If you can, use a copper bowl for whipping egg whites for meringues, sponge cakes, and soufflés. Chefs have known about the beneficial effects of copper on egg foam for over 200 years, but only recently have scientists learned why copper helps egg whites hold their peaks without weeping or coagulating.
Why Copper Bowls Are Best for Whipping Egg Whites
The mechanical action of whisking denatures protein in egg whites, stiffening the structure and forming strands that trap air. If you go too far, egg whites easily overheat and the foam falls, leaks liquid, or coagulates into a clumpy mess.
Forming a foam and peaks takes a bit longer using a copper bowl, but it stabilizes the whites due to formation of a copper-conalbumin complex and changes in sulfhydryl reactivity.
When you whisk egg whites (or use the mixer) a few copper ions migrate from the bowl into the whites. These copper ions form a golden yellow conalbumin-copper complex. This conalbumin-copper complex is more stable than conalbumin on its own, so the egg whites are less reactive to temperature.
Copper also causes disulfide-linked dimer formation, which decreases surface tension and elasticity. However, the effect on disulfide bonds isn’t the main factor in increasing egg white stability.
Iron and zinc also form complexes with conalbumin. However, stainless steel bowls don’t confer the same benefit as copper bowls because these other metal complexes don’t stabilize the foam.
Copper Bowl Alternatives
Copper bowls are expensive and require maintenance. So, if you don’t want to invest in a special item just for egg whites, consider adding acid to egg whites instead. Cream of tartar is the best substitute for a copper bowl because it does not add extra liquid to a recipe. However, lemon juice or vinegar work almost as well. In all cases, the weak acid lowers egg white pH, which slows disulfide bond formation and preserves foam stability.
- Add 1/8 to 1/4 teaspoon of cream of tartar per egg white or 1/2 teaspoon per cup of egg whites.
- Use 1/4 to 1/2 teaspoon of lemon juice per egg white.
- Add 1/4 to 1/2 teaspoon of white vinegar per egg white.
Other Important Factors for Whipping Egg Whites
The three other factors that help get the best results when whipping egg whites are the bowl, cleanliness, and egg white temperature.
- Generally speaking, metal and plastic bowls are superior to glass bowls for whipping egg whites because the glass surface is so flat that the foam has a harder time sticking to it and holding its shape. Choosing a bowl that is neither too big nor too small also helps get perfect peaks.
- While plastic bowls work great for whipping egg whites, they are not an ideal choice because it’s harder to make certain they are free of any traces of fat or oil. When whipping egg whites, be scrupulously clean. Don’t touch the business part of the whisk or mixing, the inside of the bowl, or the egg whites because oils from your skin can make them fall flat.
- Room temperature egg whites fluff up more quickly than cold egg whites, while cold whites maintain their structure better. So, room temperature egg whites are preferable for baking recipes, while colds egg whites are best for meringues.
References
- Luck, P.J., Foegeding, E.A. (2008). “The Role of Copper in Protein Foams”. Food Biophysics 3: 255–260. doi:10.1007/s11483-008-9060-0
- McGee, H., Long, S.; Briggs, W. (1984). “Why whip egg whites in copper bowls?”. Nature 308: 667–668. doi:10.1038/308667a0
- Phillips, L.G.; Haque, Z.; Kinsella, J.E. (1987). “A Method for the Measurement of Foam Formation and Stability”. Journal of Food Science 52(4): 1074-1077. doi:10.1111/j.1365-2621.1987.tb14279.x
- Sagis, Leonard M.C.; de Groot-Mostert, Aliza E.A; et al. (2001). “Effect of copper ions on the drainage stability of foams prepared from egg white”. Colloids and Surfaces A: Physicochemical and Engineering Aspects 180(1-2): 163-172. doi:10.1016/S0927-7757(00)00734-2
- Vega, C.; Sanghvi, A. (2012). “Cooking Literacy: Meringues as Culinary Scaffoldings”. Food Biophysics 7: 103–113. doi:10.1007/s11483-011-9247-7