Vol. X · Emulsions, Foams and Gels Plate 10 of 11ice cream, freezing point, sugar, ice crystals, overrun, sorbet, scoopable, churning, water benchMH

Ice Cream.

At −18 °C, part of the water in good ice cream is still liquid. That unfrozen syrup is what lets a spoon in.

Ice cream is four things at once. Ice crystals, which should be small — under about 50 µm — so the tongue does not feel them. Fat droplets from cream, partly clumped by churning as in whipped cream (X.7), which hold the structure and feel rich. Air bubbles, beaten in as it churns: the overrun, 25–100 % of the mix's volume. And all of them held in a concentrated sugar syrup that never fully freezes.

That syrup is the key. As water freezes out as ice, the sugar left behind is more concentrated and its freezing point falls further (III.7): at any temperature, the mix settles at the share of ice whose remaining syrup freezes at exactly that temperature. More sugar means less ice and softer ice cream; too little and it sets rock-hard; too much and it will not firm up. Alcohol does the same even more strongly. Churning while freezing keeps the crystals small. The workshop freezes a mix.

Below: the share of the water frozen as ice against temperature for a mix with the sugar you choose and for leaner and sweeter mixes (water bench: freezing point of the remaining syrup from sucrose water activity), with the freezer and serving temperatures marked.

What you will learn
Workshop
Water bench: the unfrozen syrup's freezing point from the water activity of sucrose solutions (Norrish equation); the mix's sugars (sucrose plus lactose and others) counted as sucrose — a simplification that slightly overstates the ice. Water share of the mix 62 % unless changed by the sugar.

Sugar sets the scoop

A mix with 20 % sugars starts to freeze at about −1.9 °C; at −18 °C about 86 % of its water is ice in this model and 14 % is still a thick syrup, which is what lets a spoon in. A lean mix of 14 % sugars is 91 % ice at −18 °C and sets much harder; a sweet one of 26 %, 80 %, and stays soft. That is why home-made ice cream often needs ten minutes out of the freezer, and why recipes that cut the sugar come out rock-hard unless something else (glucose syrup, a spoon of alcohol, milk powder) takes its place.

Keep the crystals small: chill the mix thoroughly before churning, churn until thick, then harden it quickly in the coldest part of the freezer, and avoid thawing and refreezing, which lets small crystals melt and large ones grow — the coarse, icy texture of old ice cream. Sorbets, with no fat, depend on sugar alone (about 25–30 %); granita is deliberately icy, scraped as it freezes. Custard ice creams use egg yolk as an emulsifier (X.2) and must be cooked to at least 71 °C for safety (XVII.6).

At the stove

Freeze two small portions of the same sweetened cream, one with a tablespoon of extra sugar. After a night, try to scoop each straight from the freezer.

Before mechanical refrigeration, ice cream was churned in a bucket of ice and salt: the salt lowers the ice's melting point to about −20 °C (III.7), cold enough to freeze the sweetened mix. Chinese records describe chilled milk desserts in the Tang dynasty.

Terms

Overrun
The air whipped into ice cream as a percentage of the mix's volume.
Freezing-point depression
The lowering of the freezing point by dissolved sugar or salt.
Sorbet
A frozen, churned syrup of fruit and sugar with no dairy.
Hardening
Freezing churned ice cream quickly to firm it without growing large crystals.
Ice cream is ice, fat, air and syrup. Sugar sets how much water is ice at a temperature, and so how soft it is.

Sources

  1. R. S. Norrish, "An equation for the activity coefficients and equilibrium relative humidities of water in confectionery syrups", J. Food Technol. 1, 1966 (K = −6.47 for sucrose).
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.