Though proteins are responsible for giving foams structure, they can also destabilize them. If whipped too long, the proteins form tight bonds that squeeze out the water. This causes the foam to lose volume, become grainy, and separate into a dry froth and runny liquid. There are simple ways to prevent this catastrophic collapse:
The presence of copper or silver in foaming egg whites binds to reactive sulfur groups. This renders the proteins unable to form other strong bonds, which prevents proteins from getting too close to each other and collapsing the foam. For this reason, the use of a copper bowl for whipping egg whites has long been recommended. If a copper or silver plated bowl is not available, the cook may add a pinch of powdered copper supplement to obtain the same result.
Bonds with sulfur groups can also be prevented by adding an acid such as 1/8 tsp cream of tartar or 1/2 tsp of lemon juice. The acid boosts the number of available hydrogen (H) ions in the mix, making it harder for the S-H groups to shed their hydrogen and form strong sulfur bonds with anything else.
Other things to keep in check include egg yolks, fats/oils, and detergent. Traces of these three things interfere with protein bonding. Their presence does not prevent a foam from forming, but it takes longer and the quality suffers. Fats and yolks can be added once the foam is formed.
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