1. What Marshmallows Are: Sweet Foam Made of Sugar, Bubbles and Gelatin
A marshmallow is a sweet made by supporting bubbles of water and air with a base of sugar and corn syrup, then stabilizing them with gelatin [3]. Even in home recipes, the method involves whipping gelatin, egg whites, and sugar, then cooling them to harden [2].
The average size of these bubbles is reported to be about 20 to 25 micrometers [3]. Since this is about 0.02 millimeters, the bubbles are roughly 1/40th to 1/50th the size of a single millimeter.
The marshmallow is lightened by whipping air into it [3], and some explanations state that more than half of its volume is air [4]. For our purposes here, we will accept that it is "about half" air.
According to food composition tables, the water content in 100 grams of marshmallow is about 18.5 grams [1]. This means water makes up less than 20 percent, revealing that it is surprisingly low in moisture.
2. How a Microwave Oven Heats the Water in a Marshmallow
A microwave oven emits electromagnetic waves (microwaves) with a frequency of 2.45 gigahertz. The energy from these waves is easily absorbed by water and turns into heat [5].
As noted earlier, the water content of a marshmallow is less than 20 percent [1]. This water heats up, and the heat then transfers to the surrounding sugar and air. This process leads to the next stage of expansion.
3. How Water Vapor and Warm Air Push the Bubbles Outward
Some of the water inside the marshmallow evaporates, adding gas (water vapor) into the bubbles. Additionally, the warmed gas pushes outward more strongly [6]. It is explained that the warm water vapor also heats the sugar [7].
There is also the concept of thermal expansion of air. Gases try to expand when their temperature rises (Charles's Law) [6]. The air inside the bubbles behaves the same way. Combined with the increase in water vapor, the bubbles are pushed outward.
Both the increase in water vapor and the warming of the air work together to expand the bubbles.
4. How Softened Sugar Lets the Bubbles Stretch
Even if the bubbles try to expand, they will not stretch much if the surrounding sugar remains hard. When the sugar is heated and becomes slightly soft, the walls stretch along with the expansion of the bubbles [7].
A useful guide for this is the "glass transition." Sugar mixtures have a temperature at which they change from a hard, glass-like state to a soft state (glass transition temperature). In a study measuring marshmallows containing gelatin, the temperature at which this change finishes was reported to be around 57°C (about 135°F) [9]. This value comes from the paper's summary, and the detailed figures were not verified in the main text. However, this is a value for a specific research formulation and may not match the temperature of store-bought marshmallows.
5. Why Marshmallows Shrink After Cooling, and Why Explanations Differ
When taken out and cooled, the bubbles shrink, and the sugar becomes hard again [7]. There is also an explanation that marshmallows heated in a microwave lose moisture and become crisp and hard [7].
However, descriptions in sources differ. Another source states that the enlarged marshmallow shrinks when cool and becomes a sticky lump [8]. Because the result can change depending on the heating time and conditions, whether it ends up hard or sticky cannot be decided here.
6. How Air Pressure Also Makes Marshmallow Bubbles Expand
If you put a marshmallow in a bottle and pump the air out, the air inside the marshmallow expands, and the marshmallow grows larger. When the air is returned, it goes back to its original size [8].
This is another example showing that the air inside bubbles expands if the surrounding air pressure drops, even without raising the temperature. In the case of a microwave, it is thought that the rise in temperature and the increase in water vapor happen together.
7. How to Observe Marshmallow Bubbles Yourself
What you can do today: Tear a marshmallow with your hands and observe if you can see small holes in the cross-section, and see how it returns when pressed with your finger. If you want to see how it expands, an adult must operate the microwave. Keep the heating time short, and an adult must watch closely during heating. Do not let children do this alone. Heated sugar becomes very hot, so do not touch it until it has cooled sufficiently.
If you want to investigate further: You can check the water content in food composition tables, or if you have different types of marshmallows, you can tear them apart and compare the cross-sections. For Japanese readers, the explanation of microwave ovens by the Electromagnetic Field Information Center (Source [5]) is easy to read.
Safety note: If you want to observe the marshmallow after heating, an adult must operate the microwave and heat it for only a short time. An adult must watch closely during heating. Wait until it has cooled sufficiently before doing the same observation, and compare it to the state before heating.