The short answer
The real story of frozen custard vs ice cream is smaller than most people expect. In the United States, both are covered by the same federal standard of identity, 21 CFR 135.110. Neither one lacks a legal definition. They share the same rules for milkfat, total solids and weight. The single difference the rule draws is egg yolk.
If a frozen dessert has more egg yolk than the ice cream limit allows, it can no longer be sold as plain ice cream. It has to carry one of the custard names instead.
What the federal rule says
The egg yolk line sits in 21 CFR 135.110(a)(2). Here is the text exactly as the eCFR gives it:
“Except in the case of frozen custard, ice cream contains less than 1.4 percent egg yolk solids by weight of the food, exclusive of the weight of any bulky flavoring ingredients used. Frozen custard shall contain 1.4 percent egg yolk solids by weight of the finished food: Provided, however, That when bulky flavors are added the egg yolk solids content of frozen custard may be reduced in proportion to the amount by weight of the bulky flavors added, but in no case is the content of egg yolk solids in the finished food less than 1.12 percent.”
The naming rule is in 21 CFR 135.110(f)(1):
“The name of the food is ‘ice cream’; except that when the egg yolk solids content of the food is in excess of that specified for ice cream by paragraph (a) of this section, the name of the food is ‘frozen custard’ or ‘french ice cream’ or ‘french custard ice cream’.”
So “french ice cream” on a label is not a style claim. It is one of three legal names for frozen custard.
Fat and solids
Both desserts must contain at least 10% milkfat. For ice cream, the floor drops to 8% of the whole when bulky flavors such as chocolate, fruit or nuts are added. Both must also carry at least 1.6 lb of total solids per gallon.
These are floors, not targets. A maker can go higher on fat in either dessert. That means the word custard on a sign tells you about the egg yolk, and nothing more, as far as the law goes.
Egg yolk changes the flavor. The Guelph e-Book lists an egg flavor defect, caused by too much egg in an ice cream that is not meant to be a custard, and says it resembles French vanilla ice cream. In a custard, that eggy note is the point. In a plain ice cream, it counts as a flaw.
Air, weight and serving temperature
Air whipped into a frozen dessert is called overrun. The federal rule does not set overrun directly. It sets weight instead: both ice cream and frozen custard must weigh at least 4.5 lb per gallon.
Here our sources do not line up neatly, so we give each figure with its source:
- The eCFR sets the 4.5 lb per gallon floor. By our own arithmetic, at a typical mix density that caps overrun near 104%.
- H. Douglas Goff’s University of Guelph e-Book gives about 100 to 120% overrun for standard ice cream, and 25 to 50% for super-premium.
The top of Guelph’s standard range sits above our arithmetic cap, so read both numbers as rough guides, not a single answer.
For frozen custard, we found no sourced figure for air content. The federal rule does not set one, and it varies by maker.
Serving temperature works the same way. Guelph gives about -16 °C (3 °F) as a typical serving temperature for ice cream. We found no sourced serving temperature for frozen custard, and the federal rule does not set one.
How each is made
Ice cream starts as a liquid mix of milk, cream, sugar and other solids. According to Guelph, the mix is frozen while being stirred in a barrel freezer, which works air in and keeps ice crystals small. It is then hardened in a blast freezer at -30 to -40 °C.
Frozen custard follows the same path. The only change in the recipe is egg yolk in the mix, enough to reach the 1.4% egg yolk solids the rule asks for.
At home, that means the same machine works for both. You decide on custard or ice cream when you build the base, not when you churn it.