Making the perfect hard-boiled egg
On Sunday, many will be celebrating Easter by decorating a few eggs. But there’s more to hard boiled eggs than tossing them in some hot water.
Nina Barrett knows boiling the perfect egg can actually be intimidating. That’s why she chose the subject to kick off Eight Forty-Eight's new series Fear of Frying.
Over the coming weeks Nina will tackle topics that might cause a bit of kitchen anxiety – sharpening knives or catering a dinner party. But first – the incredible, edible, hard-boiled egg.
VOICE OF JULIA CHILD: Boiled eggs! Poached eggs! Fried eggs! Scrambled eggs! These are breakfast eggs. We are doing eggs today, but we’re not doing any of these on the French Chef.
BARRETT: Julia Child skipped right over egg-boiling on her way to demonstrating the much more forgiving omelet.
VOICE OF JULIA: Well, that didn’t go very well. See when I flipped it, I didn’t have the courage to do it the way I should have….But you can always pick it up and if you’re alone in the kitchen, who is going to see?
BARRETT: Unfortunately egg-boiling is unforgiving. Even if you’re alone in the kitchen when you mess it up, you’re not going to be able to fix the results, because it’s not an art: it’s a science. So in the absence of guidance from Julia, I turned to thermodynamics for some eggspert advice.
Monica Olvera de la Cruz, head of the Materials Research Center at Northwestern University, is a physicist who specializes in “soft matter.”
BARRETT: When we say “soft matter,” we’re not talking about egg yolks, normally.
OLVERA: Well it does have egg yolks, and especially does have what is called a “gelation process.” When something gels, you make Jell-O, it turns more solid, it has a chemical reaction that is called a gel, and a gel is made of long molecules like these polymers that are entangled with each other and cross-linked, so they don’t flow any more like a liquid.
Scientists who do this at the atomic and molecular level call it nanotechnology. We are going to do it in Monica’s Wilmette kitchen, where she and her Northwestern colleague William Kung will explain how what we call “hard-boiling” an egg means controlling the process by which these reactions take place.
OLVERA: So, we’re going to put a large and a small room temperature — two and two, and then two and two. We’re going to do four! Nice! We have data to plot!
Two variables that can impact the cooking process are the size and the starting temperature of the eggs. To be helpful, William has brought along the differential equation developed by British physicist CDH Williams to determine the time required to boil an egg, given its size and initial temperature.
KUNG: The first order of partial derivative with respect to distance at the quantity the radius squared times the temperature gradient is equal to the temperature gradient itself times some proportionality construct…
I’m ready to whip out my calculator, when William begins to backpedal. Apparently the equation, impressive-sounding as it is, assumes that the egg will be perfectly spherical, and fails to take into account that the yolk and the white conduct heat at different rates.
So instead we’re going to use the recipe I was given in culinary school. We put one jumbo and one extra-large room temperature egg in one pot, and one jumbo and one extra-large refrigerated egg in another, cover them by about an inch with tap water, and put them on high heat. Fairly quickly, both pots fill with tiny bubbles, and William points out that bubbles are also forming on the surface of the eggs.
KUNG: Basically the egg shell is very porous, and right inside the egg shell, there’s a lot of little air bubbles. So as you boil the egg and bring the temperature up, the air bubbles want to expand. If the temperature from the starting temperature goes too fast, the air is going to want to expand too fast, hence it’s going to leak right through the pores and crack the shell.
All four of our eggs come safely to a boil without cracking, but the cold eggs take about two minutes longer. The recipe says to only let them boil 30 seconds, then seal the pots with, turn the burner off, and let them sit for 15 minutes. This is when, according to William, all the true molecular gastronomy is taking place. Eggs whites, he says, consist of proteins curled up in little balls, suspended in water.
KUNG: So when you cook the egg, you heat up the temperature, so we’re breaking the chemical bonds within each of these protein molecules. So they become long, flexible polymer chains. In addition, the heat is going to drive the further introduction of chemical bonds between neighboring long, linear, polymer molecules, so they form a network. It’s a process called gelation. So the longer you cook it, the more you’re going to add chemical bonds to it, and that’s why it becomes rubbery when the egg is overcooked.
The fact that the eggs are just sitting in the water, rather than actually boiling, minimizes the chance that we’ll overshoot the mark. It also minimizes the chance that iron in the yolk will react with sulfur in the white to make that greenish tinge around the yolk, another sign of overcooking.
After 15 minutes, we run cold tap water into both the pots for five minutes to cool them down slowly. Again, we want to minimize the chance that a sudden change in internal and external temperatures will cause the shells to crack. And now, the moment of truth, when we find out: can physics help us boil the perfect egg?
OLVERA: So let’s just start with the large ones. I don’t want to jump to any conclusions…Let’s see. They are beautiful, these eggs! I’m sorry, they are beautiful. Maybe I put too much salt.
KUNG: They’re good!
OLVERA: Mmm hmm! Very good!
Actually, it turns out all you really have to do is follow the instructions to the letter — and take your results with no more than a grain of salt.
For WBEZ, I’m Nina Barrett, wishing you all bon appetit!
Instructions for hard-cooked eggs
Paging through a few different cookbooks will show you that experts differ about how to produce the perfect hard-cooked egg. The recipe we used in the WBEZ segment was given to me when I was in culinary school at the Le Cordon Bleu College of Culinary Arts in Chicago by my chef-instructor, Brian J. Karam, who told me it’s the method he’s come to rely on after years of trial and error.
If you’re making Easter eggs, be aware that there’s a trade-off you might have to make between producing an egg that won’t crack — nice for presentation — and one that’s easy to peel. The instructions below minimize the chance of cracking because they reduce the thermal shock to the egg as it cools off — but the longer you leave the eggs in cold water, the harder it becomes to peel them. If easy peeling’s what you want, many sources, including the January/February issue of Saveur magazine, suggest tapping the shells with a spoon or rolling them on the counter before shocking them in cold water.
For the scientific perspective on egg-boiling, see the equation posted online by CDH Williams.
1. Begin with a room-temperature egg (the shell is less likely to crack)
2. Put in pot and cover with water to cover an inch over the eggs. (If you put in too much water, it will take too long to come to a boil and they’ll overcook)
3. Bring to a boil on FULL HEAT (so it happens fast)
4. Let boil 30 seconds, then cover pan and turn off heat. Need nice tight seal so no steam escapes.
5. Leave it on the stove top for 15 minutes.
6. Put under cold running water (without dumping out the cooking water) for 5 minutes to cool.
Music Button: Beastie Boys, "Egg Man", from the CD Paul's Boutique, (Capitol)