Not Exactly Egg Pie

• If you wanted to, you could fill a pastry crust with savory ingredients and then add beaten eggs without any milk or cream at all. The result would be tasty enough, more or less a frittata in a crust, but it wouldn't be a quiche. The eggs would have a much firmer consistency, like scrambled eggs or an omelet, and if you overbaked them even slightly, they'd become tough and rubbery. They'd also cook much more quickly than the crust, so you'd have to bake the crust separately beforehand. A quiche is altogether more delicate and elegant, and cream is the reason.

Of Cream and Custards

• In a good quiche, neither the eggs nor the cream dominates, forming instead a well-balanced partnership. The cream lends the mixture richness and finesse, while the eggs thicken the cream into a delicate soft custard. The proteins in the egg try to become firm in the oven's heat as they would in a skillet, but they're too diluted by the cream. Instead, as the molecules of protein cook and bond to each other, they form a delicate web that traps the cream into a soft gel. Most recipes adhere to a time-honored ratio of two eggs per cup of cream or milk, but you can vary that ratio to make the filling softer, firmer or richer as you wish.

Half-and-Half

• You could achieve the correct quiche texture with almost any liquid, if you were prepared to experiment, but dairy products add flavor and richness to the mixture. Heavy cream brings plenty of both, but at a significant cost in fat and calories. Ordinary milk has much less fat, but also -- unfortunately -- less flavor as well. The half-and-half called for in many recipes is a compromise, a middle ground between virtue and pleasure.

Substitutions

• If you're deep into quiche-making and discover you're out of half-and-half, don't despair. You can use ordinary milk, heavy whipping cream or light coffee cream, with similar results. You can also simulate half-and-half by mixing equal parts milk and heavy cream, or adding a splash of heavy cream to your light coffee cream. Evaporated milk will also work in a pinch, though it lends a slight "cooked" flavor to the finished quiche. If you need to omit the half-and-half because of a dairy allergy or lactose intolerance, use a non-dairy milk such as soy milk, almond milk or -- if it's compatible with your flavors -- coconut milk. Don't use sweetened or flavored milk.