By Harold McGee
French or custard ice cream contains an additional ingredient: egg yolks, as many as 12 per quart/liter. The proteins and emulsifiers in egg yolk can help keep ice crystals small and the texture smooth even at relatively low milk-fat and high water levels; some traditional French ice cream mixes are a crème anglaise made with milk, not cream. A mix that contains yolks must be cooked to disperse the proteins and emulsifiers (and kill any bacteria in the raw yolks), and the resulting thickened, custard-like mix makes an ice cream with a characteristic cooked, eggy flavor.
A distinct style of custard ice cream is the Italian gelato, which is typically high in butterfat as well as egg yolks, and frozen with little overrun into a very rich, dense cream. (The name simply means “frozen,” and in Italy is applied to a range of frozen preparations.)
Reduced-fat, low-fat, and nonfat ice creams contain progressively less fat than the 10% minimum specified in the commercial American definition of ice cream. They keep their ice crystals small with a variety of additives, including corn syrup, powdered milk, and vegetable gums. “Soft-serve” ice cream is a reduced-fat preparation whose softness comes from being dispensed at a relatively high temperature (20–22°F/–6°C).