Proper custard comes from an iron lung
To make true Milwaukee-style frozen custard, you’ll need an iron lung -- fortunately, not an actual pressure ventilator, but a seriously hardcore ice cream machine nicknamed for its formidable shape and size.
One reason to employ these behemoths is overrun, the technical term for the amount of air whipped into a frozen dessert. With a lot of standard-fare supermarket ice creams, you end up paying as much for the air as you do actual product: if an ice cream has 100% overrun (which is not an atypical number) a full half of the packaged product is actually nothing but the stuff you breathe.
Custard’s overrun, however, should hit at less than 25%, according to Linscott of Gilles Frozen Custard. “Quality custards have less air whipped into them. Some places go higher, but I don’t know many that go much lower [than we do],” he says. The end result: a dense solidity that ice cream wishes it could claim.
These machines also make custard terrifically fast. At Gilles, that means the finished product is ready less than twelve minutes from the time the mix is poured into the machine -- although the exact time varies by flavor. Recipes that contain more sugar take a little longer to finish (which makes sense when you think about how much harder it would be to freeze an ingredient like sugar versus, say, cream). The iron lung’s quick-freeze method is also key to keeping the product’s ice crystals small, which is what gives custard its iconically silky, super smooth texture.
Next, the custard is sent down a chute into a dipping freezer, where it’s ready to be dished up. Because it’s served as soon as it’s made, the temperature of the custard is higher than that of scooped ice cream, which has to be kept in a freezer before serving. This makes for a softer product, similar to soft-serve ice cream -- although thanks to the low overrun and small ice crystal formation, you end up with a completely different, infinitely more luxurious texture.
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.