Air is an ingredient
Ice cream is a foam: tiny air bubbles, ice crystals and fat droplets held in a thick sugar syrup. The air whipped in while the mix freezes makes it soft and light; without it, ice cream would be a hard, icy block. The amount of air is measured by the overrun, the percentage increase in volume over the mix.
The formulas
By volume: overrun = (volume of ice cream − volume of mix) ÷ volume of mix × 100
By weight, more practical at home: fill the same cup first with mix, then with ice cream, and weigh both:
Overrun = (weight of mix − weight of ice cream) ÷ weight of ice cream × 100
The share of air in the finished ice cream is overrun ÷ (100 + overrun).
Example
A cup holds 250 g of mix and 167 g of freshly churned ice cream:
- Overrun: (250 − 167) ÷ 167 = 50%
- Air: 33% of the volume
- One quart of mix gives about 1.5 quarts of ice cream
Typical overrun
- Gelato: about 20 to 35%, churned slowly, dense and served warmer
- Homemade and premium ice cream: 25 to 50%, rich and creamy
- Soft serve: about 30 to 60%
- Standard commercial ice cream: 50 to 100%
US federal rules require ice cream to weigh at least 4.5 pounds per gallon, which effectively caps overrun at about 100%. Comparing the weight of two tubs of the same size is a quick way to see which contains more air.
What affects overrun
- Machine: home machines with slow paddles reach 20 to 40%; commercial continuous freezers can inject air under pressure.
- Churning time: longer churning adds air, up to a point.
- Recipe: milk proteins, fat, egg yolks and stabilizers help hold air bubbles; too much fat churned too long turns to butter.
- Temperature: a well-chilled, aged mix whips better.
Measure the overrun of each batch to adjust your recipe and compare results consistently.