IN THE KITCHEN : Hard-Boiled Detective Meets Shy Molecule
Some people lust after foie gras ; others yearn for perfectly cooked hard-boiled eggs. The latter are probably more difficult.
To tell the truth, the matter had never really crossed my mind until a friend began rhapsodizing about these hard-boiled eggs she had eaten two or three years ago. “They were perfectly cooked,” she said, “firm, but still moist . . . you know, not rubbery. And the yolks were this deep-gold color, without that any of that awful green outside. How come you never see hard-boiled eggs like that?”
The one or two times every year I want a hard-boiled egg (Easter not included, since those eggs rarely even get cracked, much less eaten), I cook the eggs the way my mom taught me: Put an egg in water, bring to a boil, cook until done.
The flaw in this technique is obvious: How do you know when the egg is done? Well, when it seems as if it should be. It’s like boiling rice with the lid on; if you can’t see what you’re cooking, you’re left only with instinct. And when you cook something only once or twice a year, your instincts are bound to be a bit, shall we say, rusty.
But it sounded like an interesting problem. Since eggs cook at a fairly constant rate, and since boiling water stays at a fairly constant temperature, cooking time should be the only variable. Just as there is a legendary 3-minute soft-boiled egg, it seems there must be a 10- or 15-minute hard-boiled egg.
So I got 15 eggs, numbered them with a magic marker, covered them with cold water and brought them to a boil in The Times Test Kitchen. After the water reached a boil, I pulled the eggs out at one-minute intervals, in numerical order, and chilled them in an ice-water bath. When all the eggs had been cooked, I cracked each egg open and compared the results.
I got something that seemed like an answer--a cooking time of somewhere between 13 and 14 minutes yielded a hard-boiled egg that could be considered “perfectly cooked,” depending on whose taste you’re cooking them to.
But when I tried to repeat the process using just four eggs, the times were off by as much as two minutes. The problem? Water reaches a boil depending on how much solid matter it has to heat. What’s more, the culinary definition of a boil is fairly elastic, ranging from the 185-degree simmer to the 212-degree roll. My first batch was maintained at a pretty steady simmer. I had to answer the phone while the second batch was cooking and returned to find the water raging away at full tilt.
To eliminate those problems, I decided to start the eggs in boiling water, hoping the temperature would stay more constant. But this time, no matter how golden the yolks were, there was no way these eggs could be considered well cooked, much less perfectly cooked. In the first place, there was definite “banding” of the yolks--the inside might be golden, but the outside would be pale and on the verge of being overcooked. What’s worse, half the eggs had popped their shells, spewing little plumes of egg white. What’s more, the whites were rubbery.
So I retreated to the library to try and figure out exactly what it is that happens when an egg is cooked. It turns out--the egg business being a sizable industry--there is quite a body of scientific literature on the subject, ranging from “Quantitative Analysis of Gelation in Egg Protein Systems” to “Influence of Three Organic Acids on the Quality Characteristics of Hard-cooked Eggs.”
I took the easy way out, choosing a 1983 American Scientist article called “The Culinary Alchemy of Eggs,” by Arthur E. Grosser, a professor of chemistry at McGill University who specializes in molecular beam kinetics.
It turns out that there’s a lot more to cooking an egg than meets the eye. “An egg protein is a shy molecule,” Grosser writes, “and when left all alone it will curl up into itself, coiling into a ball.” But when heat is applied, the proteins react. “As the temperature increases, the action becomes more energetic until a molecular demolition derby is under way. The weak internal bonds of the egg protein can no longer hold the ball together and it opens out into a floppy streamer.”
When this happens, “it exposes a soft underbelly of richly seductive targets for chemical attack and bonding. As these protein streamers bump into each other, they immediately bond and form strong links . . . until there is a three-dimensional protein network, semirigid and resilient.”
Grosser’s article also explains why my eggs had cracked--it was a simple thermodynamic reaction. Every egg has an air pocket inside. When that air pocket is quickly heated it expands, cracking the shell and spewing the white. In my first tries, starting the eggs in cold water, the air pocket was heated more gradually, allowing the extra air to bubble off slowly through the porous shell. You can accomplish much the same thing by pricking a small hole in the broad end of the shell to let the air escape.
And that pesky green ring? Grosser says it’s a chemical reaction. As the protein in the egg white warms during cooking, it gives off a small amount of hydrogen sulfide gas. When that gas meets the iron particles present in every yolk, it forms iron sulfide--the greenish band. Fortunately, this is one of the last of the chemical reactions involved in cooking an egg. Don’t overcook and you won’t get a green band.
Still, I was no closer to my perfectly cooked egg. Just as I was ready to throw up my hands and give up, a colleague came into the kitchen. “You know the best way to cook hard-boiled eggs?” she asked. “The way my mom taught me: Start them in cold water, bring them just to a boil and then turn the heat off right away and let the eggs sit. When the water is cool enough to put your hand in, take the eggs out. Works every time.”
At this point, I was willing to try anything. So I stuck an egg in a pot and gave it a shot. When the water had cooled just enough that I could reach my hand in, I pulled the egg and cracked it open. The yolk was a lovely even golden yellow, the white was tender. Excited, I tried again, this time with a half-dozen eggs and letting the water cool even more. The same result.
The more I thought about it, the more sense it made. The key factor in this cooking method is a very gentle heat curve. After all, it doesn’t take much heat to cook an egg--the proteins become firm-tender at 160 degrees--and as long as the eggs don’t get much beyond that temperature, they won’t overcook.
Theoretically, then, you could leave an egg in 159-degree water forever and it wouldn’t get tough. The eggs heat up at a slow rate and--just when those proteins get going really crazy--begin to cool again, slowly. Using this method, by the time the egg yolks get to that perfect temperature, the water surrounding them is already cooling to below it.
What could be easier?