Vol. XVII · Eggs and Dairy Plate 9 of 10soufflé, egg-white foam, rising, gas expansion, steam, oven, setting, collapse, 舒芙蕾, cheese souffléMH

Soufflés.

A soufflé does not rise because it is magic, or fall because someone slammed the door. It rises because its bubbles get hot, and falls because they cool.

A soufflé starts as a thick base — a béchamel with cheese, a fruit purée, a pastry cream — lightened with stiffly whisked egg whites (X.6) folded in so as to lose as little air as possible. In a hot oven, two things inflate the bubbles. The air in them expands in proportion to its absolute temperature (Charles's law). And the water around them evaporates into them: near 100 °C the vapour pressure of water approaches the pressure of the air, so a bubble that is mostly steam can grow several-fold. Meanwhile the egg proteins set from the outside in (VI.3), giving the rising foam a structure.

Out of the oven the reverse happens at once: the steam condenses and the air contracts, and only the set protein holds the shape up. A soufflé that is fully set falls less but is drier; one that is soft and creamy in the middle — the French ideal — falls faster. That is why it must be served at once. Japanese 舒芙蕾 pancakes and Chinese 戚风 cakes use the same physics with more structure. The workshop heats a bubble.

Below: the volume of a bubble of air and water vapour, relative to its size at room temperature, against its temperature (ideal gas with water's vapour pressure, exact), and the theoretical rise at the temperature you choose.

What you will learn
Workshop
Exact for an ideal bubble at constant pressure (101.3 kPa): dry air expands with absolute temperature, and the bubble also holds water vapour at the saturation pressure for its temperature (IAPWS), so volume ∝ T ÷ (P − psat(T)). Real soufflés rise much less — bubbles burst and merge, and the setting proteins limit expansion — but the steep rise near 100 °C is real.

Hot bubbles rise, cool bubbles sink

Heated from 20 to 90 °C, dry air in a bubble expands only to 1.24 times its volume; but because water evaporates into the bubble, a bubble in a moist batter can in principle grow to about 3.9 times, and at 60 °C to 1.38; at 98 °C the ideal bubble would be 18 times its size. Real soufflés rise to perhaps twice their raw height, because bubbles burst, merge and escape and the setting egg holds them back, but the steep growth of steam near 100 °C is why the rise happens in the last minutes and why the centre's temperature matters so much.

Method: butter the dish and coat it with sugar or grated cheese so the batter can climb; make a thick, well-seasoned base; whisk the whites to firm, glossy peaks (not dry); stir a quarter in to loosen the base, then fold in the rest; fill the dish, level, and run a thumb round the rim so it rises straight; bake on a low rack in a hot oven (about 190–200 °C) without opening the door for the first two-thirds of the time; serve the moment it is risen and browned, with the centre still slightly creamy. A collapsed soufflé can rise again in the oven: the gas law works both ways.

At the stove

Bake a cheese soufflé and look through the oven window: most of the rise comes in the last third of the baking time, as the centre heats past 70–80 °C.

The French soufflé, "blown", appears in recipes from the eighteenth century; Antonin Carême in the early nineteenth refined it, and it became the test piece of French restaurant kitchens — partly because it cannot wait for the diner.

Terms

Soufflé
A baked dish raised by an egg-white foam.
Charles's law
A gas's volume at constant pressure is proportional to its absolute temperature.
Vapour pressure
The pressure of water vapour in equilibrium with water at a temperature (III.2).
Folding
Combining a foam with a base while losing as little air as possible.
A soufflé rises as its bubbles heat — air expands and water turns to steam, steeply near 100 °C — and the egg sets; it falls as they cool. Serve at once.

Sources

  1. IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 (saturation).
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.