The Igneous Petrology of Ice Cream
It was hot out last weekend. Some of you might scoff at what I consider “hot”, but the glorious thing about Seattle is that the entire city seems willing to join me in whining and wilting whenever the temperature breaks 80 (that’s 25 of your Earth units). Naturally, I spent a lot of time thinking about ice cream.
Ice cream is an igneous rock. You begin with a liquid slurry containing a hodgepodge of chemicals, and by bringing it below its freezing point, you create something solid – or at least solid-ish. Good ice cream or sorbet needs a little give, a bit of liquid remaining between ice crystals so that you can comfortably dig into it with a spoon. This is what it looks like:
From Clarke, 2003, “The Physics of Ice Cream” Physics Education 38 (3)
Compare that to a thin section of glassy lava from the Pacific Northwest:
Much like igneous rocks, the same liquid mix can turn out very differently depending on what happens while it is freezing. The goal of most ice cream and sorbet is to have a smooth and creamy texture, which would be ruined by the presence of large ice crystals. To achieve this, you want to cool your ice cream so quickly that the crystals don’t have time to grow. However, there’s a lot of energy involved in the transition from liquid to solid water, and a home ice cream maker can’t do the heat transfer quickly enough to keep the ice crystals small. The alternative approach is to keep the mixture stirred while it freezes, so that you can avoid growing giant crystals where it’s coldest. So you have to sit there and turn the crank until your arm is sore (or invest in a fancier machine that will do the stirring for you).
Or, you could acquire some liquid nitrogen – pouring a -321° liquid (that’s a mere 77 Earth units above absolute zero) into your ice cream mix will freeze it so quickly that stirring is neither difficult, nor tremendously important. The crystals will be tiny no matter what. This method has the added advantage of being able to freeze mixtures whose melting point is below what you can get with home freezers and rock salt, enabling such monstrosities as a silky-smooth 80 proof rum raisin.
The difference between ice cream and sorbet is that ice cream contains cream and sometimes eggs or other emulsifying agents, while sorbet is just fruit, water, and sugar. The presence of fat and emulsifiers in ice cream provides another way to control the crystal structure (and another way to ruin the texture, if you let the fat globules get too big) but there’s no direct analog for this in igneous petrology.
Occasionally, you do want a dessert with a sharper texture to it, or maybe you’re just too lazy to stir very often. That’s how you make a granita: Leave the sugar slurry in the freezer, so that the crystals have time to grow, but interrupt the process every half-hour or so, so that they don’t get too big. You end up with a slushie or crystal mush.
Granite… or granita? Magma that is left to cool slowly underground will also produce large crystals.
Coming tomorrow: The metamorphic petrology of ice cream, or, why I need to stop cooling off by staring absent-mindedly into the freezer.