Frozen Custard is basically a premium ice cream. Even though it uses more milk and less cream than regular ice cream, because egg yolks are also added, the Frozen Custard ends up smoother and thicker than regular ice cream. The eggs that are used are pasteurized first.
The United States Department of Agriculture (USDA) actually has a legal definition of Frozen Custard. It must contain 10% butterfat, and 1.4% egg yolk solids. If ingredients such as fruit and nuts are added, the egg yolk solid content can be reduced to 1.2 %.
Frozen Custard has less air (what the industry calls “overrun” ) than ice cream, so it is heavier — by volume, an America ½ gallon of frozen custard will weigh more than an American ½ gallon of ice cream. Frozen Custard will also hold its shape better. It can be served at 25/26 degrees F (-3 C), while ice cream is best served about 10 degrees F (-12 C.)
Interestingly, or confusingly, French Vanilla Ice Cream has the same legal requirements as for Frozen Custard. Depending on the manufacturer, the French Vanilla Ice Cream may be the same as what another manufacturer calls Frozen Custard. Or, it may be whipped to introduce air, while Frozen Custard is not generally whipped.
Frozen Custard was mentioned in the Chester Times of Pennsylvania on 30 January 1895 as a menu item at the Aubrey Hotel. The Coshocton Daily Tribune of Ohio in July 1910 gives a recipe for Frozen Custard that calls for a cup of cream and 3 beaten egg whites to be added to a quart of vanilla custard, then frozen.
Despite this, Kohr Brothers in America claims to have been the first to make Frozen Custard — on Coney Island in 1919. It was Archie C. Kohr, they say, who added eggs to what was essentially ice cream to make it stay firmer a bit longer in the heat.