The scientific secret to perfectly foamed eggs

This story originally appeared on FiveThirtyEight and is reprinted with permission.

This is The Digest, a new FiveThirtyEight column exploring the science, history and economics of food. We're still working out the essential ingredients to make this just right - we welcome your feedback and suggestions! You can email me, leave a note in the comments, or find me on Twitter. Now, on to this week's main course.

Eggs are a marvelous ingredient. They can be used to make food dense and rich or light and fluffy. They feature in both breakfast and cocktails. And the yolks are responsible for mayonnaise, the great unifying condiment. The whites, which can go from a dense liquid of mostly water and protein to a mountain range of liquid foam, can be loaded with sugar and made into a variety of delights. All you need is a few minutes of patience and muscle.

Harold McGee, perhaps the best-known kitchen chemist in the United States, often tells the story of his history with egg whites as a moment of breakthrough, the moment when nature and science first defied his expectations. What's more, it happened not once, but twice, as he tried to figure out the ruse behind a long-standing kitchen magic trick: making structurally sound foam using nothing but egg whites, a whisk and a copper pot.

The way McGee tells it, he first encountered the idea that egg whites should be whipped in a copper pot, rather than glass or other metal, from Julia Child's writing - and promptly rejected the advice. The justification she offered (that the pot acidifies the egg white) wasn't scientifically sound (copper doesn't change the pH of a solution). But then, in researching his first book, he stumbled upon "Ptissier," or "The Pastry-cook," an engraving that was first published in 1771 and shows a young boy whipping eggs in a copper pot. He decided he must at least test the method, given that chefs had been recommending it for centuries. Much to his surprise, there was a remarkable difference between using a copper or a glass vessel. Whipping the egg whites in copper took nearly twice as long, but the foam took on more structured forms that were difficult to over-whip, an ever-present threat when using a glass bowl.

That we can make liquid mountains out of egg white hills is a matter of chemistry. The long proteins in the egg whites have sections that are drawn to water and others that are not. As the whisk introduces air into the mix, it unfurls these compacted proteins. The hydrophilic sections start connecting with the water, while the hydrophobic sections cling to air bubbles. More and more of these new bonds form, and the egg white begins to grow until it becomes a mountain range of foam.

But one can whip too many times. The proteins also form bonds with one another, and as the number of bonds grows, they will eventually squeeze out the water and air, leaving a pool of liquid and a blobby mess of protein where once there were beautiful peaks and valleys.

To make sense of the copper bowl's mysterious powers, McGee theorized that one of the proteins, ovotransferrin, would attach to copper from the bowl. That prevented it from unfurling and would keep the whole mass resistant to over coagulating.

He sent the theory off to the journal Nature, where it was published in 1984, and then immortalized it in early editions of his classic tome "On Food and Cooking." And there it stood until around 1994, when McGee noticed something that made his theory untenable: Egg foam was also more stable if mixed in a silver bowl, and silver doesn't bind to ovotransferrin.

And so he did what any good scientist would: He re-examined his theory and came up with a new one.

One of the bonds formed between the proteins is the result of electron clouds that are knocked around, leaving unstable sulfur groups that are inclined to form bonds with one another. McGee now believes that the copper (or silver) attaches to the unstable sulfur atoms so that other protein strands won't. This limits the number of bonds, making it difficult to overwhip the egg whites. This also explains why adding acids (such as lemon, vinegar or cream of tartar), as many cookbooks suggest, helps prevent overcoagulation.

When I found a copper pot at a thrift store not too long ago, I decided to run a little experiment and try out the various methods of making egg foam that are described in cookbooks. I tested whipping plain egg whites in a stainless steel bowl, egg whites in a copper bowl, and then egg whites in a steel bowl with three different additions: cream of tartar, a ground-up copper supplement pill, and lemon.

Doing this experiment, I was reminded how satisfying it is to take a ball of condensed liquid and fluff it up to eight times its original size. It's such a marvel with a plain egg white that it made the difference between using a steel or copper bowl all the more notable. It took about twice as long to whip the whites in copper, as McGee had described, but when they were good and whipped, they formed sweeping swirls that stayed in form as long as I left them. The egg whites in the stainless steel bowl, however, turned into a rather clumpy mess that started to separate within a few minutes. The difference was sort of like soft serve in an air-conditioned store versus soft serve from a truck on a sweltering hot day.

Both the cream of tartar and the powdered copper made liquid peaks equally beautiful. They did tweak the flavor ever so slightly, but it was nothing that a little sugar (or a lot, in the case of a meringue) wouldn't mask. The lemon only worked marginally better for me than the plain egg white on stainless steel. (In other words, it was a relative flop.) Since the cream of tartar has the benefit of making the foam extra white without chipping away at my lovingly used (read: heavily scratched) copper bowl, that will be my preferred egg foam sidekick from now on. Now that I've started, I can't get enough. Meringue, anyone?

Morsels

• The Baltimore City Council's Health Committee has a hearing Tuesday about some controversial legislation proposed earlier this year. It would require businesses selling or advertising sugar-sweetened beverages to post health warnings that the drinks contribute to tooth decay, obesity and diabetes. Among other testimony, the committee will hear about a study that found parents were 20 percentage points less likely to choose a sugary beverage for their kids if the packaging had a warning label.


• There are just 24 days until a Vermont law takes effect and requires companies to label foods containing genetically modified organisms (GMOs). Congress is trying to get its own bill through the House and Senate first, but that's looking increasingly unlikely.


• The man himself, Harold McGee, has a new story in Lucky Peach on how sometimes perception is reality when it comes to food.