Egg Foams in the Kitchen: Meringue

In the last post, we covered eggs’ many roles in cakes, cookies, muffins, and breads. But eggs are even more versatile when we consider what they can do with air. Think about it: egg whites on their own are dense and chewy. But with air (and sugar), they become light, crisp, melt-in-your-mouth meringue cookies. Add a few more ingredients, and we can make macarons, pavlovas, cake frostings, soufflés, and sponge cakes. In this post, we’ll explore how meringue forms and how we adjust its texture for different bakes. You might find it helpful to review protein structure and what an egg contains before diving in.

Meringues are egg with air.

At its most basic, a meringue is just an egg with a lot of air bubbles. But this description belies how extraordinary the foaming process is. Not all liquids can hold air—when you run water into a glass or whisk it really hard, bubbles quickly float to the surface and burst. Eggs are unique because they contain an assortment of proteins. Some of them cause the egg to solidify with heat, but others, such as globulins and ovotransferrin, hold air bubbles in the water of the egg to create an egg foam.

Egg proteins hold air bubbles in an egg foam.

We can think about all proteins as strings of amino acid beads with different chemical properties. Some beads love water and seek it out, while others avoid it. In an egg, which is mostly water, protein strings are wadded up so that water-loving, hydrophilic amino acids are on the outside, where they can interact with water, and water-avoiding, hydrophobic amino acids lie inside. This is the protein’s native, folded state, shown in the illustration below.

When we whisk an egg, two factors unwind or denature the protein wads. First, the physical agitation causes the proteins to unfold. And second, whisking incorporates air into the egg. Suddenly, there’s more air and less water around the proteins. In their scramble to find more water, water-loving amino acids leave formation and expose the hydrophobic amino acids that were hiding in the core of the protein wad. Unprotected, the only way the hydrophobic amino acids can continue to avoid water is to turn toward the air bubble. In order to appease both the water-loving and the water-avoiding amino acids, the protein strings ultimately unfold into new shapes around an air bubble, as shown on the right in the drawing below. The amino acids then form new chemical bonds to each other, which stabilizes the proteins’ new shapes.

Egg proteins rearrange around air bubbles and stabilize them in water.

As we continue to whip the egg, the volume of the egg foam increases. The protein-coated bubbles become more densely packed, and the liquid from the egg stretches thinner between them. Since less water is left between the air bubbles, the foam becomes drier. Overwhipped egg whites separate into liquid and dry, crumbly foam because all the water gets squeezed out, shown in the right-hand picture below. Whipped properly, however, an egg white contains enough water and protein to reach eight times its original volume, and all of that expansion is due to the incorporation of air!

When we first begin to whip an egg, there’s a lot of water between the protein-coated bubbles (left). As we add more air, there’s less room for water (center). Eventually, the proteins from different bubbles can bond so tightly the water is squeezed out (right).

Meringues are stabilized egg foams.

Impressive as this transformation is, it’s useless if we can’t preserve it. If we just whipped an egg white and put it in a bowl, the water would trickle out until our foam collapsed into a puddle. And because water is so thin, there wouldn’t be much time to serve the foam before this happens. (Also, a plain, albeit airy, egg white probably doesn’t taste that exciting to begin with.)

The good news for us is that we can preserve egg foams. Heat, sugar, and other ingredients transform our fragile clouds of protein, water, and air into stable meringues that can withstand the pressures of folding and piping or stand for days without collapsing. These strategies reduce the surface tension of the water so that it doesn’t pull down on itself so strongly, increase the viscosity of the liquid to slow it from draining out of the foam, and reinforce the protein network holding the air. You’ll notice that we’ve already discussed many of these ingredients and their general effects on proteins, but here, we will explore their specific interactions with egg proteins and their effects on meringues.

Increasing meringue stability decreases meringue volume, lightness, and tenderness.

Before we get into the details, let’s consider some general properties of egg foams and meringues. If the air bubbles in a foam are small and even in size, the meringue is fine. Because of the distribution of the bubbles, fine meringues produce a more uniform texture than coarse meringues. Meringues can be coarse if they’re beaten at too high a speed or with too thick a blade, or they can coarsen over time if the liquid walls in the meringue are too weak. To prevent this from happening, meringues need a certain degree of stability. This is essentially the meringue’s ability to keep ahold of its air and stay standing. We’ll see that strategies that contribute to stability typically work in direct opposition to qualities such as high volume, lightness, and tenderness, which are also desirable.

Like many choices in baking, the right balance between these properties depends on what you’re making. If you’re piping hundreds of meringue cookies that need to last for days, the meringue has to be more stable than one that’s spread on a pie, baked, and served within hours. Ultimately, there is no “best” meringue recipe. It depends wholly on your recipe and needs!

Factors that increase meringue stability

Heat coagulates egg proteins to reinforce structure.

One of the most frequently used methods to stabilize an egg foam is with heat. Remember that an egg contains several different proteins, only some of which participate in foaming. The rest, like the proteins that solidify an egg as it cooks, don’t unfold around air bubbles when we whip an egg. Only at high enough temperatures will they denature and coagulate, solidifying the shape and structure of the egg. In the case of a meringue, this shape just happens to contain a lot of air bubbles.

Heat coagulates egg proteins to reinforce meringue structure.

Protein coagulation in meringues works the same way as it does in any other recipe with eggs or meat. As the proteins set, they draw together. This can cause meringue to shrink as it cooks, just as a chicken breast would. In baking, shrinking can be especially frustrating when meringue pie toppings draw back from the edge of the crust. (We’ll discuss a way to prevent this with starch further down!) The dense protein structure can also tear in unappealing chunks as the meringue is sliced. And of course, just as a well-done steak is chewier than a rare steak, the additional structure from the coagulated protein toughens the texture of the meringue.

Heat acts on air and water.

Heat also alters other components of the meringue. As with any baked good, the trapped air bubbles expand until the coagulating proteins solidify the egg. Remember that this process, leavening, increases volume and lightness. Many cake recipes, such as Japanese cheesecakes, Japanese soufflé pancakes, and sponge cakes, rely on meringue to provide the air bubbles that ultimately raise the cake and provide its tender, ultra-airy texture. Other structural ingredients in these bakes, such as the starch in flour, also set with heat to reinforce the meringue structure.

In addition, heat causes water to vaporize, which dries the meringue and solidifies it further. Undercooked meringues contain too much water, which drains and weeps into a puddle. Overcooked meringues, on the other hand, force water out because the proteins link together too tightly. This creates beads of liquid at the surface of the meringue. Meringues baked at too high a temperature can also crack if the outside sets while the inside is still expanding. (We saw something similar in low-sugar muffins.) Thus, it is important to use an oven temperature and baking time that will cook the meringue fully and evenly.

In the oven, air expands until egg proteins coagulate to set and reinforce the meringue’s structure. Water also vaporizes to dry the meringue.

Heat changes meringue texture.

The timing of the heat also affects the texture of the meringue. French meringue is whipped first, then baked, so the meringue contains plenty of air for a light texture. In contrast, Italian and Swiss meringues are partially cooked (with hot syrup or over a double boiler, respectively) before they are fully whipped. Since the heat coagulates some of the proteins, the egg can’t stretch as far to hold air, and the resulting meringue is denser. However, the resulting structure can stand for days; plus, the heat helps sugar dissolve.

Sugar strengthens structure and protects against overwhipping.

Every meringue contains sugar. Soft meringues, which is what pie toppings and cakes typically use, are softer in texture (surprise!) and contain about an equal weight of sugar and egg whites. That’s the lower limit for sugar in meringues—any less and the foam will not hold. The upper limit is about twice the weight of the egg whites, which creates hard meringues for pavlovas and cookies. Eggs don’t contain enough water to dissolve much more sugar. As we’ve discussed, sugar has a strong attraction to water that gives it countless roles in baking. In meringues, sugar increases stability.

When we add sugar, it draws water and dissolves. Some of this water comes from proteins that have already aggregated around air bubbles. By drying these proteins, sugar makes them slightly more rigid and stable. The dissolved sugar also creates a thick syrup that is slower to drain, so the meringue retains its volume and matrix of fine air bubbles. The viscous liquid also protects the proteins from becoming overwhipped, dry, and inflexible. Remember that overwhipping occurs when the egg contains so much air that there is no room left for water in between air bubbles. With sugar, the liquid becomes thick enough to stay in between the bubbles and keep the proteins lubricated. In fact, sugar can bring meringues back from the brink of overwhipping: some recipes for meringue-based cookies whip the eggs to dry, crumbly peaks, then quickly add sugar to rescue the foam before it separates permanently.

However, stability comes at a cost to volume and lightness. The sugar syrup doesn’t stretch as thin as water, so the meringue can’t accommodate as much air. Sugar also draws water from proteins that are still wadded up, which makes it more difficult for them to unfold and aggregate around air bubbles. For this reason, the earlier sugar is added in the whipping process, the finer the foam and the firmer, denser, and creamier the meringue. The later the sugar is added, the softer the meringue.

Sugar continues to strengthen the meringue as it cooks. Because sugar holds onto water, the meringue dries more slowly, which gives the coagulating egg proteins more time to link together and set. When a hard meringue fully dries in the oven, the sugar crystallizes into thin strands to provide structural support, as shown below. Soft meringues retain some water after baking, so the sugar doesn’t crystallize, but it continues to dry and strengthen the proteins.

Sugar dissolves in the water of the egg (left). After baking, it crystallizes in hard meringues to reinforce structure (right).

Sugar should dissolve into the meringue.

Keep in mind that we only get extra stability if the sugar dissolves. Solid sugar crystals can’t interact with proteins, and their weight can deflate the foam. After baking, these chunky crystals taste gritty in contrast to the thin threads of recrystallized sugar. And large sugar crystals attract water from the air, which condenses into drops of liquid on the surface of a baked meringue.

As we’ve discussed, there are several ways to encourage sugar to dissolve. To start, we can add sugar gradually to give it time to dissolve. We can also consider crystal size. Granulated sugar crystals are relatively large, and although they will dissolve in soft meringues, they create a coarser foam. In hard meringues, granulated sugar will not fully dissolve. Instead, we can use superfine sugar, which is ground more finely and dissolves more readily. Superfine sugar should be substituted one-to-one for granulated sugar by weight. You can buy it, or you can make your own by grinding granulated sugar in a food processor. Note that superfine sugar is different from powdered sugar. Powdered sugar is ground even more finely, but it’s mixed with cornstarch to prevent clumping. While starch isn’t necessarily detrimental to meringues, it does change their texture, so powdered sugar should be used intentionally. (We’ll discuss starch in more detail further down!)

Granulated sugar (left) is coarser than powdered sugar (right).

We can also use heat to dissolve sugar. Italian meringues start by heating sugar and water into a syrup that’s then added to the eggs. Swiss meringues heat egg whites and sugar together before beating the mixture into a foam. As we discussed above, these techniques use high sugar concentrations and partial protein coagulation to create denser, more stable meringues.

Some other functions of sugar in meringue

As in all baking, sugar adds sweetness and physical mass. Sugar also contributes to browning, which can dramatically highlight ridges and peaks in meringue. In hard meringues, crystallized sugar adds crunch.

Acids speed up foaming and prevent overwhipping.

In addition to sugar and eggs, most meringue recipes contain a touch of cream of tartar, lemon juice, or vinegar. All of these ingredients are acidic, and they are interchangeable in meringue recipes. As we’ve discussed, acids unwind and denature proteins by disrupting amino acid bonds. When we add acid to our eggs, the proteins denature and aggregate around air bubbles more quickly, which stabilizes the foam. Acids are usually added to the eggs at the start so that the egg proteins start unfolding as soon as possible.

Acids also prevent egg proteins from bonding too tightly to each other. In the world of proteins, the strongest bond that can form is called a disulfide bridge. We briefly mentioned these in the context of gluten, where they create a tight gluten structure ideal for bread doughs. However, we don’t want too many disulfide bridges in egg foams. If the proteins are linked too tightly to each other, they won’t be able to stretch as air expands in the oven, and the bake won’t rise.

In order for a disulfide bridge to form, two cysteine amino acids, which have a sulfur attached to a hydrogen on their ends, must first lose their hydrogens and then link their sulfurs, as shown below. (“Disulfide” means “two sulfurs.”) By definition, acids add hydrogens to solution, and if there are lots of hydrogens floating around, it’s harder for the hydrogen on the cysteine to leave. Even if it does manage it, another one quickly takes its place. With hydrogens constantly attached, the sulfurs can’t bond, the protein network remains elastic, and the foam expands in the oven.

Disulfide bridges form when sulfurs (S) from two cysteine amino acids lose their hydrogens (H) and form a bond. Acid and copper prevent this from happening (not shown).

Copper also prevents overwhipping.

Copper also interferes with disulfide bridge formation. That’s why copper bowls are used for meringues: microscopic flecks of copper get whisked into the eggs and bind tightly to sulfur. With copper bound, sulfur can’t bind another sulfur to form a disulfide bridge. Because copper and acid perform similar functions in egg foams, you only need to use one or the other in your meringue.

Copper also binds the coagulating egg protein conalbumin and increases the temperature at which it coagulates. This means that, in the oven, air bubbles have more time to expand before the meringue gets hot enough to solidify, so the final product has greater volume.

Starch dries meringues and changes their texture.

Starch isn’t always used in meringues, but it does add stability. Remember that starches are long chains of sugar molecules organized into compact granules. When heated, these granules absorb water and swell. Starch is more often used in hard meringues, where it absorbs water to dry, strengthen, and stiffen the meringue. One way to add starch to your meringue is to use powdered sugar, which contains about three percent cornstarch.

Shirley Corriher shares another fascinating use for starch in meringue pies. Remember how heat coagulates egg proteins in meringue, which can cause shrinking and tearing? Starch can prevent both these problems by acting as a physical barrier to protein coagulation: if there’s a big starch granule between two proteins, they can’t tighten very far. And if the egg proteins in a meringue can’t tighten, the meringue won’t shrink. The looser protein clumps also slice through cleanly and have a more tender texture.

For this method to work, the starch must be swollen before the egg proteins coagulate. Cornstarch doesn’t swell until too high a temperature, so it has to be cooked with water first. The paste of swollen cornstarch granules can then be added to a meringue. In contrast, tapioca starch swells at a lower temperature than egg proteins coagulate, so it can be added directly to a meringue with sugar. As the meringue bakes, the tapioca starch granules swell first, and then the proteins coagulate around them.

Solid fat and gelatin solidify in cold temperatures to reinforce structure.

Some meringues are never heated. Instead, they set in the refrigerator. For these recipes, meringue is folded into solid fat or gelatin, which congeals in the refrigerator to reinforce the meringue structure. In chocolate mousse, for example, melted chocolate combines with the water lining the meringue’s air bubbles. The cocoa butter in the chocolate hardens around the air in the refrigerator to create a stable, light dessert.

Chocolate mousse is meringue stabilized by chocolate.

Factors that decrease meringue stability

Fats and emulsifiers interfere with the formation of structure.

Fats can be mixed into meringue after it’s whipped to reinforce it or to tenderize a cake, but fat in unwhipped eggs limits the final volume of the meringue. This makes the final product denser and tougher.

From previous posts, we know that fats coat proteins and prevent water from reaching them. Without water, the proteins can’t unfold and aggregate around air bubbles. This is one way that fats prevent eggs from foaming. But in meringues, the bigger problem is that fats compete with egg proteins for space along the edges of air bubbles. Since fats are hydrophobic, they use the air to avoid water. Emulsifiers are also attracted to air bubbles because their water-loving and water-avoiding ends can sit perfectly at the interface of water and air. However, unlike proteins, fats and emulsifiers cannot form bonds. They weaken the egg foam because they take up space at the edges of air bubbles without contributing to their stability.

Lipids coat proteins and prevent them from unfolding and aggregating (right). They also compete with proteins for space at the surface of air bubbles (left). Since lipids cannot form bonds the way proteins can, they destabilize meringue.

Egg yolks destroy an egg white meringue, but they foam, too.

All lipids (that is, solid fats, oils, and emulsifiers) decrease meringue stability, but the combination of lipids in egg yolks is one of the most detrimental for a meringue. That’s why sponge cakes require clean beaters for the egg whites: you don’t want to contaminate them with the egg yolks or fats from the rest of the cake batter. There are many strategies to avoid yolk contamination. Cold, fresh eggs, for example, are easier to separate. An additional bowl can be used to separate each individual egg so that one broken yolk doesn’t contaminate a bowl full of whites. Cracking the egg on a flat surface prevents eggshell fragments from piercing the yolk membrane. Find and use the strategies that work for you. One myth regarding fat contamination is that you should never use plastic because it can trap fats that will leach into your eggs. Although plastic can trap fat, it won’t release enough into your meringue to significantly destabilize it. A clean plastic bowl will work just fine if nothing else is available.

Note that egg yolks can foam—their lipids just limit the meringue’s final volume, which can lead to a low rise and a dense, tough texture if a recipe was developed for egg whites only. Egg whites can reach eight times their original volume, but whole eggs can only expand about threefold (and that’s with help from heat, sugar, and a little patience). Génoise is a cake leavened with whole egg foams, and it is perfectly light because the recipes are developed for whole eggs.

Egg yolks are also capable of foaming by themselves—after all, they do contain proteins. Their limiting factor is actually their water content: they don’t contain enough liquid to hold many air bubbles! With the help of wine (which contains water, alcohol, and acid), heat, and sugar, egg yolks can be beaten to four times their volume to create zabaglione and sabayon.

Water adds volume but dilutes proteins.

Unlike egg yolk foams, egg white foams don’t need additional water. However, we can use water to soften the texture of an egg white meringue. By adding water, we dilute the egg proteins and weaken the meringue’s structure. The extra liquid also stretches further to accommodate more bubbles and more volume. On the flip side, some bakers like to leave separated egg whites in the refrigerator for at least a day to decrease their water content. This process, called “aging,” is typically used to give macarons extra stability.

Salt prevents protein bonding.

In “Proteins in the Kitchen,” we learned that salts, like acids, affect protein structure. Salts dissolve into charged ions, which interfere with the interactions of charged amino acids. In the case of meringues, salt ions prevent charged amino acids on different protein strings from binding to each other, thus decreasing their aggregation around air bubbles and the overall foam stability. For this reason, salt is often added to the other ingredients in a recipe and combined with the meringue only after the eggs are fully whipped.

Other considerations (or not) for an egg foam

Fresh eggs or old? Cold or room temperature? Which whisk do I use? How fast do I whip? And for how long? Does it actually matter? Let’s get into it.

Egg freshness and temperature don’t matter a ton.

There are pros and cons for both fresh and old eggs. Fresh eggs have stronger yolk membranes, so they’re easier to separate. Fresh egg whites are also thicker, so the meringue’s volume is somewhat limited. Old, thin egg whites drain more quickly and are less acidic, so meringues made from old egg whites are less stable. Ultimately, any egg white will work. But if you have to choose, use your fresher eggs for the extra stability. Most eggs sold at the grocery store aren’t farm-fresh, anyway, so any loss in volume is negligible.

Many recipes specify that egg whites should come to room temperature (about 70°F/21°C) before beating. Although the proteins in warmer whites are wadded less tightly and thus will unfold and aggregate faster, the difference in whipping time between cold and warm eggs isn’t that significant with modern electric mixers. The eggs will quickly warm up as they’re beating.

Beating utensils and speed affect foam stability.

To speed up the beating process, use a whisk with more tines. That way, each stroke adds more air. For a fine-textured foam, thin wires (like what you’d find on a whisk) are better than thick blades (like the paddle attachment on a stand mixer). And whipping at a medium-high speed for a longer time will produce finer bubbles and a more stable foam than whipping at a high speed for a shorter period of time.

Beating time depends on your bake.

The ideal amount of air for a meringue depends on the recipe. The longer we beat, the drier the foam and the stiffer the proteins. If we’re making an angel food cake, where the meringue is combined with other ingredients, we need a less beaten foam that retains enough moisture to fold smoothly into the batter. A wetter meringue also has more flexible proteins that stretch around expanding air bubbles in the oven for a higher rise. On the other hand, the meringue for cookies or pavlova needs to be dry and stiff enough to hold its shape. These meringues are beaten longer, which means their proteins are more rigid and inflexible, so they don’t rise as high in the oven. Fortunately, volume isn’t as important a consideration for these bakes.

This castella cake is leavened by a soft meringue barely whipped to stiff peaks. A wet meringue folds more easily into the batter and has the flexibility to accommodate air expansion during baking.

We can even selectively dry some proteins in our meringue to create different shapes. French macarons develop their signature feet because they sit at room temperature for a while before they’re baked. During this time, the surface of the macaron batter dries, and the proteins there stiffen. When the macaron is baked, the meringue can’t expand upward, so it expands downward instead to create the feet.

Macaron feet develop because the surface of the batter is dried before baking.

Recipes often use peak stiffness as a measure of how much to beat the foam. Soft peaks fold over because there’s still a lot of water between the air bubbles. At stiff peaks, the foam is about ninety percent air. There’s minimal water between the air bubbles, and the proteins on adjacent bubbles start to stick to each other. The foam is dry and stiff enough for the tip of the peak to stand straight up. Most recipes don’t go beyond this point—the proteins on adjacent bubbles will start to bind too tightly and squeeze water out.

Another way to describe these stages is “slipperiness.” A foam with soft peaks is still wet enough to slide around the bowl. As the meringue stiffens, it becomes dry enough to stick to the bowl, even if you turn it upside-down. And if water starts to separate out of the foam, it will start to slip again.

Meringues are versatile.

Whew! This was a long post with a long list of ingredients and considerations. But it’s important to remember that eggs are not finicky. They come out of the shell with all the equipment they need to hold a lot of air. In college, my two roommates and I whipped a meringue in a plastic bowl with the cheapest whisk we could find at the grocery store. It turned out just fine. (Our arms, on the other hand, are a different story.) But with some extra knowledge about how meringues work, we are better equipped to understand recipes for everything from meringue pies to angel food cakes to dacquoise and beyond. Better yet, we can adapt these recipes to fit our own kitchens, personal tastes, and specific needs.



References

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

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

McGee, H. On Food and Cooking; Scriber: New York, 2004.

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

Saffitz, C. Spongecake is simple, versatile, and delicious if done right. Twin Cities, 2021.

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