Roasting a marshmallow over a barbecue

Science of roasting marshmallows – Over a campfire or barbecue

Want to sound smart during your next barbecue or outdoor outing? Tell your fellow roasters what happens when roasting those marshmallow you’re making over the campfire or barbecue! Don’t know what happens?! Good thing we’ve written this post on exactly that topic :-).

Even though not everyone will admit, roasting marshmallows is just as must fun for adults as it is for kids. Somehow placing those marshmallows on a stick, slowly turning them around in the heat, seeing them brown slowly and taking them out of the heat just before they burn is a great way to end a barbecue (unless you’re more of a cookie person, in that case you should end with a barbecue cookie!). While you relax, a lot of chemistry is going on right in front of your eyes though.

What happens to a marshmallow above the fire?

Let’s start by listing what we can see happening:

  1. At first nothings seems to happen with the marshmallow. It just sits on a stick.
  2. But then, all of a sudden, if you don’t watch out, things start moving quickly. It starts with the marshmallow browning slightly.
  3. Depending on your marshmallow type it might also get softer at this point, worst case: leaking off the stick!
  4. Keep on turning at that point because browning will move quickly and if not turned quick enough, it might get black on one side!
Roasting a marshmallow over a barbecue - start
The marshmallows have just started roasting over the fire.

What is a marshmallow?

In order to explain why and how all of this happens you need to know what a marshmallow actually is (we also have an in-depth marshmallow science version). Every recipe and product is different, but the basis of a marshmallow is:

  • Sugar: marshmallows cannot be made without sugar, sugar forms the basic structure of a marshmallow.
  • Egg whites: although not all commercial marshmallows still contain egg whites (it is more efficient without), egg whites are often used to create that foamy texture we’re looking for in marshmallow
  • Gelatin: or another gelling agent which will stabilize the marshmallow (in the olden days marshmallow root was used, hence the name)
  • Water: don’t forget the water, it’s the water that keeps everything together

Now that we’ve got that refreshed, let’s go back to that campfire and see what happens to these four components above the fire.

Science of browning in roasting marshmallows

Once you’ve rotated your marshmallow around for a couple of times, you will notice that is starts to brown slightly. As with a lot of browning processes in food, the browning of marshmallows is caused by two processes:

  1. Caramelization
  2. Maillard reaction

Caramelization occurs when sugar is heated to very high temperatures, well above 100°C. The high heat caused the sugar to melt and subsequently brown. The Maillard reaction on the other hand can occur at far lower temperatures and is a browning reaction caused by a reaction between sugars and proteins. Both reactions are chemically very complex, so we won’t go into further detail there.

So why doesn’t browning start immediately? That’s because the marshmallow has to heat up first. Also, it will need to lose some moisture on the outside. This moisture loss is necessary since both browning reactions only occur slowly when it’s too moist. Also, if there’s too much moisture instead of heating up the marshmallow further, you’re just using that energy to evaporate water.

Melting of roasted marshmallows

If at some point now you stop turning the marshmallow and look away, you might find your marshmallow falling off your stick.

So why do some marshmallows actually melt or at least soften during roasting? This is because of the gelatin that becomes softer as it warms up. Gelatin is a gelling agent which means it can form a gel. However, the gelatin gel is very heat dependent. Gelatin, which is a protein, will melt again at higher temperatures. Other gelling agents, such as agar agar, do not have this heat reversibility.

The gelatin helps holding the marshmallow together. Once the gelatin melts, the sugar syrup and egg proteins will not be strong enough enough to keep the foam up, the air bubbles will start escaping.

Also, sugar dissolves better in its liquids at higher temperature. As a result, if there’s some undissolved sugar, it will dissolve again when it comes warm, making everything go more runny.

Roasting a perfect marshmallow

So knowing all this, how should we make that perfect roasted marshmallow?

First of all, do not try to roast marshmallows over a very high heat. Since marhmallows contain so much sugar, the browning can go super quick. A high heat will probably burn the marshmallow before you have time to rotate it well.

Also, don’t place the marshmallow too close to the fire! For the exact same reason as mentioned above, the marshmallow will burn before it will brown nicely.

Keep on turning that marshmallow. You want an evenly brown marshmallow and don’t want the marshmallow to leak of your stick.

Roasting marshmallows over a campfire - ready
Almost ready!

The importance of orientation on the stick

Last but not least, let’s quickly discuss the best way to place those marshmallows on a stick! As you can see in the photos in this post we’ve oriented the marshmallows in two ways: in the direction of the stick and perpendicular to the stick.

Those positioned in the direction of the stick worked a lot better. We could heast just about all surfaces evenly. However, those sitting on the stick in a perpendicular manner had a lot of area that could not be heated well. As a result, we had some more white spaces and a little less brown crunch.

Have fun

And most of all, remember to have fun when you’re roasting marshmallows. It’s great science happening in front of you, so whether you want to impress your friends, teach your kids or just want to enjoy a melty brown marshmallow, enjoy!

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