Think you need to be a molecular gastronomist, pr a chef at a fancy restaurant to try your hand at making food foams?
Nothing could be further from the truth! Even though a lot of articles online write about these delicate, innovative foams in food, we humans have been making foams for our food for a lot longer than the past few decades.
Foams are everywhere in food. Marshmallows, bread, ice cream, the white foam layer on top of a beer, meringues, whipped cream, chocolate mousse. They are all every day examples of foams in food and no matter how fancy the foam, the the science behind it remains the same.
What is a foam?
Almost intuitively, you would likely describe a food foam as a very light food, considering its volume. Foams do indeed have a very low density (which is the weight per volume). A large volume of foam can still be very light.
Foams are an example of a dispersion. In a dispersion one material is mixed in another material, but they stay distinctly separate. In the case of a foam a gaseous material (e.g. air or carbon dioxide) is dispersed throughout another material which is a liquid or a solid.
The presence of that gas is why the density of a foam is so low. Gases have a very low density, far lower that than of most liquids and solid. As such, just how light your foam turns out depends on how much gas you manage to get into your foam.
An egg white foam for instance can easily increase in volume 6- or 8-fold. That means that in the lower range of those increases only 1/6th of the foam is made of the egg white whereas 5/6th is air! As a result this foam is super light and airy. A light chocolate mousse on the other may be doubled in size (although this depends a lot on the recipe you’re using!). It only contains 50% air and as such is a lot denser.
Types of foams
There are a lot of different types of foams and they cover a wide range of characteristics as you could see in the egg white foam vs chocolate mousse comparison.
As we mentioned before, the gas can either be trapped in a liquid or in a solid. These are appropriately named liquid and solid foams. As you can imagine, trapping a gas in a liquid might be easier, but, the gas is prone to escaping from the liquid again (e.g. in the case of an Italian meringue). In the case of a solid on the other hand, the gas is pretty much trapped in place (you won’t see a properly baked bread collapse).
How a foam collapses over time
If the solid is stable and strong enough, a solid foam will not deform over time Just image seeing your bread slowly collapse in the days after you’ve taken it from the oven. If it collapses that would be due to spoilage, not because of an unstable foam structure.
That is not the case for most liquid foams though. You might have come back to your freshly whipped cream a day later and notice it has shrunk in size. It’s even more apparent if you used whipped cream from a pressurized canister. That tends to collapse in a matter of minutes or at least within the hour.
The reason for this collapse is that the gas in the foam is slowly leaving the liquid. Since the liquid by itself is not strong enough to hold its shape, that will mean it slowly sinks in. There are several scientific mechanisms at play here of which we’ll highlight the three most important ones for foams.
Foams will almost always contain smaller as well as larger sized bubbles. Ostwald ripening describes how the smaller bubbles in a foam will merge with larger bubbles over time. This is because gas slowly diffuses through the liquid separating the two bubbles, moving from the smaller to the larger bubble.
This movement of gas is caused by a pressure difference inside bubbles. The pressure within smaller bubbles is higher than that in larger bubbles. As such, there is a driving force for the gas to diffuse from one to the other bubble.
Another way a foam can lose its final structure is by two bubbles merging with one another. In this case the film that separates the two bubbles breaks which causes the two to come together and form one bubble.
Whereas Ostwald ripening is driven by a pressure difference and diffusion of gases, coalescence is governed by the strength of the liquid layer between the bubbles. Even though the effect is the same of the two, this difference in mechanism means that you will have to use different tools to prevent either from happening.
Cocoa particles can sink out of a chocolate milk system through a process called sedimentation. The particles sink because of a difference in density. The same can happen to the air in a foam. However, instead of sinking down, the gas floats upwards.
In order for sedimentation to happen it must be easy enough for gas bubbles to move through the rest of the foam. In the case of a beer foam this is not a problem. In a matter of minutes the whole beer foam will have disappeared. In an ice cream though sedimentation is almost non-existent. The gas bubbles are fixed in place.
Liquid can slowly flow down driven by gravity. The effect is the same as for sedimentation. However, as was the case for Ostwald ripening and coalescence, the underlying mechanisms are different, resulting in different ways to prevent them.
Making and stabilizing a foam
Now that we know how a foam disappears over time, let’s look at how it gets made in the first place. Making a foam requires some way to incorporate gas into a liquid or solid (although in food that solid is often a liquid at the point of adding the gas, only turning solid later).
Using a (electric) whisk to beat air into an egg white or milk foam is one way. This would be described as using mechanical energy to incorporate a gas, in this case air, into the foam.
Another method is to form that gas from within. This is what you do when using yeast. baking soda or baking powder. Yeast produces carbon dioxide through fermentation and baking soda and baking powder produce gas through a chemical reaction.
At a larger scale, some more force can be used to introduce air into a liquid or solid. Air can be injected under pressure for instance. The overall effect though is the same, gas bubbles become dispersed throughout a liquid or solid phase.
If you’ve every tried to make a water foam, you know that just adding mechanical energy isn’t enough to make a good foam. No matter how hard you mix, pure water won’t form a good foam. The gas and water split almost immediately after stopping any whisking. In order to make a good foam you need something that helps hold onto that gas within the liquid.
It is why whisking milk will give some sort of a foam and an egg white gives an even better foam. They both contain molecules that help form this foam: proteins.
In order for a proper foam to form you need something that helps lower the interfacial tension which makes it easier to break up bubbles into even smaller bubbles. Also, you need something that helps all these freshly formed bubbles from coming together immediately. This is where surfactants come in.
Surfactants helps the gas and liquid phase to come together. In food most surfactants can do this because of their structure. Surfactant molecules contain a section that loves to sit in water (hydrophilic) and a section that does not appreciate water (hydrophobic).
Lecithin is an example of such a molecule. It has a head and tail structure. The head is hydrophilic whereas the tail is hydrophobic. Once a gas bubble sits within water with lecithin mixed through that lecithin will organize itself along the border of that gas bubble, stabilizing that bubble.
Proteins work in a similar way. Their molecular structure is a lot more complicated, however, it works in a similar way. The protein has hydrophilic and hydrophobic sections that sit on the interface of the gas/liquid interface and help to stabilize the air bubbles.
To close off, you may by now be wondering how you can prevent your foam from collapsing and encourage its formation in the first place. Using the mechanisms discussed above, there are several options!:
- Thickening the liquid between the air bubbles, increasing its viscosity. This makes it hard for liquid to move down and for bubbles to travel up. It’s one of the reasons you add sugar to a meringue!
- Solidify the foam once the gas has been trapped in. This is what you do for a chocolate mousse. After making the mousse you leave it to cool so the chocolate in the mousse hardens out, stabilizing the system.
- Prevent the liquid from going anymore by gelling it. This is what stabilizes marshmallows! It’s also why marshmallows shrink when you roast them, the gelatin that gels them loses its gelling property to gas gets out more easily!
- Solidify the foam by baking it and drying it out. It’s why French meringue is more stable than Italian and Swiss merinque and why taking an uncooked cake out of the oven means disaster (all the gas leaves the unbaked cake!).
Of course, in some cases, your best shot at preventing collapse is to just eat it fast enough. Don’t leave that whipped cream standing around for too long. Just enjoy it with a piece of apple pie. And if you’re eating in a fancy restaurant serving you a light and airy foam. You’d better get at it fast, those disappear pretty quickly!
Decoding delicious, Egg Foams, link
Logson, J., How to make modernist foams, Amazing food made easy, link
ScienceDirect, Ostwald ripening information page, link, visited March-2020
The Incredible egg, Foams, 2013, link
Walstra, P., Physical Chemistry of foods, Marcel Dekker Inc, 2003, ISBN: 0-8247-9355-2