the pectin and the cell walls are no longer held together tightly. When we bite into the apple, our teeth separate individual cells. The scaffolding just falls apart. We don’t break any cell walls, and we perceive a mealy, drier texture.

In fresh apples, pectin holds the cell walls together, so cells break when we bite them. In older apples, pectin is broken down, so individual cells separate when we bite into them.

Sugar protects cell wall structures.

Heat also disintegrates pectin by converting it into a compound that dissolves in water. When we cook fruits and vegetables, the cell wall scaffolding falls apart. This might be desirable for fruit purées, but it’s not good for fruit fillings. The key to fruit that holds its shape is sugar. When fruit is cooked in a concentrated syrup, sugar diffuses into the fruit, holds onto water molecules, and prevents the water from dissolving the pectin. Sugar does the same thing for the pectin released from mashed fruit in jams and jellies, which helps them to thicken and set. In protecting the pectin glue, like a waterproof finish on wooden scaffolding, sugar maintains both the structure and texture of fruit. (Sugar is also the difference between mushy refried beans and individual baked beans.)

When my mom first dictated to me her recipe for Chinese red bean dessert soup (紅豆湯), she stressed that the sugar had to be added at the very end, else the beans would never soften. I’ll be sharing a comparison of these beans cooked with and without sugar soon. But if we’re baking a fruit pie, we can use sugar to enhance the fruit’s structure and texture. I’ll be sharing an apple pie filling in the next post where sugar concentrates flavor, preserves crunch, and reduces wateriness.

Conclusions

Sugar, as a hygroscopic compound and a source of energy, has roles that extend to meringues, yeast breads, and fruit. After exploring some of the ideas in this post with recipes for fruit pie fillings and red bean soup, we’ll focus on another important place for sugar in the dessert world: candy.



References

BeMiller, J. N. An Introduction to Pectins: Structures and Properties. American Chemical Society, 1986.

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

Corriher, S. Getting the Texture You Want When Cooking with Fresh Fruit. Fine Cooking, 2001, 46.

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

Freeman, S.; Quillin, K.; Allison, L. Biological Science, 5th ed.; Pearson: New York, 2014.

Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry, 6th ed.; Freeman, W. H. & Company: New York, 2012.

Leave a Reply

Your email address will not be published.

%d bloggers like this: