We ain't afraid of no yolks
Fat, as you’ve surely heard time and again, equals flavor. This axiom is largely true: delicious heavy cream, for instance, contains at least 30% butterfat, whereas skim milk contains less than .5% -- which is why it’s so bland and watery. So you’d be forgiven for thinking that more butterfat is always the answer to better flavor. That axiom isn’t always true in frozen applications, however. “With higher butterfat, custard loses its velvety texture and starts getting greasy,” Linscott says.
And so, frozen custard mix is required to have at least 10% butterfat, generating a mouthfeel that’s rich -- but greaseless. Custard also must contain 1.4% egg yolks: fewer eggs, and it’s considered ice cream; less butterfat, and you’ve got “ice milk,” which sounds like it would be Ned Flanders’ favorite non-popsicle frozen dessert. The FDA regulates these ratios, determining what can be called “frozen custard,” just like it regulates everything else.