Sugar, salt and alcohol lower the freezing point
Also searched as: freezing point, ice cream stays soft, salt on ice, why ice cream does not go hard
In short: Dissolved sugar, salt or alcohol get in the way of water as it freezes. It only freezes at lower temperatures, and part of it stays liquid.
Like a dance floor full of people: the more strangers standing in between, the harder it is for the couples to find each other.
Simplified model
- -21 °Cice with salt: this cold the brine can get
- -18 °Cfreezer
- -5 °Csweet ice cream stays scoopable
- 0 °Cpure water freezes
At 0 °C: pure water freezes.
What happens
Water molecules have to line up in a lattice to freeze. Dissolved particles get in the way, so the solution only freezes below 0 °C. In ice cream, part always stays liquid, so it stays scoopable. The other way round, salt melts ice: ice with salt turns into a brine that can get as cold as about minus 21 °C.
Why it matters
Explains why low-sugar ice cream turns rock hard, why a splash of alcohol keeps sorbet soft and how to make ice cream in a bag with ice and salt.
Rules of thumb
- Ice cream and sorbet need enough sugar, otherwise they turn hard.
- A tablespoon of spirits keeps sorbet softer; too much stops it from freezing.
- Ice with salt chills faster and deeper than ice alone.
Levers
In practice
Desserts and sweets
Sorbet made from very sweet fruit turns out softer by itself than one made from lemon.
- Harold McGee: On Food and Cooking (2004)