Using a process called overrun, air is blended into the mixture of ingredients until its volume increases by approximately 20%. By comparison, ice cream may have an overrun as large as 100%, meaning half of the final product is composed of air. The high percentage of butterfat and egg yolk gives frozen custard a thick, creamy texture and a smoother, softer consistency than ice cream. Frozen custard is most often served at –8°C (18°F), warmer than the –12°C (10°F) at which ice cream is served.
Another difference between frozen custard and ice cream is the way the custard is frozen. The mix enters a refrigerated tube and, as it freezes, blades scrape the product cream off the barrel walls. The now frozen custard is discharged directly into containers from which it can be served. The speed with which the product leaves the barrel minimizes the amount of air in the product but more importantly ensures that the ice crystals formed are very small.
Frozen custard is usually prepared fresh at the place of sale, rather than stored; however, it is occasionally available in supermarkets or by mail order. Generally, modern frozen custard stands provide only three different flavors per day: vanilla, chocolate, and a unique “flavor of the day.” Flavor-of-the-day calendars are usually made available by the store either in paper form or online. The older vintage custard stands tend to have a dozen or so standard flavors.
During the 1933 World's Fair, frozen custard was brought to Chicago. By the 1940s, frozen custard stands could be found throughout the East and Midwest of the United States.