Their expansion is caused only indirectly by the eggs they contain; cooking vaporizes the water, which puffs up the dough.
A QUICHE MUST NOT BE COOKED TOO LONG, or it will lose its smooth consistency. It will not be perfectly moist (or chevelotte, as they say in some parts of Lorraine), hence the culinary rule: Cooking must stop when the quiche begins to rise. Rise? Why does it rise? And why is this a sign that the quiche is done?
To answer the question, let us examine another dish that also expands: the quenelle, a cousin of the German Knödel, or dumpling. Whether the quenelle is made of finely minced fish or meat, the flesh has in most cases been mixed with cream, eggs, and panada (in this case a dough obtained by kneading flour with boiling water). Once poached the quenelles are placed in a sauce and, if all goes well, they puff up, even when the eggs have not been beaten.
The quenelle and the quiche therefore have cream and eggs in common. Which of the two ingredients is necessary to make them rise? Heating the cream does not appear to make it expand. But a glance at recipes for petits choux, composed of moist bread and eggs, reveals that the egg is the common denominator of dishes that puff up. Some chefs knew this a century ago: The anonymous author of a cooking manual published in 1905 wrote that “dishes that contain eggs can puff up.”
Why should eggs have this property of expansion? A moment’s reflection yields an insight that even long experience making quiches, varying the proportions of egg white, yolk, cream, and bacon, may not: Although there is no reason why air bubbles should spontaneously appear in the heated egg, eggs do contain water (almost 90% in the case of the white, 50% in the yolk), and this water evaporates during cooking. Anyone who has ever fried an egg has seen the white part rise on contact with the hot skillet. Steam becomes trapped under the coagulated layer and pushes it up.
Perfectly Puffed Up
Now that we know the reason for the egg’s expansion, how can we maximize it? If one were to broil an egg white, cooking it from above, the water in the top part would evaporate and escape without puffing it up. By contrast, if one heats the white from below, as in the case of a fried egg, one observes the expansion I have just described. Dishes that are supposed to puff up therefore must be heated from below. Soufflés? Put the ramekin on the floor of the oven. Macaroons? Quenelles? Use a bottom-heated metal plate instead of a broiler pan.
How much can a dish that contains egg swell up through the vaporization of water alone? An egg white, which weighs about 30 grams and therefore contains about 27 grams of water (not quite an ounce), can generate more than 30 liters of steam (almost 32 quarts). Why, then, should such expansion be limited to soufflés, macaroons, pains à l’anise (sometimes called jumbles or knots in English-speaking countries), cheese-filled puff pastries, and the like? Because a significant part of the moisture is lost by diffusion through the upper part of the dough; indeed, careful scrutiny of these items as they cook reveals the escape of vapor bubbles. Maximum expansion would require an impermeable upper layer that keeps all the moisture inside.
Return to Quiche
Let’s come back to quiche. Why is its expansion a sign that it is done? The liquid that runs around the bacon and into the pastry is composed mainly of fat, water, and proteins. The proteins coagulate as a result of cooking, binding together to form a gel, a lattice that traps the water and fat. The more water there is, the softer the gel. (By contrast, meat, fish, and eggs whose water has entirely evaporated are tough solids.) In other words, although it does puff up a quiche, the evaporation of water is also a signal that the quiche is beginning to lose its tenderness. So there is some truth to the old saying.
But is it the whole truth? No, for the puffing up occurs only if one cooks the quiche by heating it from below. And so the maxim must be revised to say that a quiche is done when, having been heated from below, it begins to puff up. Then, and only then, will it be chevelotte.