Up to not too long ago, I’d consider myself a muffin fan over a cupcake lover. I never really understood the craze around cupcakes with their (too) sweet toppings. But, that changed after seeing yet another season of Sugar Rush on Netflix.
During the first round of this baking show, four pairs of bakers have to make cupcakes. These cupcakes tend to look amazing and, seem well balanced in flavor. This made me want to try making cupcakes, more specifically: cupcakes with a Swiss meringue buttercream frosting. Not only does making this buttercream regularly lead to overflowing mixers (fun to see, but more on that later), it also tends to get great reviews from the judges.
What’s more, after looking into it a little, it’s full of some great food science phenomena. Looking for a way to ‘practice’ phase transitions, protein denaturation, melting points, and dissolving molecules. In other words, a worthy topic to research further.
What is a Swiss meringue buttercream?
Before starting the Swiss meringue buttercream journey, I had to choose a flavor and figure out what it was I needed to make this cupcake topping. Seeing my love for chocolate flavors in general, I quickly landed on a chocolate version. Apart from requiring chocolate, I needed:
- Egg whites: these are heated with the sugar and then whipped up nice and fluffy (it’s what make it a Swiss meringue)
- Sugar: to dissolve in the egg whites and sweeten the whole thing
- Butter: to actually make a buttercream, you’d need butter!
For a full recipe, scroll down to the bottom of this article, for now, we’ll first look into the different steps and the science involved in each.
Step 1: Heating up egg whites & sugar
For making my first Swiss meringue buttercream, I had to start by heating egg whites and sugar to 66°C (150°F). For doing this on a small scale, you’d typically use a double boiler. A bowl with egg whites and sugar sits on top of a pot of boiling water. While stirring continuously, to prevent part of the mixture from overheating and some of it not heating enough you gently heat the mixture up.
Despite the seemingly simple step, a lot is going on within your egg white sugar mixture at this point.
Dissolving your sugar
My Swiss meringue buttercream recipe uses roughly equal amounts of egg whites and granulated sugar. Egg whites consist of mostly water (roughly 90% of the egg white), the remainder is mostly proteins).
If you’d mix equal amounts of room temperature water and sugar together, chances are most of the sugar will not dissolve. If you’d taste a spoonful, it will likely be grainy because of the sugar crystals still in the water. For a smooth buttercream, you want all the sugar to be dissolved.
Sugar dissolves better and faster in water at higher temperatures (which is why that teaspoon of sugar in your hot tea is gone within a matter of seconds). By heating up your egg white and sugar mixture you ensure the sugar dissolves fully in the water within your egg whites.
Tip: If your buttercream still turns out grainy because of the sugar, you could try using icing sugar. Icing sugar is ground granulated sugar and has smaller sugar crystals. This makes it even easier to dissolve in water!
Denaturing the proteins
Apart from water, the egg whites consist of about 10% proteins. This is a mix of various different proteins that all behave slightly differently. Proteins are large complexly folded up strands of amino acids. When proteins denature they unfold. As a result, their functionality changes. In the case of an egg, it’s what causes the egg to cook and solidify when you boil or fry it. It is also what helps create a foam when whipping an egg white.
The second most prevalent protein in egg whites, ovotransferrin (also called conalbumin) denatures (through unfolds) through the impact of heat. Starting at roughly 60°C (140°F) the proteins will denature. If you’re making an egg white foam like our Swiss meringue buttercream, that helps the final foam to be extra sturdy. You’re partially cooking and slightly setting the foam. It makes your overall frosting a lot more stable.
If you control the heat well, you will also kill off microorganisms (mostly Salmonella) during this first step. That said, it’s a fine balance to get this right, so if you’re not fully comfortable with this, use pasteurized egg whites. These have received a heat treatment before being packaged, killing off potentially harmful microorganisms.
Step 2: Whipping the egg whites – Aeration
Once your egg whites and sugar have reached the target temperature, it’s time to start whipping those egg whites! As you go, the egg whites with sugar will turn whiter in color and become larger and larger in volume. They will also become slightly shiny.
This is the step where, if you’ve watched the show Sugar Rush, you can regularly see bowls of mixers overflowing with whipped up egg whites. It can be hard to judge just how much your egg whites are going to increase in volume, especially if making large batches!
Proteins can denature because of heat (which is what happened to the ovotransferrin). Another way to cause denaturation of some proteins is agitation. Whipping egg whites is a perfect example of agitation. The main protein in egg whites (ovalbumin) is especially vulnerable to agitation, in other words, it will now unfold as well.
So why is this unfolding so important? As mentioned earlier, proteins are long chains of amino acids. Parts of a protein tend to like to sit in water (hydrophilic), parts of it do not (hydrophobic). When the protein unfolds while you’re whisking your egg white, those hydrophilic parts will continue to sit in the water of the egg white. However, the hydrophobic parts will want to align themselves with the air bubbles in the egg white. This is what helps stabilize these bubbles, and ensures they don’t immediately disappear again (as they would in pure water).
During whisking, you’re introducing a lot of air bubbles in the egg whites. These bubbles need to be held onto and the denaturation by both heat and agitation, help out here!
Step 3: Whipping in the butter
Once your Swiss meringue has been whipped up nicely, it’s time to add the last core ingredient: butter. If you’re using ‘regular’ butter, it will be an emulsion of milk fat, water, and a few proteins. Butter contains upwards of 80% fat, but the exact quantity depends on your butter and probably the country you live in, since legislation varies. The fat in butter is mostly solid in the fridge, partially solid at room temperature, and completely liquid at about 40-45°C (104-113°F).
Butter can be a sensitive ingredient. Once butter has melted completely, it won’t come back completely into its original form if not cooled well. The water will have escaped from the water, causing the two phases to split.
Melting (some) of the butter (phase transitions)
This is important to keep in mind when making this buttercream. You don’t want to add the butter when the Swiss meringue is too hot. Too much of the butter will melt and there’s a chance the buttercream splits. It’s why you start adding the butter once the Swiss meringue is still warm to the touch, but not hot anymore.
You add the butter piece by piece, to make it easier for the mixer to mix the butter through completely. Also, this will help to soften the butter, the Swiss meringue, still being somewhat warm, will soften and melt the butter enough for it to fully incorporate homogeneously.
Step 4: Folding/Whisking in optionals
Once your sugars, egg whites and butter have all been incorporated, you’ve got a Swiss meringue buttercream. However, you can change flavors and textures slightly. You can add flavors as well as colors, both of which can be added just before adding the butter, or right after.
The recipe we used below uses chocolate to add a chocolatey flavor to the buttercream and make it a nice brown. You could also add in things like nut butters at this point. Your quantities will differ per buttercream type. Always make sure the buttercream remains sturdy enough, adding too much watery liquid for instance will make the buttercream unstable. So be careful in adding too much fruit purees or juices.
Using (& Softening) buttercream
When you’re making buttercream you’re melting a lot of the fats in your butter. The buttercream that you end up with will be soft and silky. It will be pretty easy to pip in a piping bag or scoop however you want. However, once you’ve left your buttercream in the fridge for a few hours, you will notice it has hardened up considerably!
In the fridge, all those butter fats have solidified. It’s likely your buttercream is now too hard to pipe. If you’ve stored your buttercream in a bowl, you can solve this by putting it over a double boiler for a short while. You want to warm it up just enough for the butter to soften again, without melting all the fats (it might split) or further cooking the egg white proteins.
Warm it up slightly and then use your stand mixer or electric mixer to whip it all up consistently again. If it’s still too hard, re-heat gently and try again!
- 55g granulated (regular) sugar
- 2 egg whites
- pinch of salt (optional_
- 155g of unsalted butter (ideally, at room temperature and chopped in smaller pieces)
- 140g dark chocolate (use milk chocolate if you prefer sweeter and less chocolatey)
- Melt your chocolate, either using a double boiler or microwave (if using the latter, heat in short burst and stir in between to prevent it from burning). Leave to sit on the countertop to cool back down.
- Mix the sugar and egg whites in the bowl of a stand mixer (if you're using a handheld electric mixer, use a regular stainless steel bowl).
- Place the bowl on top of a pan with boiling water. Whisk the mix continuously, while measuring the temperature. Once the mix has reached 66C (150F), take the bowl off the heat and start whisking the mixture with your stand mixer (whisk attachment) or electric mixer at a high speed.
- The egg white mixture will turn whiter and increase in volume. Continue whisking until the mix has cooled down considerably, the bowl shouldn't be hot to the touch anymore but is still slightly warm.
- Add the butter in piece by piece. Don't add all the butter in at once, it makes it more likely to split. Continue whisking while adding the butter.
- Once all the butter is in and when the mixture has cooled down to just above room temperature, turn off the mixer.
- Take the still liquid, but cooled down, chocolate and fold it into the swiss meringue buttercream. You can use the stand mixer or electric mixer again for fully incorporating the chocolate.
- Take the buttercream from the bowl and either scoop onto whatever you're planning to decorate or use a piping bag to decorate.
Egg Safety Center, Cooking eggs to the right temperature, link ; provides recommendations on minimum cooking temperatures for various egg formats
Kimihiko MIZUTANI, Yong CHEN, Honami YAMASHITA, Masaaki HIROSE& Shigeo AIBARA (2006) Thermostabilization of Ovotransferrin by Anions for Pasteurization of Liquid Egg White, Bioscience, Biotechnology, and Biochemistry, 70:8, 1839-1845, DOI: 10.1271/bbb.60003, link
Klaus Roth, Proteins Present in Egg White — Part of the Boiled Eggs Article, 22 Feb 2012, Chemie in unserer Zeit/Wiley-VCH, link
B. Srilakshmi, Food Science, 2003, ISBN: 9788122414813, p.126, link