For a long time, a common recommendation was to use a copper bowl to whip egg whites into a foam. In fact, special bowls were made just for this practice and it was long a part of French tradition. This is no longer recommended as often today, sometimes because there is fear of copper contamination, which is not really a problem, but mostly because copper bowls are expensive and hard to maintain; and there are easier alternatives to get the same effect. But, what were the reasons for doing this in the first place?
Well, one thing to recognize is that this practice came about before anybody really knew anything about the properties of egg whites and there is evidence that copper was a recommended material for egg white whipping in the early 1700’s, and maybe even earlier. Many cookbook authors seem to think that the copper bowls were used because chefs thought they increased the volume of the foam. So, when cooks experiment with copper bowls and other bowls, such as stainless steel or even glass, and come up with the same amount of foam for the same amount of egg white, they determine that the copper bowl thing was a myth.
However, it seems the main reason chefs loved copper was that it produced a more stable and glossy foam, with less grains. In other words, there is less chance of a broken meringue with curdles. These kind of concerns made much more sense when egg whites had to be whipped by hand using an almost super-human whisk action. I doubt anyone reading this has ever tried that, but if you ever did, especially on restaurant scales, you’d probably understand why chefs might look for any advantage at all, even if they are imagined. Even the hand-crank egg beater probably rendered the type of bowl used less important, let alone modern electric mixers. Even so, there are certain surfaces that work best, and since hardly any of us own copper bowls any more, stainless steal is probably the best.
Plastic tends to have a bit of fat clinging to its porous surface, which can disrupt an egg white foam. And glass or porcelain are too smooth, causing the egg whites to slide down the sides of the bowl instead of clinging, which helps support the foam. And to stabilize the foam, today, we are usually instructed to add a bit of cream of tartar. 1Brown, Amy C. Understanding Food: Principles and Preparation. Belmont, CA: Wadsworth Pub, 2011. 2McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004. 3Hillman, Howard. The New Kitchen Science: A Guide to Knowing the Hows and Whys for Fun and Success in the Kitchen. Boston: Houghton Mifflin, 2003. It is difficult to say whether copper would really make a big difference. But what are the scientific reasons, if any, for it’s use?
Scientific theories as to why copper bowls are better for whipping egg whites are several, and there seems to be little agreement, but I’ll give the most popular one here.
One of the first things to do, however, is to figure out how egg whites form a foam in the first place, and what causes this foam to break. When we whip the whites, we are forcing air into them. But, at the same time, we are subjecting the proteins to a physical stress that causes the proteins to unfold and become less compact, which is called denaturing. The air itself also applies a force to the proteins, pulling on them, so as to help unfold them. There are different kinds of proteins in egg whites. Some of them are not affected much by whipping, but the ones that are, such as globulinsand ovotransferrin, have parts that are attracted to both air and water.
When you whip, you are creating a mixture of water, air, and unfolded proteins which easily form bonds with each other, creating a sort of network of proteins. The proteins tend to orient themselves with their water-loving parts pointing toward the water, and their air loving parts sticking into the air. So this forms a network of bubbles, the proteins holding the air in the bubbles and keeping the water out by creating a kind of film. This makes a foam that can be six to eight times the volume of the original egg whites. Anything foreign, such as a bit of egg yolk, that gets into the whites can disrupt this foam, and temperature can also play a role, but if you whip enough, you can eventually make a foam even out of contaminated egg white, although perhaps not a perfect one. 4 McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004. 5Brown, Amy C. Understanding Food: Principles and Preparation. Belmont, CA: Wadsworth Pub, 2011.
So, think of an egg white foam as a network of unfolded protein molecules holding air and water between them. But the proteins can bond together in many different ways, at different sites. The more you whip, the more bonds form, and when too many bonds start forming, the proteins become too jumbled up and start squeezing out the water, collapsing the network and causing the foam to “break.” In practical terms, this is when the whipped egg whites suddenly start collapsing, get grainy or curdle, and start separating into a runny liquid and a dry froth. So, in order to make a nice stable foam, you have to have just enough protein bonds, and prevent the accumulation of excess bonds.
One way in which copper bowls might help this is by preventing a certain type of bond. As above, there are different ways the proteins can bond, one to another. One of these ways is through the bonding of reactive sulfur groups on the proteins. These bonds form when the sulfur-hydrogen groups (S-H) on two different proteins shed their hydrogens and form a sulfur to sulfur bond with each other. This is one of the strongest type of protein bonds that can occur, so when they occur, the proteins are starting to get a bit too friendly and jumble together too tightly. Copper is said to react with sulfur groups and form tight bonds of its own with them, which basically blocks them from forming bonds with other sulfur groups on other proteins. By bonding with sulfur groups in this way, copper bowls block these strong sulfur bonds between proteins, creating a more stable foam, one that doesn’t collapse. It is claimed that a bit of powdered copper added to the egg whites does the same thing, and that silver has the same properties.
We are often warned that copper bowls should not be used because of copper contamination. It is claimed that this is why copper bowls for whipping egg whites fell out of favor. But in truth, the amount of copper that gets into whipped egg whites from a copper bowl is tiny and poses no danger (copper is a trace element in human nutrition so a little bit is okay).
And now we can start to understand how the addition of cream of tartar, or another acid, such as lemon juice, helps. In order for the sulfur on different proteins to bond with each other, as explained above, they have to get rid of a hydrogen (H) ion. When you add an acid, you are adding a bunch of hydrogen ions. All those extra H’s in the egg whites makes it difficult for the sulfur groups to give up their own hydrogens. When the hydrogens on the sulfur groups cannot be shed as quickly, the sulfur bonds cannot form. Although this does not completely eliminate the bonds from forming, it makes them form very slowly, allowing you to create a more stable and glossy foam, without grains. 6McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004.
Sources [ + ]
|1, 5.||↲||Brown, Amy C. Understanding Food: Principles and Preparation. Belmont, CA: Wadsworth Pub, 2011.|
|2, 6.||↲||McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004.|
|3.||↲||Hillman, Howard. The New Kitchen Science: A Guide to Knowing the Hows and Whys for Fun and Success in the Kitchen. Boston: Houghton Mifflin, 2003.|
|4.||↲||McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004.|