Now reading The Science of Eggs

The Science of Eggs

Harold McGee on egg temperatures, pH, cooking methods, and color.


This article first ran in Lucky Peach #4: The American Food Issue. Get yourself a copy hereSubscribe today for more great stuff like this. 

I’ve loved the slick-rich contrasts of eggs cooked in the shell ever since tasting my first deviled egg (made with bottled French dressing) decades ago. I’m not a fan of the modern immersion-circulated “63-degree” egg and its ilk, cooked so far below the boil that both yolk and white are creamy. It’s good to see the classic mollet egg—boiled for 5 or 6 minutes—begin to work its way back into salads and noodle dishes.

But the big advantage of the 63-degree egg is that you don’t have to peel it. Just crack the shell and most of it slides right out. With other gently cooked eggs in the shell, the white is tender and solid rather than semiliquid, and peeling remains an unpredictable and frustrating job. Sometimes the shell comes away cleanly, but sometimes it clings to the white, gouging or tearing divots in it. In a given carton there’ll usually be both easy eggs and recalcitrant eggs, and you can’t tell which is going to be which.

Over the years I’ve tested many different methods for making eggs easier to peel. The latest is the oral method, much demonstrated on the Web: you partly peel a hard-cooked egg at both ends, then place your mouth on the narrow end and blow the egg through the other.

Blowing eggs is certainly quick and showy, but not especially hygienic. And stubborn eggs still pop your ears before they pop their shells. No more reliable is pre-poking the shell, boiling with baking soda, or shocking in ice water.

After blowing failed to be the answer, I decided that it was time to solve the peeling problem for real. How tough could it be?

Well, the egg turns out to be a very sophisticated package—still mysterious and not so easy to manipulate. I did finally come up with a reliable peeling method, but it takes some pretty inelegant, brute-force chemistry, and it’s kind of messy. But entertainingly so.

I started by revisiting the scientific literature on egg-peeling to see what solid information we have these days. It comes down to one not-so-new fact: eggs become easier to peel as they age. Freshly laid eggs slowly lose moisture and carbon dioxide through their porous shells, and the loss of carbon dioxide causes their whites to become more alkaline. The pH of the egg white of a freshly laid egg starts out at a slightly alkaline 7.5. Food scientists have found that when the pH rises above 9, the egg becomes easy to peel.

Apparently, the change in pH makes the egg-white proteins adhere less strongly to the thin, tough membrane attached to the inner surface of the shell, but we still don’t know how or why. It might involve a well-documented stabilization of the major egg-white protein, or the recently discovered “peri-albumen layer,” a microscopically thin envelope around the egg white that disappears in a few days as a fertilized egg develops.

In any case, the fresher a farm-fresh egg, the more likely it is to peel ugly.

The change in pH of an egg white from 7.5 to a peelable 9 takes a few days at room temperature, but it takes much longer in the refrigerator. Cold temperatures helpfully slow general deterioration, but they also slow the egg’s loss of carbon dioxide and the rise in pH and peelability. According to a couple of recent studies, a refrigerated egg may take 2 or more weeks for its albumen pH to reach 9. So even when they’re not especially fresh, the eggs we buy may not have become alkaline enough to peel cleanly.

The challenge is to raise the pH of eggs reliably and quickly, faster than you’d do it by leaving them in the back of the fridge for weeks. My first thought was to store them at room temperature, or place them in a warm oven to speed their aging. This does work, but it still takes several days for enough carbon dioxide to pass through the membranes and shell. Then I tried immersing eggs in a very alkaline solution at pH 12, to raise the inner pH artificially. That also worked, and also took days.

I realized that the shell is the main obstacle to changing the chemical environment within. How could I make the inner egg more accessible to an alkaline bath? By starting with alkali’s opposite: acid. I found that I could etch away much of the shell by immersing it in acid for about an hour. This acid treatment also conveniently makes the shell weaker and easier to break during peeling. Once the shell’s been thinned out by acid, a few hours in an alkaline solution is enough to raise the pH of the egg white just beneath the shell.

The entire process takes about 4 hours—maybe 30 minutes of real attention—and a couple of odd ingredients. One is citric acid, the main acid in citrus fruits. Pure citric acid is used in Middle Eastern cooking, and is much stronger than vinegar. It’s available online, and is often sold in ethnic markets or in the kosher section of supermarkets as “sour salt.” Buy plenty; you’ll need almost half a pound for a dozen eggs.

The second ingredient is baked baking soda. By spreading baking soda out on a baking sheet and heating it in a 350°F oven for a couple of hours, you transform sodium bicarbonate into sodium carbonate. Baking soda itself is a weak alkali that can’t create a pH above 9, but a carbonate solution can get up to pH 12.

To treat a dozen eggs, dissolve about a cup (200 grams) of citric acid in a quart of water. Use a bowl large enough that you can keep all the eggs immersed. They’ll quickly fizz over with tiny bubbles of carbon dioxide created from the acid’s reaction with their shells’ calcium carbonate. Then the eggs will rise to the surface and start doing barrel rolls as the bubbles grow and detach from the shells.

As the shells continue to dissolve and some of their protein is released, the bubbling creates a foamy scum. To speed up and even out the shell etching, gently rub the scum off each egg and back into the liquid every 15 or 20 minutes. (If your hands have any recent nicks, wear gloves or expect some stinging.)

After about 45 minutes, start checking the eggs. When they’ve developed little clear spots where the acid has etched away nearly all of the shell, remove and rinse them. Don’t let the spots grow into large clear patches. All the eggs should be well etched in 45 to 90 minutes.

At this point, immerse the etched eggs in a solution of baked baking soda—4 teaspoons (10 grams) dissolved per quart of water. Let them sit for at least 2 hours—or as many as 6 hours—gently stirring every 15 or 20 minutes. After the soak, rinse them off and either proceed to the cooking or refrigerate them for later use.

I’ve handled several dozen eggs this way, all as fresh as I could find at stores and farmers’ markets, and only a couple of eggs tore when I peeled them. The acid-alkali method works pretty well. The weakened shells often peel off in long, papery spirals.

Whether you etch the shells or not, once you cook eggs it’s best to immerse them in cold or iced water and let them cool down before trying to peel them. This forms the white and further helps it resist tears and gouges. Gently tap or squeeze the shell all around to fracture it into many small pieces. The egg’s narrow end is especially vulnerable, so press the shell down on the very tip with one finger while peeling up to it.

If you’re adventurous, let the eggs sit in the soda solution overnight or longer before cooking them. When the pH of the outer white approaches 12, it develops an opalescent translucency and stronger flavor. You’re on the way to the Chinese century egg and real edginess.

Oh, and while we’re talking eggs, a quick note about yolk color: a deeply colored yolk is beautiful to behold and reflect on, but it doesn’t really tell you anything about how the hen was raised, or how flavorful or nutritious the egg is. Sorry. It’s true that uncaged farm hens and backyard hens accumulate yellow-orange pigments from green plants, and maybe from bugs, that battery hens don’t. But feeds for battery hens routinely include ingredients for coloring their yolks that are rich in the same or similar vitaminrelated pigments (mainly lutein and zeaxanthin, both important for keeping our eyes healthy). Among those ingredients are yellow maize, meals made from marigold petals, alfalfa, grass, algae, paprika extract, and (in Europe, not the United States) synthetic pigments.

Egg producers in different regions formulate their feeds to match consumer preferences, which are measured on scales like the Roche Yolk Color Fan. (There are similar color scales for salmon flesh). According to a recent European review, Ireland and Sweden prefer light yellow yolks of Roche 8–9, France and England a deeper 11–12, and northern Europe and Spain an orange 13–14.

A number of other factors also affect yolk color, including the hen’s breed, age, and other feed components. So the only way to know where a beautiful yolk got its color is to know the hen’s owner, and to ask what it eats.

The best way to highlight yolk color is to avoid cooking the egg fully. Just as happens in the white, the heated proteins form microscopic aggregates that scatter light rays and eventually turn the liquid into a cloudy solid. The more moist you leave the yolk, the clearer and deeper its color will remain.