The Science of Marshmallows (and a Recipe)

Whether you want to roast marshmallows, dunk some marshmallows in a warm cup of chocolate milk, or just want to eat marshmallows; you might just want to know what marshmallows are, how they’re made and why marshmallows are made the way they are. If you’re even more ambitious, you might even be considering those marshmallows for roasting, dunking or eating! And when making marshmallows, it really helps if you understand what the role is of all those ingredients and steps.

Therefore, this post is completely dedicated to marshmallow science, how they’re made (when made at home) and why they’re made that way. Only interested in a recipe? No worries, scroll down to the bottom of this post.

What is a marshmallow?

From a scientists perspective a marshmallow is a foam. More precisely, it is a foam with air bubbles, stabilized by sugars and proteins (egg & gelatin). The sugar & protein mix is strong enough to keep the bubbles apart and prevent the air from escaping, but soft enough to create that gooey marshmallow texture.

The sketch on the right side is a very simplified sketch of a marshmallow. We’ll have a look at each of these three to see how they make the marshmallow. But before we dive in, a quick reminder of how marshmallows are made:

1. A sugar syrup is made by boiling water & sugar
2. Egg whites are whipped up with a little sugar to make a foam
3. The sugar syrup is poured onto the egg foam
4. Dissolved gelatin is added at the end

The sugar syrup in marshmallow

Marshmallows contain quite a bit of sugar (just have a look on your marshmallow pack). The sugar is essential for creating structure in marshmallow. Without it, the marshmallow wouldn’t be stable (nor would it taste like anything).

Sugar in a marshmallow is added in the form of a syrup, a mix of water and sugar. This syrup is slightly thicker than normal water, making it less runny. This makes it a little harder for air bubbles to travel through the syrup (and thus leave the marshmallow). In a lot of other foams this characteristic of sugar is used, for instance in Italian meringue or a crispy baked meringue.

So sugar prevents water and air from leaking out too easily, but it has another important role: sugar syrup introduces heat into the marshmallow. A sugar syrup is always hot when added. This heat is required to ‘cook’ the marshmallow. Without this ‘cook’ it wouldn’t be as strong because the egg proteins wouldn’t have firmed up. But we’ll come back to that when discussing the role of egg proteins!

Boiling a sugar syrup

In marshmallow recipes you tend to find quite precise instructions on how to boil the sugar syrup. Best are those which mention a temperature the sugar syrup has to reach. Why’s that?

Since we want to get the right amount of water in the marshmallow and enough heat, it is important to boil the sugar syrup to the correct temperature. When boiling a sugar syrup water will evaporate and concentrate the syrup. The higher the temperature, the less water is left. The boiling temperature will tell exactly how much water is still left. Since measuring temperature is quite easy, it is a great measure to test whether enough water has been boiled off. If too much water is present, the marshmallow will be too liquid, if not enough is there, it will be rock hard!

Type of sugar in marshmallows

Homemade marshmallow recipes tend to use both granulated sugar and glucose syrup (or another sugar syrup type). There is a good reason for not only using granulated sugar.

Granulated sugar is pure sucrose. When a sugar syrup with only sucrose and water cools down only a small disturbance is enough to recrystallize this sugar. Glucose syrup on the other hand contains some large carbohydrates besides the glucose itself. These large molecules will prevent the sugar from recrystallizing. As a result you have a smoother syrup and it’s easier to make.

Since glucose syrup by itself can be very sticky and gooey, often a combination is used.

Egg in marshmallows

A sugar syrup itself cannot form a foam. In order to make that foam, we need something that can ‘catch’ air bubbles until we add the sugar to help them stabilize them. As we’ve discussed before, egg proteins are very good in this. If you whip up an egg white it will form a foam by itself. This is because of the proteins in the egg white. These proteins get unravelled by the whisking and the sections of proteins that like water will sit around water, whereas the air loving sections sit next to the air. This keeps the air bubbles at bay for a while, but not permanently.

For making these foams we do not want to use the fats that sit in the egg yolk. These fats will destabilize the foam only. So we use egg whites to start the foam we need for marshmallows.

However, the egg white in marshmallows don’t just create the foam, they also help stabilizing it once the hot sugar syrup is poored in. Most proteins will denature when heated up to temperatures above approx. 70°C (which temperature exactly depends on the protein). When egg proteins denature they will thicken and set (think of boiling or baking an egg!). So, when the hot sugar syrup is added to the egg protein foam this is exactly what will happen. The egg protein will set and this will stabilize the foam the egg has just made. The set proteins will make it a lot harder for air to escape, or for water to move around. If egg proteins aren’t heated, the foam that was made will collapse by itself.

Last but not least: Gelatin & marshmallows

Even though egg proteins stabilize a foam, they are not strong enough for a long enough period. Just using egg whites is enough when making an Italian meringue, but not when making marshmallows that have to be stored for a long period.

This is where the gelatin comes is. Gelatin molecules are also proteins, but they are a little stronger when it comes to gelling. They can form very strong gels. By adding the gelatin the marshmallow will stay stable for a long period of time. In a previous post we’ve discussed gelatin and how it works in more detail.

Making marshmallows – the recipe

Now that you know what all the ingredients do in a marshmallow, it’s time to make marshmallow. In the scheme below you will find an illustration of all the different steps.

Marshmallow science explained

Prep Time: 30 minutes

Cook Time: 30 minutes

Total Time: 1 hour

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Ingredients

Sugar syrup

• 150ml water
• 250g sugar (regular, granulated)
• 150g glucose syrup

Foam

• 1 tbsp sugar
• 2 egg whites
• 10g gelatin (take care though, the amount strongly depends on the type of gelatin you're using, you might have to experiment a little; we've used gelatin sheets, not powder)
• 100g cornstarch
• 100g icing sugar

Instructions

1. If your gelatin requires to be pre-soaked in cold water, do so at the start.
2. Place the water, sugar and glucose syrup in a pan and bring to the boil. Boil until the liquid has reached 120 degrees Celsius (= 248 F).
3. In the meantime, whisk up two egg whites until you've got a stable foam (easiest to do this in a stand mixer). Add the 1 tbsp of sugar and whip up again until a strong foam has formed. The sugar should allow the foam to become a little larger and more airy.
4. Once the syrup has reached the required temperature, slowly pour it onto the egg white foam, while continuously whipping (you should keep on whisking, best to use a stand mixer for this). Keep on whipping the mixture until the bowl has cooled down to 'hand temperature'.
5. In the meantime, prep the gelatin. Take the pre-soaked gelatin out of the water and squeeze off the excess water. Place the gelatin in a small pot on a low fire and slightly heat the gelatin. You will see the gelatin dissolve completely. Take from the heat as soon as it has completely dissolved.
6. Add the dissolved gelatin to the bowl, whip for a little longer before spreading out on parchment paper.
7. Mix the icing sugar & cornstarch and immediately coat the freshly spread out marshmallow. Add enough to coat all sides.
8. Cut into pieces once cooled down slightly more and immediately coat the sides with the starch+icing sugar mixture.

Making marshmallows – tips to get started

A first warning, if you want to make marshmallows yourself, it really really helps if you have a stand mixer. If you don’t, get someone to help you since you’ll have to pour hot syrup and mix at the same time.

Also, marshmallow tends to stick to just about everything, it’s a sugar syrup so be prepared. Make sure you have plenty baking paper (I don’t like using plastic foil, it tends to stick more) and a mixture of corn starch with icing sugar (use a ratio you like, 50/50 is good to start, if you want it less sweet, increase the portion of corn starch. Use the baking paper to spread on the marshmallow and immediately coat the marshmallow with that mix of starch and sugar. That will prevent it from sticking to everything. Once they’re cut into smaller pieces, again, immediately coat!

Once you’re got this ready, you’re all set!

Quick update here, I’ve also tried making marshmallows using marshmallow root instead of gelatin, so real marshmallow marshmallows :-)!