roasted marshmallow

What Happens When Roasting Marshmallows?

You may not want to admit it, but isn’t roasting marshmallows just as must fun for adults as it is for kids. Isn’t it delightful to see the marshmallows shiver, sweat, turn brown, hopefully without burning or falling in the fire!?

The heat transforms a soft white pillow candy into a super soft, almost liquid, brown snack with a thin crunchy outer layer. We have the Maillard reaction, and the special properties of gelatin to thank for these delightful changes.

Let’s roast a marshmallow

If you’re close to a fire, take a marshmallow and roast it. What do you see? Probably something along the following lines:

  1. At first, nothing seems to happen. The marshmallow just sits on a stick above the heat;
  2. But then, things start moving quickly. It starts with the marshmallow browning slightly;
  3. It will also get softer, at some point almost falling off the roasting stick;
  4. Some smoke may come from the marshmallow;
  5. Maybe your marshmallow bubbles a little;
  6. Until it’s turned a nice even brown color, with an ever so slightly crispy outside, ready to be devoured.

In short, your marshmallow turns soft, brown, and crunchy, and may bubble and steam.

Roasting a marshmallow over a barbecue - start
Roasting marshmallows over hot coals.

Maillard reactions turn marshmallows brown

So why does all of that happen? Let’s have a look at the color changes of a marshmallow first. If you look at the pack of your marshmallow or your own marshmallow recipe, you’ll probably see at less the following ingredients:

Gelatin is what makes a marshmallow bouncy (learn more here). Both gelatin and egg whites make the marshmallow so light and airy. They also both contain a lot of proteins, a group of common molecules in food. It so happens that if you heat proteins in the presence of the right types of sugars, they’ll react.

This reaction is called the Maillard reaction and it’s a very common chemical reaction in food. The warmer the food, the more likely the Maillard reaction is to occur.

The rate of the Maillard reaction also depends on the amount of moisture present. Too much moisture slows down the reaction. It’s why boiling potatoes in a pot of water won’t turn them brown. But place them in an oven at the same temperature, and they will turn brown, be it slowly.

When the marshmallow is hot and dry enough, it starts to brown. Once it has started, the reaction can proceed very rapidly.

lemon meringue tart cut in two
Italian meringue contains very similar ingredients to marshmallows. Hence, it browns easily as well if you blow torch it. Notice how the outside has also dried out a little?

What about caramelization?

Another way for marshmallows to turn brown is through caramelization. Caramelization only requires sugars to occur, it doesn’t need the proteins. However, caramelization only starts at very high temperatures. As such, when proteins are present, most of the browning takes place because of the Maillard reaction.

Melting gelatin makes them gooey

Browning a marshmallow evenly is easy at the start, but becomes harder as you go. At some point the marshmallow turns so soft, that it’s at risk of actually falling from the stick. Only proper roasting skills will keep it on.

To explain, we again have to look at the gelatin. At room temperature it’s the gelatin that ensures the marshmallow holds its shape. Gelatin does so by forming a gel. Its protein molecules hold onto water and form a network that makes it harder for water molecules to escape. This gel however is what scientists would refer to as thermo-reversible. That is, it can be broken down by warming up the gel, and re-formed by cooling it down again.

So, as you heat your marshmallow the gelatin starts to melt and lose its hold of the water molecules within. As a result, the marshmallow becomes softer and gooey.

This thermo-reversibility is one of the things that sets gelatin apart from many other gelling agents. If you’re roastig marshmallows made with agar-agar for instance, you’ll not see this effect.

Hot sugar syrups help the gooeyness

Even though gelatin is the main culprit when it comes to melting marshmallows, sugar plays a role as well. Marshmallows are made with concentrated sugar syrups. These syrups help to thicken and stabilize the marshmallow as well. At room temperature, the syrup is very thick and viscous. However, at higher temperatures, the syrup will become a lot thinner and more fluid. Softening the marshmallow.

Newsletter

Want to be updated on new food science articles? Subscribe to our weekly newsletter

Evaporating moisture makes them crunchy

A well-roasted marshmallow seems to have formed a skin on the outside. It may even be a little crunchy. We have the evaporation of moisture to thank for this. During roasting, the marshmallows become quite warm. So much so that water, which boils at 100°C (212°F), starts to evaporate quite quickly. Foods can become crunchy when they no longer contain a lot of water, which we discussed in great detail when analyzing crispy chicken skin.

That smoke and those bubbles are probably water

As long as you’re not burning your marshmallow, the ‘smoke’ that comes from your marshmallow is mostly made up of evaporating water. And the bubbles that appear on the surface of your marshmallow? Evaporating water as well. The bubbles form when the water vapor can’t escape the marshmallow well. It blows up parts of the marshmallow until the bubbles burst and the vapor is set free.

Roasting a perfect marshmallow

So let’s translate these phenomena into practical advice for roasting a perfect marshmallow. It starts by balancing time & temperature. You don’t want to wait hours for the marshmallow to roast, nor do you want it to roast too quickly. Once that Maillard reaction is going, it’s easy to go too fast and burn the marshmallow.

Assuming you’re roasting your marshmallow above a fire, that means you’ll have to optimize two factors:

  1. Temperature of the fire itself; it’s almost impossible to roast a marshmallow above a blazing hot fire. The chemical reactions simply proceed too quickly.
  2. Distance to the fire; the further away from the heat source, the colder it will be. Find a spot that doesn’t burn the marshmallow, while not taking forever either. And remember, whereas it may take longer to get started, once that marshmallow is warm, things happen rapidly. So you may start a little closer to the fire than where you end.
  3. Type of fire; last but not least. It’s very hard to roast a marshmallow in dancing flames. They aren’t just very hot, they also move all the time. Making it hard to sit at a fixed distance. Instead, opt for roasting above some still hot coals, or kindling wood. They still provide more than enough heat, but provide a more stable heat source.

Rotating for an even heat

Much as how you’d continuously rotate a rotisserie chicken, you do have to heat your marshmallow if you want all sides cooked evenly. It’s a matter of distributing heat evenly. The temperature difference between the fire and non-fire sides of a marshmallow is pretty big!

Also, don’t stack your marshmallows too closely. They won’t turn brown where they touch.

Roasting marshmallows over a campfire - ready
Almost ready!

Have fun

Roasting marshmallows is a great moment for a good old food science experiment. Vary the distance to the fire, test different types of marshmallows, or rotating speeds. There’s so much you can experiment with. All good reasons to roast plenty of marshmallows! And most of all, remember to have fun when you’re roasting marshmallows (even if you don’t want to admit it’s fun).

Newsletter Updates

Enter your email address below to subscribe to our weekly newsletter

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.