By Harold McGee
The consistency of a custard can be firm or soft, slick or creamy, depending on its egg content. The greater the proportion of whole eggs or whites, the firmer and glossier the custard. Extra yolks, or using yolks alone, will produce a more tender, creamier effect. A custard to be served in the container it was cooked in can be as soft as the cook desires. Those that are to be turned out of a container for serving must be firm enough to stand on their own, which means that they must contain either some egg whites or at least 3 yolks per cup/250 ml of liquid (the LDL-bound yolk proteins are less efficient networkers than the free-floating albumen proteins, so we need more of them to make a firm gel). The replacement of some or all of the milk with cream reduces the proportion of eggs required for a given firmness, since cream contains 20 to 40% less water and the egg proteins are proportionally less diluted. Unmolding is easiest from a buttered ramekin, and when the custards have been allowed to cool thoroughly; cooling firms protein gels.