Water evaporates upon contact with the heated sides of the ramekin and causes the soufflé to rise.
HOW CAN ONE MAKE A PERFECT SOUFFLé every time? The question is not only of practical interest to cooks. Measuring the pressure inside a cheese soufflé and the loss of water during cooking will also help food scientists understand the dynamics involved.
Contrary to a widely held belief, this dish is within the reach of beginners. In the case of a cheese soufflé, for example, begin by preparing a béchamel sauce, heating flour and butter, and then adding to this milk and grated cheese. Next, off heat, fold in egg yolks and (very carefully) beaten egg whites. The result is cooked in the oven at a temperature of 180–200°C (about 375–425°F), depending on the size, for twenty to thirty minutes.
It has long been supposed that soufflés rise because they contain air bubbles that expand upon heating. A simple calculation shows that this effect can generate an expansion of only about 20%. However, expert pastry cooks are perfectly capable of making soufflés that double or triple in volume.
Soufflés rise because of the vaporization of the water found in the milk and eggs. The proof is readily seen: When one cuts into a soufflé with a knife in order to share it with a party of delighted dinner guests, a cloud of steam bursts forth. At exactly this moment, to everyone’s great dismay, the soufflé collapses.
A Model Soufflé and a Soufflé Model
What is the maximum possible expansion of a soufflé? A simple thermodynamic model is useful in thinking about this question. The warm air inside the oven transfers its heat first to the ramekin and to the upper part of the soufflé, establishing a temperature gradient between the periphery and the center of the mixture. When the temperature reaches 100°C (212°F) near the sides of the ramekin and at the soufflé’s upper surface, the water inside evaporates and a crust is formed.
This account is supported by internal temperature readings. When one sinks the probe of a thermocouple (a very precise sort of thermometer) into a soufflé during cooking, one finds that the upper crust has the same temperature as the oven and that at the center the temperature falls before rising again near the bottom.
Moreover, this measurement reveals a hidden dynamic at work in soufflés. If the probe is placed at a fixed position in relation to the ramekin, it registers first an increase in temperature, then a slight decrease or leveling off, and finally a jump back up to about 70°C (158°F). What is happening is that as the heat gradually becomes distributed throughout the soufflé, the evaporation of the water as it comes into contact with the bottom of the ramekin produces bubbles that push the whole of the soufflé upward, which in turn causes cold layers of the mixture to rise to the level of the probe. These layers nonetheless continue to heat up, and the cooking is done when the eggs have thoroughly coagulated, at about 70°C.
In Search of New Heights
How does the water evaporate? Comparison of two soufflés that are identical except for the egg whites used in each suggests an answer: A soufflé with very stiff egg whites rises higher than one whose whites have been whipped for a shorter time because steam bubbles have a harder time penetrating the stable foam of the vigorously beaten whites. Thus leavening is caused by at least two things: the formation of steam bubbles and the trapping of this moisture in the body of the soufflé, in areas where the temperature is sufficiently great to avoid recondensation.
How much water is needed to increase the volume of a soufflé to its greatest extent? Weighing a soufflé before and after cooking reveals that about 10% of its mass has been lost, which is to say that for a soufflé that weighs 300 grams (about 10 ounces), 30 grams (about 1 ounce) can evaporate. Keep in mind that 1 gram of liquid produces 1 liter, or slightly more than a quart, of vapor. Nonetheless, not all the water in a soufflé remains trapped inside; otherwise the internal pressure would exceed a hundred atmospheres. Recent measurements have shown that the pressure increases during cooking by only a few dozen millimeters of mercury, which proves that only part of the evaporated water is retained; the rest escapes in the form of bubbles that eventually burst at the surface of the soufflé.
This suggests that the way to obtain a perfectly leavened soufflé is to heat the bottom of the ramekin, to use very firm whipped egg whites, and to seal the surface in order to prevent the release of the bubbles formed inside. How would one go about doing this? One possibility would be to place the soufflé under a broiler before putting it in the oven. This method has the additional advantage that the soufflé then rises in a regular fashion and, when it is done, has a smooth golden glaze on top that promises a rich flavor.