Marshmallow Expansion in Microwave
Name: Colleen F.
Why do marshmallows puff up when heated in a microwave
instead of melting like other candy?
Marshmallows expand because they are made of foamed sugar. Microwaves heat the
water in the foam and the trapped air causes the marshmallow to swell up. As soon
as the microwaves are turned off, the foam collapses into a gooey puddle when the
expanded air no longer feels the heating effect of the water.
It's a race against "time". Marshmallows are a foam of air entrapped in
sugar and other ingredients. When you put the marshmallow in the microwave
two things happen as it heats up. The trapped air expands with increasing
temperature and at the same time the marshmallow matrix begins to soften.
Since you observe expansion, heating the entrapped air is winning. Should
you keep heating however the marshmallow will probably eventually collapse.
Because marshmallows are mostly air, which is composed of gases. Gases
expand when heated. If you left the marshmallows in the microwave long
they probably would melt.
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Update: June 2012