fold
Gently combine a light, airy mixture (whipped cream, beaten egg whites) into a heavier one using a scooping, turning motion -- meant to preserve the air already whipped in, unlike stirring or whisking.
How to do it
Work in a bowl noticeably larger than the mixture itself -- a cramped bowl encourages aggressive stirring, which is exactly what folding is trying to avoid. Stir a portion of the lighter mixture into the heavier one first in a fairly ordinary way; this 'sacrificial' addition lightens the base and makes the rest of the folding easier. Add the remaining lighter mixture, then use a broad spatula to cut straight down through the center, sweep across the bottom of the bowl, and lift the mixture up and over itself. Rotate the bowl slightly and repeat. Stop as soon as the mixture looks reasonably uniform -- a few faint streaks left behind are better than mixing so thoroughly that all the air comes back out.
Common pitfalls
- Stirring in circles -- it knocks the air back out almost immediately.
- Trying to combine everything in one pass -- a heavy mixture can crush a light, airy one before it's incorporated.
- Over-folding -- past a certain point, folding just becomes another form of ordinary mixing.
- Under-folding out of caution -- large unmixed pockets can bake unevenly.
Why it works
Whipping egg whites or cream builds a foam -- gas bubbles dispersed through a liquid or semi-solid structure -- and those bubbles are mechanically fragile. Vigorous mixing makes them collide, merge, and escape; folding cuts down on that shear force while still gradually blending the two mixtures together. In a cake or soufflé, those trapped bubbles expand as the surrounding gas warms during baking, so they directly determine the finished volume and lightness of the result.
Equipment
A large, flexible silicone spatula is ideal, since it sweeps cleanly across the bottom and sides of the bowl; a broad metal spoon can also do the job. The bowl itself should be clearly larger than the mixture, since a tight fit tends to push you toward more aggressive stirring than the technique calls for.