Learn the science behind:
Every time we whip up some egg whites into a white, light, and airy foam we’re a little amazed. It’s pretty special how such a simple ingredient can hold on to so much air! Even more special, when you consider that chickens don’t really need that foaming ability to grow chicks out of their eggs.
These light and airy foams are powerhouses when it comes to holding on to air. Something that isn’t all that easy to do. So far the only other food ingredient that comes close is aquafaba.
There are tons of ways to let the airiness of an egg-white shine and see the science in action. But, the simplest one is by making it into a meringue. In meringues, sugar and egg white (and maybe some heat) work in tandem to create an array of foams that are even more stable and airy than those made with just egg whites. We’re going to dive into the science behind these foams to give you the tricks and tools to make even better meringues going forward.
Table of contents
- What is a meringue?
- Why egg whites foam so well
- How sugar stabilizes an egg white foam
- The role of heat in a meringue
- Other ways to (de)stabilize meringue foams
- Types of meringue – French, Italian, Swiss
- Further reading
What is a meringue?
Meringues are light, fluffy, edible foams, traditionally made from egg whites and sugar. Some are crisp, whereas others are soft and shiny. Some are aked, whereas others are ‘cooked’ and yet others receive no heat treatment at all. If no color is added, meringues are white(-ish).
You’ll find meringues in a range of foods, though most commonly in sweets/desserts. In some applications, you’ll eat the meringue as such but they’re also often used as a topping (on pie or eggnog), or as part of a dish (e.g. pavlova or in your chocolate mousse).
Meringues start out by whipping up liquid egg whites into a foam. By whisking an egg white you’re incorporating air bubbles into the liquid egg whites. Sugar is either added at the start, or later while whipping. Meringues might undergo a heat treatment (or not).
It’s a foam
Scientifically speaking, meringues are foams. Foams consist of two separate immiscible phases: one liquid or solid, the other one being a gas. In a foam, the gas is dispersed throughout the liquid (or solid) phase.
The tricky thing about foams is that most aren’t stable over time. The individual gas bubbles that are dispersed throughout the solid or liquid want to merge together again, energetically that’s more favorable. As a result, a lot of foams collapse over, caused by the gas leaving the other phase. When making meringues, you’ll find that a common theme is how to stabilize that foam for long enough! Sugar plays a key role here, but so does a heat treatment.
Why egg whites foam so well
The starting point for most meringues is an egg white. Understanding why that egg white can get so foamy is crucial to understanding the science of meringues. Once you start whipping that egg white it starts to become foamy. This is quite a unique property. If you’d try the same with pure water, you wouldn’t even get close to getting the same airiness! But why?
To answer that question, we have to look at the composition of the egg whites themselves. Aside from water, most of an egg-white is made of proteins, about 10%. Egg whites barely contain any fat or carbohydrates and it’s the proteins that give the egg white its special properties.
Proteins are a very common type of molecule in food (as you can learn in our Food Chemistry Basics Course). They are made up of long chains of building blocks called amino acids. These long chains curl up and fold in very specific ways which make the proteins do their job (e.g. as an enzyme).
When proteins are unfolded you expose all the amino acids that make up the protein. Different amino acids have different properties, for foam stabilization in meringues their hydrophobic vs. hydrophilic properties are important. Parts that are hydrophobic do not like to sit in water, whereas the hydrophilic amino acids prefer to sit in water. Proteins can have sections of both.
Egg white contains a mixture of proteins, the most prevalent one being ovalbumin. What’s special about this protein is that it can be denatured (unfolded) quite easily, simply by using some mechanical force (here’s where a whisk comes into play). This in and by itself is quite special. Not a lot of proteins denature as well as this protein does when agitated mechanically. A lot only denature when exposed to heat, others barely denature at all!
By whisking and thus unfolding the protein, you expose the long chain of amino acids that makes up a protein. Parts of this unfolded protein chain like to sit in water, whereas other parts do not (hydrophilic vs. hydrophobic sections). The protein will arrange itself in such a way that most sections can sit in their preferred space.
Aligning on the interface
In the case of an egg-white foam this means that the hydrophilic parts will align themselves along the water present in the egg. The hydrophobic parts on the other hand will arrange themselves alongside air bubbles, since they don’t like to sit in water. As a result, these proteins lie on the interface between water and air. As such, they stabilize the air bubbles within the water, it will be harder for the air bubbles to ‘meet’ one another again. The structure and composition of egg white proteins are such that they are really good at this job, far better than many others!
That said, egg white proteins aren’t strong enough to hold on to the air forever. Over time, the air bubbles will coalesce and the egg white foam will collapse again. This is where additional stabilization of the meringue comes into play.
We’ve discussed the science of foams and why and how they collapse over time in far greater detail in a separate post.
Egg whites have traditionally been the go-to ingredient for meringues because of those special proteins. There aren’t a lot of ingredients that can compete. So far, the ingredient that gets closest is aquafaba. Aquafaba is the leftover liquid from cooking chickpeas. It also has surprisingly good foaming properties, though it does still fall a little short compared to egg whites. That said, you can make good meringues with it. The science behind those meringues is pretty similar to those made with egg whites, as we’ve explored in more detail here.
Apart from this naturally occurring product, ingredient manufacturers have been working on finding other alternatives. Knowing how egg whites work, they’ve developed a soy based protein that can also be whipped up (e.g. Versawhip 600K). Here the soy protein has been modified in such a way that it can also be whipped up (whereas ‘regular’ soy protein wouldn’t!).
How sugar stabilizes an egg white foam
Even though egg whites are great at foaming up, they need additional help to create a foam that’s stable over time. This is where sugar comes in. Sugar will dissolve in the water (liquid) from the egg white. As a result, it increases the viscosity of the watery phase. If you’ve made sugar syrups before, you will have noticed that higher concentrations of sugar make thicker, more viscous syrups.
By increasing the viscosity of the water phase, the sugar makes it harder for the air bubbles to merge together and escape (read more about those mechanisms here). As a result, an egg white foam that contains a good amount of sugar (a small pinch won’t help you here) will take a lot longer to collapse than one that doesn’t contain any sugar.
Powdered sugar vs granulated sugar
Depending on the type of meringue you’re making, you might be adding quite a lot of sugar to your meringue. Ideally, most (if not all) of that sugar dissolves in the water of the egg white. This way you won’t have any grittiness from any remaining sugar particles. In some recipes, however, the concentration of sugar is very close to the maximum amount of sugar that can dissolve in water at room temperature. This can make it challenging to dissolve all the sugar.
This is where choosing an appropriate sugar particle size comes in. Smaller particles of sugar dissolve in water more easily. Larger particles can take a lot longer to properly dissolve. It is why recipes for very smooth, delicate meringues will often call for using icing sugar. The icing sugar consists of very small sugar crystals that dissolve quite rapidly!
You will mostly see this for meringues that don’t heat the sugar and/or the egg whites before or during whipping (e.g. French meringues, see below). If your meringue involves a step in which you heat the sugar in a liquid this is less important. The heating step increases the speed at which sugar dissolves, making it easier to dissolve larger particles!
Since you want to dissolve all this sugar (else it will become gritty) recipes tend to call for using powdered (icing) sugar. The only difference between powdered and granulated sugar is the particle size. That of powdered sugar is a lot smaller and as a result, it dissolves a lot more easily.
When to add the sugar
When making a meringue you could add all the sugar with the egg white from the get-go and whisk them together. However, you could also whip the egg whites first and add your sugar later. The order of addition can impact the result of your meringue and will depend on the type of meringue you’re making. There isn’t a right or wrong, both will make meringue, just a slightly different texture. We won’t dig into the details here, but you can find more information in articles digging deeper into specific types of meringues (e.g. Swiss or Italian meringues).
The disadvantage of using sugar syrups
Since the sugar needs to increase the viscosity of the water naturally present in the egg white, it’s best to use dry sugar instead of a natural sugar syrup such as maple, rice or agave syrup. Sugar syrups already contain a lot of water themselves and won’t be able to increase the viscosity of the egg white enough.
If you’re using egg white powder instead of liquid egg whites you could give these sugar syrups a shot! Since you will need to add water anyway, adding (part of) the sugar as a liquid can make a stable meringue. You will need to tweak ratios to get that optimal ratio of water : proteins : sugar.
Bonus: Sweetness & Browning
Sugar is mostly there to help stabilize that meringue, however, it does more. First of all, it sweetens the meringue and thus provides some flavor. Second, it makes it easier for the meringue to turn brown, e.g. upon blow torching the meringue. The sugar can caramelize because of the heat, but, more importantly, the sugar + proteins in the egg white can react in the Maillard reaction, also causing the meringue to turn brown!
The role of heat in a meringue
Even adding sugar to a meringue won’t make it stable for that long a time frame. Over time, most raw meringues will still collapse, often in a matter of hours. This is where the third major factoring for stabilization comes in: heat. It can help stabilize a meringue in several ways.
Cook the proteins
Those egg white proteins have been denatured during whipping, causing them to unravel and unfold and align themselves along the water:air interface. However, you can denature them more permanently by heating the proteins. This is what happens as well when you cook/boil an egg. The egg white transforms from transparent into white. Most of this transformation is due to the proteins ‘cooking’. Once cooked, there is no way you’ll get back to your initial egg white anymore, it’s been changed permanently.
By fully cooking your proteins in the meringue, they can be stable for a very long time (although generally, it still is important there’s not too much water, see below since that will keep it somewhat unstable). When making French meringues (see below) you’re using this technique.
Dissolve (more) sugar
At higher temperatures, you can dissolve more sugar in less water. Some meringues use this aspect by making a hot, very concentrated and thick sugar syrup. By adding this hot syrup you can both add a lot of dissolved sugar and use the heat of the syrup to ‘cook’ some more of those proteins.
As long as you have a foam of air bubbles in a liquid (e.g. water) it is almost inherently unstable. However, if you get rid of enough of liquid (aka water) in your meringue you can create a solid foam. In a solid foam the gas bubbles are dispersed into a solid phase (in this case a matrix of mostly sugar and proteins and just a little water). Bubbles are pretty much trapped here.
You achieve this by first making the meringue and then subsequently baking the meringue, to evaporate any excess moisture. The tricky part here is always to do it slowly though. Since a meringue is mostly sugar and proteins, it’s very prone to browning and burning. Also, the water is trapped quite well and will need time to slowly evaporate without breaking the structure. You use this ‘trick’ when making French meringues.
Other ways to (de)stabilize meringue foams
The major aspect of successfully making a good meringue is to manage its stability. You want your meringue to stay light and airy for long enough for your application. In some cases you might only need it to be stable for a few hours, in other cases you’re looking at days. Egg whites, sugar and heat are the core ingredients for managing stability. However, there are a few other factors that can either greatly increase or decrease the stability of a meringue!
How egg yolk destabilizes an egg white foam
Before we dive even deeper into the world of meringues, let’s make clear that you really should only use the egg white and not the yolk for making a meringue. A yolk also contains proteins, but apart from that, it contains fat, which egg whites don’t. This fat can ruin your meringue.
As we discussed, proteins stabilize the egg white foam by unfolding and sitting on the surface between water and air. However, if fat is present, these proteins will not just sit on the water/air interface. Instead, it will probably prefer to sit on the fat/water interface, somewhat acting like an emulsifier. This severely limits the foam stabilization properties and as a result your foam won’t be as stable and will collapse a lot more easily.
Impact of lemon juice & cream of tartar
Many recipes will call for using lemon juice or cream of tartar in your meringues. Both of these help stabilize your foam. In a lot of cases you can do without them but it will just give a slightly less stable or less firm foam.
Both work by lowering the pH-value of your egg white mixture. In other words, it makes the meringue mixture more acidic. These lower pH-values help the proteins to unfold and thus helps stabilizing your egg foam even better.
Adding gelatin (and other stabilizers)
You can of course stabilize the meringue even further by adding other ingredients. A common ingredient you’ll find is gelatin. It’s what helps transform meringues into a marshmallow! Even though this is a suitable way to stabilize a foam for a long period of time, you wouldn’t call these foams meringues anymore. You’ve simply made another dish.
The reason an ingredient such as gelatin works is that it literally ‘gels’ the meringue. The gelatin molecules form a complex entangled network of molecules that help to stiffen the water phase. It becomes even harder for individual gas bubbles to travel through the liquid phase, stabilizing the foam for a long period of time!
Adding butter (buttercream meringue)
This stabilization trick is similar to that of gelatin, you’re adding another ingredient to help add stability to your foam. However, instead of adding a gelling agent, here we’re adding an ingredient that turns solid over time. Butter, but also chocolate for instance, can be melted down and mixed through the meringue. They will set and solidify again upon cooling. As a result, you now have gas bubbles trapped in a solid phase which makes it even more stable over time.
Types of meringue – French, Italian, Swiss
There are a lot of different types of meringues. A common way to classify them is by origin (though whether that’s truly their origin, who knows): French, Italian & Swiss meringue. Whereas the names aren’t always used consistently, these three types do distinguish between three key ways to stabilize a meringue.
French = Baked
The French version is quite a stable type of meringue, thanks to the fact that it’s been baked in the oven. As most meringues do, the French version starts out by whipping egg whites and adding sugar to create an even fluffier foam. The foam is then baked in the oven (see recipe at the end of this post). Depending on how long you bake them for they either turn out completely crunchy, or still have a slightly soft center.
Before placing the French meringues in the oven you had a liquid phase (sugar + water) in which you had dispersed a gas phase (the air). By baking them in the oven you evaporate a lot of the moisture from the liquid phase. As a result, it turns solid and crispy. Solid foams are a lot more stable than liquid foams. The gas bubbles are trapped more securely and will have trouble finding one another. As long as you store French meringues away from moisture (which can soften the meringue again) they can be stored for a long time.
Italian = Hot sugar syrup
An Italian meringue starts by making a hot, concentrated sugar syrup. By making the sugar syrup you dissolve any and all sugar crystals. This ensures you get a super smooth meringue.
Once the sugar syrup is ready (which depends on your recipe) you add it to freshly whipped up egg whites. The heat from the sugar now partially cooks the egg whites, helping its stability and increases the viscosity of the water phase thanks to all the dissolved sugar!
Swiss = Au-bain marie
Last but not least, in the so-called Swiss version you gently heat up the egg whites and sugar together while whipping it. Once they’ve reached the temperature you’re aiming for you increase whipping drastically. This meringue definitely isn’t crispy, but it is very light and airy and even stable enough to pass through a piping bag without losing all the air again.
Of course, within these different types, there are again countless possible variations. You can add colors, or flavors, change cooking times and ratios of ingredients. However, all follow the same science that we just discussed, so you should be good to go to let those egg whites shine in a new meringue!
Cook’s Illustrated, What’s the Difference Between French, Swiss, and Italian Meringues?, link
Fabio Licciardello, Pierangelo Frisullo, Janine Laverse, Giuseppe Muratore, Matteo Alessandro Del Nobile, Effect of sugar, citric acid and egg white type on the microstructural and mechanical properties of meringues, 2011, Journal of food engineering, link
K. Lomakina, K. Mikova, A Study of the factors affecting the foaming properties of egg white – a review, 2006, Czech J. Food Sci., 24: 110–118., link
Harold McGee, On food and cooking, 2004, p. 108-110
Modernist Pantry, Versawhip 600K, link ; want to try egg less meringues? this is one of the commercially available alternatives
Vassilios Raikos, Lydia Campbell, Stephen Euston,Effects of sucrose and sodium chloride on foaming properties of egg white proteins, 2007, Food Research International, DOI: 10.1016/j.foodres.2006.10.008, link
Nicole Rees, How to make meringues, from Fine Cooking #128, pp. 78-85, link
The Science of Cooking, What does salt do to a meringue mixture?, on Exploratorium, link