❄️ Freezing
Freezing a milk mixture: with stirring it becomes ice cream; in moulds without stirring, kulfi.
Freezing while stirring is the key step in making ice cream. A mix of milk, cream, sugar and stabilisers is first pasteurised by heating it above 80 °C, and then homogenised. Next it is kept cold, below 5 °C, for at least 4 hours. During this time the proteins and stabilisers swell, and the fat partly hardens.
Then the mix goes into a freezer, a chilled cylinder with scraper blades. Water freezes into crystals on the walls, the blades scrape them off at once, and the mix is stirred all the time and filled with air. This keeps the ice crystals small and the ice cream smooth. The volume grows as well: with 100% “overrun”, half of the ice cream is air. About half of the water freezes in the freezer. Finally the ice cream is “hardened” in tunnels with air at about −40 °C.
In the past, ice cream was churned by hand in a bucket of ice and salt: salt lowers the melting point of ice, so the mix gets colder. Hand-cranked ice cream makers were invented in the 1840s, and the first continuous freezer appeared in 1913. Indian kulfi is an exception: it is frozen in moulds without stirring, so it is denser.
How it connects to milk
❄️ Freezing → gives → 🍦 Ice cream → made from → 🥛 Milk
Links
- What it yields
- 🍦 Ice cream, 🍨 Kulfi
Frequently asked questions
Why is ice cream stirred while it freezes?
To keep the ice crystals small and to work air into the mix. Without stirring you would get a hard block of ice with large crystals.
How much air is in ice cream?
It depends on the type. With 100% “overrun”, the mix doubles in volume, so about half of the ice cream is air.