Egg-White Foams.
One egg white holds seven times its volume of air. A speck of yolk can stop it holding any.
Egg white is about 90 % water and 10 % protein. Whisking drags air into it as bubbles and stretches the proteins at each bubble's surface, where they partly unfold (VI.1) and bond to one another into an elastic film: the foam. The longer it is whisked, the smaller and more numerous the bubbles and the stiffer the foam, through foamy, soft peaks (the tip curls over) and stiff peaks (it stands up). Beat on and the proteins bond too tightly, squeeze out water and the foam turns dry, grainy and collapses: over-beaten.
Three additions change the story. Fat — a trace of yolk, a greasy bowl — sits in the bubble surfaces and stops the proteins bonding; the foam will not rise. Sugar dissolves into the film, making it thicker and slower to drain, so a meringue is glossy and stable, but it slows whipping: add it gradually once soft peaks form. Acid (cream of tartar, lemon) or a copper bowl limits the bonds that over-beating makes. The workshop whips a white.
Below: the volume of foam against whisking minutes for a plain white, one with a trace of yolk, one with sugar and one with cream of tartar, and the state of yours (colloid bench, teaching model).
- Explain how whisking makes a foam.
- Recognise soft and stiff peaks.
- Use sugar and acid to stabilise a foam.
- Avoid fat and over-beating.
Clean bowl, then sugar
Whisked four minutes, a clean white reaches about 624 % overrun in this model; with a trace of yolk, only 306 %, and it will not hold a peak. Whisked on to twelve minutes it is over-beaten and falls back to 419 %; with a pinch of cream of tartar the same twelve minutes still gives 699 %. Fifty grams of sugar per white makes the foam smaller but far steadier: it holds about 80 minutes before weeping against 20 for plain white.
For meringue: separate eggs cold (the yolk is firmer), whisk the whites at room temperature in a clean glass, metal or copper bowl wiped with lemon; start slowly until foamy, add cream of tartar, whisk to soft peaks, then add sugar a spoonful at a time and whisk to stiff, glossy peaks. Fold whites into a batter (soufflé, chiffon cake) at soft-to-firm peaks, not stiff, so they can stretch further in the oven. Chinese 戚风 and Japanese 舒芙蕾 pancakes depend entirely on this foam.
Whisk two egg whites in separate clean bowls; add a tiny drop of yolk to one before starting. Whisk both the same way for the same time. One rises to stiff peaks; the other stays a loose froth.
Terms
- Overrun
- The extra volume of a foam as a percentage of the liquid it started from.
- Soft peaks
- Peaks that curl over at the tip.
- Stiff peaks
- Peaks that stand straight up.
- Cream of tartar
- Potassium bitartrate, an acid salt that stabilises egg-white foams.
Sources
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
- H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.