Ice cream consists of three basic elements: ice crystals made of pure water, the concentrated cream that the crystals leave behind as they form from the prepared mix, and tiny air cells formed as the mix is churned during the freezing.
- The ice crystals form from water molecules as the mix freezes, and give ice cream its solidity; they’re its backbone. And their size determines whether it is fine and smooth or coarse and grainy. But they account for only a fraction of its volume.
- The concentrated cream is what is left of the mix when the ice crystals form. Thanks to all the dissolved sugar, about a fifth of the water in the mix remains unfrozen even at 0°F/–18°C. The result is a very thick fluid that’s about equal portions of liquid water, milk fat, milk proteins, and sugar. This fluid coats each of the many millions of ice crystals, and sticks them together—but not too strongly.
- Air cells are trapped in the ice cream mix when it’s agitated during the freezing. They interrupt and weaken the matrix of ice crystals and cream, making that matrix lighter and easier to scoop and bite into. The air cells inflate the volume of the ice cream over the volume of the original mix. The increase is called overrun, and in a fluffy ice cream can be as much as 100%: that is, the final ice cream volume is half mix and half air. The lower the overrun, the denser the ice cream.