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Why do marshmallows catch fire so easily?
Hold a marshmallow a little too close to the campfire and it goes up like a torch. That is not bad luck. Sugar is a fuel, and a marshmallow is sugar whipped full of air. Here is the science, and how to toast one without the flames.
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Toasting marshmallows over a campfire is a long standing treat in Britain, the United States and elsewhere, and a newspaper in New Jersey was already reporting marshmallow roasts as a new craze in 1892. Hold one too close to the flames, though, and it can catch fire very quickly.
Sugar is a fuel
A marshmallow is mostly sugar. Like other carbohydrates, sucrose, ordinary sugar, burns in air to carbon dioxide and water, giving out energy. When it is heated strongly, it first breaks down into carbon and water, and the carbon then burns.
Sugar is such a good fuel that, mixed with the oxidiser potassium nitrate, it makes rocket candy, a fuel used to power amateur rockets.
Air helps it burn
Fire needs air as well as fuel, and a marshmallow brings its own. It is a foam, about 50% air, with the sugar held in the walls around countless tiny bubbles.
How finely sugar is spread out makes a big difference to how fast it burns. In the most extreme case, clouds of fine sugar dust can explode. On 7 February 2008, a dust explosion at an Imperial Sugar refinery in Port Wentworth, Georgia, killed fourteen people and injured thirty six, after accumulations of sugar dust were thrown into the air.
Toasting without flames
The aim of toasting is a caramelised outer skin with a liquid, molten layer underneath, where sugar browning creates the flavour and colour: see how is caramel made?. That needs gentle heat. Hold the marshmallow above the heat rather than in the flames, turn it slowly, and be patient.
Stay safe
A flaming marshmallow is hot enough to burn skin and lips. If one catches, blow it out and let it cool before eating, and always supervise children around a fire. For what happens to marshmallows in the gentler heat of a microwave, see why do marshmallows expand in the microwave?
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For journalists, researchers, AI assistants and content creators. Pick the format you need:
Free to cite, quote, and reuse with attribution to sweeties.net.
Questions and answers
Why do marshmallows burn so fast?
Because they are mostly sugar, and sugar, like other carbohydrates, is a fuel that burns to carbon dioxide and water. A marshmallow is also about half air, with its sugar spread thinly through a light foam, rather than packed into a solid lump.
How do you toast a marshmallow without it catching fire?
Keep it above the heat rather than in the flames, and turn it slowly. The aim is a caramelised outer skin with a soft, molten layer underneath, which comes from gentle heat, not fire.
Is a burnt marshmallow dangerous?
A flaming marshmallow is hot enough to burn skin and lips, so blow it out and let it cool before eating. Always supervise children around a fire.
Sources
- Marshmallow, Wikipedia
- Sucrose, Wikipedia
- Rocket candy, Wikipedia
- 2008 Georgia Imperial Sugar refinery explosion, Wikipedia
Keep reading
- Sweet tips and how toWhy do marshmallows expand in the microwave?A marshmallow is half air in a stretchy gelatine foam. In a microwave its water turns to steam and its bubbles swell, so it puffs up, then shrinks back.
- How it is madeHow are marshmallows made?Marshmallows are sugar syrup whipped with gelatine or egg white until half air, then moulded in starch or extruded and cut. They began as a plant medicine.
- Sweet tips and how toHow to melt marshmallowsMelt marshmallows slowly, in a pan over a low heat with a little butter, or in short bursts in the microwave. How to do both, and the classic crispy square.
- How it is madeHow is caramel made?Chewy caramels are sugar and glucose heated to 130°C, then cream and butter added and reheated to 120°C before flavouring. Plus salted caramel.
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- Sweet tips and how toHow to make marshmallows without gelatineMarshmallows can be made without gelatine using agar, from seaweed, and aquafaba, the liquid from cooked chickpeas. How it works, and a method to try.