Marshmallows Under Vacuum: Watching Trapped Air Win
A marshmallow is mostly air held in a sugar matrix. Pull the air out of the room above it and the trapped pockets expand, doubling the sweet's volume before it collapses into a wrinkled shell.
Materials
| Item | Amount | If you do not have it |
|---|---|---|
| Marshmallows | 6, fresh and soft | Stale marshmallows work but expand less and crack |
| Vacuum pump or large syringe | 1, a 100 ml syringe with the nozzle capped works | A wine saver pump and a rigid container |
| Rigid clear container | 1, with a lid you can seal | A strong plastic jar; a thin-walled bottle will implode |
| Shaving foam | a walnut-sized blob, optional | Whipped cream, which expands even more dramatically |
| Grapes or a sealed crisp packet | 1 of each, optional | A balloon with a little air in it |
| Kitchen scale | 1, accurate to 0.1 g | None; the mass comparison is optional |
Method
Weigh one marshmallow and note the mass. Put two in the container and seal it.
Watch for: A standard marshmallow is about 7 g and roughly 4.5 cm across.
Pull the pump slowly. Watch the marshmallows as the pressure drops.
Watch for: They swell smoothly and reach about twice their starting diameter before anything else happens.
Keep pumping past the point where they stop growing.
Watch for: The surface goes from smooth to wrinkled as the outer skin reaches its stretch limit, and a few cracks appear.
Open the valve and let air back in quickly.
Watch for: They collapse inward within about a second and end up smaller and denser than they started, with a dimpled crust.
Weigh the collapsed marshmallow and compare with the starting mass.
Watch for: The mass is unchanged to within 0.1 g, which proves the change was entirely volume, not material.
Why it works
A marshmallow is an aerated sugar foam: roughly 50 percent air by volume, held in a matrix of sucrose, glucose syrup and gelatin. The air pockets inside are at atmospheric pressure, about 101 kilopascals. When the pressure in the container drops, the pressure inside each bubble is suddenly higher than outside it, so the bubble expands until the elastic skin of the sugar matrix resists. The matrix behaves like a rubber membrane: stretch it a little and it springs back, stretch it past its limit and it yields and then stops recovering. When you let the air back in, the bubbles cannot re-absorb the volume they lost because the matrix has been stretched into a new shape, which is why the marshmallow comes out smaller rather than returning to its original dimensions.
Think of it like: It is a balloon in a bell jar. Let air out of the jar and the balloon inflates by comparison. Let air back in and the balloon shrinks to what it was -- unless you stretched it so far that the rubber will never lie flat again.
Clean-up and disposal
All the food items can be eaten, though vacuum-treated marshmallows are dense and chewy rather than soft. Do not put vacuum-treated marshmallows in a microwave, because the trapped air and the sugar can heat unevenly and spit. Wipe the container with a damp cloth and let it dry fully before sealing it again.
Bench notes
What makes a good sample
Freshness dominates. A fresh marshmallow has a soft, extensible matrix and stretches to about twice its diameter. A stale one has a matrix that has already partly crystallised and it cracks at about 1.4 times.
Observed / 实测Fresh marshmallows at 25 kPa container pressure reach 2.0 to 2.2 times their diameter. Stale ones reach 1.3 to 1.5 times and lose pieces.
Reading the pressure
If your pump has a gauge, note the reading at which each change happens. Swelling starts almost immediately, wrinkling begins around 40 kPa and cracking around 20 kPa for a standard marshmallow.
Observed / 实测The whole sequence from atmospheric to 20 kPa takes about 25 seconds with a 100 ml syringe on a 500 ml container.
Weak vessels and strong sugar
Variations worth running
- Run the same test with a blob of shaving foam and with a spoonful of whipped cream, and rank them by how much volume they gain.
- Put a sealed crisp packet in the container: the bag inflates rather than the contents, which shows the effect works on any trapped gas.
- Take a marshmallow out still swollen, put it straight in a microwave for ten seconds, and watch it expand again by a different mechanism.
Questions that come up on the bench
- Why does it not go back to its original size?
- The sugar and gelatin matrix has been stretched beyond its elastic limit and rearranged. When the pressure returns, the bubbles compress but the walls cannot re-inflate to their old shape, so the whole thing ends up denser than it started.
- Does the marshmallow lose any mass?
- No, not measurably over a few minutes. The change is volume only. If you leave a swollen marshmallow in the vacuum for an hour it does lose a little water, but that is evaporation rather than the pressure effect.
- Why does the container need to be rigid?
- A flexible container collapses inward as you pump, so the pressure inside never drops much. The effect depends entirely on the pressure difference between the bubble interior and the container atmosphere.