Having been around the block with the composition of ice cream over the last week, it’s worth asking at this point what type of substance ice cream actually is. Every so often I come across a summer newspaper article on ice cream where the author pronounces sagely that ice cream is classified by science as a colloid. Which is just about like saying: according to science, ice cream is a bunch of different stuff, all mixed up. Thank you, Mr. Tesla, but could you provide us with any more specific information?
To be fair, “colloid” is a more specific term than that. It basically means “one substance dispersed in another substance”. All colloids have two parts: there’s the dispersed substance (or medium or phase), and the continuous substance (or medium or phase). The dispersed substance is, as it’s name implies, that which is spread throughout the other substance, and the continuous substance is the stuff that it’s in. Clear?
Substances (broadly speaking) come in three basic forms: liquid, solid and gas. The dispersed and continuous phases of a colloid can be any one of those, and thus can be mixed and matched to make many different kinds of colloids. There are solid-in-liquid colloids (called sols of which paint is a good example), solid-in-gas colloids (an aerosol like smoke), gas-in-solid colloids (solid foams like styrofoam), and many others.
Liquid-in-liquid colloids are known as emulsions, which I yammer about quite a bit on this blog. French-style, custardy ice cream mixes are emulsions, but then so are the milk and cream that go into them in the first place. So are the egg yolks, for that matter. Gas-in-liquid colloids are called liquid foams, and ice cream is definitely one of those, since it’s got so much air in it. So then what’s the word for a colloid within a colloid within a colloid (emulsion in an emulsion in a foam)? I’m not sure there is one.
Still, I’ll go out on a limb and declare ice cream a “foam” since that’s the last stage in the ice cream making process. I know I’ve got at least a couple of real chemists out there in the crowd, anyone care to weigh in on this?