The Food Lab

Unraveling the mysteries of home cooking through science.

The Food Lab: The Hard Truth About Boiled Eggs

It's time for another round of The Food Lab. Got a suggestion for an upcoming topic? Email Kenji here, and he'll do his best to answer your queries in a future post. Become a fan of The Food Lab on Facebook or follow it on Twitter for play-by-plays on future kitchen tests and recipe experiments.

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A perfect hard-boiled egg: clear yellow yolk and smooth, tender white, all peeled perfectly. [Photographs: J. Kenji Lopez-Alt]

Back when I was a lowly line cook at a fancy-pants restaurant in Boston, as the new guy*, it was my job to wake my butt up at the crack of dawn to come in early and prep breakfast whenever one of the Beacon Hill politicians wanted to impress their campaign funders with boozy waffles and perfectly soft-boiled eggs topping their asparagus. In those days, I used the brute-force method of getting perfect boiled eggs: I boiled at least three times what I needed, knowing that at least half of them would stick to their shells and turn into egg salad sandwiches for family meal.

*Let's just call it like it is: I was the kitchen bitch.

Since then, I've had a bit of time to reflect and refine my methodology. The secret to peeling hard boiled eggs? Well "secret" might be a bit of an exaggeration. Here's the truth: there is no 100% fool-proof method, and anybody who tells you different is selling something. And I do believe I've tried them all, many, many times over. The number of eggs I've boiled over the last several years in carefully controlled circumstances numbers well into the thousands, but despite that, the best boiled egg I cooked this year is no better than the best boiled egg I cooked twelve years ago in that Beacon Hill kitchen.

That said, a bit of the old scientific method has helped to greatly increase my success rate. Finding the hard truth about boiled eggs was a tough case to, er, crack.* I can now pretty routinely produce perfectly boiled eggs with clean-peeling shells, and you can too!

*sorry, I just can't help myself!

Want the tl;dr version? Here you go. This is all you need to know:

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Lower your eggs straight from the fridge into already-boiling water, or place them in a steamer insert in a covered pot steaming at full blast on the stovetop. If boiling, lower the heat to the barest simmer. Cook the eggs for 11 minutes for hard or 6 minutes for soft. Serve. Or, if serving cold, shock them in ice water immediately. Let them chill in that water for at least 15 minutes, or better yet, in the fridge overnight. Peel under cool running water.

No baking, no pricking, no tricks, no gimmicks, that's it.

But of course, there's still a lot of eggsplaining to do.

This is not the first time I've explored boiling eggs, but this time I went all-in with dozens and dozens and dozens and dozens of eggs to get to the bottom of what really makes them tick.

Bear with me.

Old vs. Young

The age of an egg does make a difference, but only when the eggs are super fresh: freshly laid eggs are more likely to stick to the shell. In the U.S., eggs can sit for up to 30 days before being packaged, and the sell-by date can be a further 30 days after that, which means that most likely, the eggs you're getting at the supermarket are old enough that no further aging at home should be necessary. Still, it certainly doesn't hurt to look for a package with the closest expiration date.

If you're buying your eggs direct from the farmer or you keep a couple hens out back, then you may want to let your eggs sit around for a couple of weeks before using them for boiling. (And if you want to cook those super-fresh eggs, might I recommend poaching them using our foolproof method? It's almost a shame to do anything else with them!)

Much more important to the end result is the way in which the egg is cooked.

Hot or Cold Start?

Let's get one thing quickly out of the way: we're cooking eggs straight out of the fridge here. Tempering by letting them sit at room temperature doesn't make much difference, and it takes a long time to do.*

*For you Euro-types who store their eggs at room temperature, everything discussed here still applies.

More than any other factor, the thing that made the most difference in how cleanly eggs released from their shells was the temperature at which they started: A hot start produces easier-to-peel eggs. And it doesn't matter whether that hot start is in boiling water or in a steam-filled pot or pressure-cooker. They're all strikingly easier to shell than those started in a cold pot.

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Starting eggs in cold water greatly increases the chances of shell sticking.

Even with two-week-old eggs, starting cold resulted in eggs that had just over a 50% success rate for clean peeling. Eggs started in boiling water or steam came out well above 90%. I don't have a fully satisfactory answer for this phenomenon, but my thoughts are that it's somewhat like cooking a steak in a skillet. Add the steak to a cold pan and heat it up slowly, and as proteins coagulate, they bond with the metal, becoming nearly inseparable. Heat that steak fast, however, and the proteins bundle into themselves, instead of sticking to the metal. Slow-cooked egg whites bond more strongly with the membrane on the inside of an egg shell.

You might notice that this goes almost exactly against the advice I gave four years ago in my very first Food Lab column. Back then, I did not know what I do now. Back then, I recommended starting eggs in cold water and bringing them up to a simmer with the water, the idea being that they'll cook more evenly with a slow start. This is true: eggs started slow will have more tender, evenly cooked whites.

Why is that? It's because in rapidly boiling water, the exterior of an egg will cook much faster than the center.

Take a look at these eggs, which were cooked in fully boiling water for times ranging from 1 minute to 15 minutes:

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As you can see, not one of them is a perfect hard-cooked egg: the eggs go directly from having slightly translucent centers to having rubbery whites.

Here's what happens when an egg white cooks at a raging, full-on boil:

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  • From 30 -140 degrees: As it gets hot, its proteins, which resemble coiled up balls of yarn, slowly start to uncoil.
  • At 140 degrees: Some of these uncoiled proteins—called ovotransferrin—begin to bond with each other, creating a matrix, and turning the egg white milky and jelly-like (like the innermost layers of egg white in the 3-minute egg above).
  • At 155 degrees: The ovotransferrin has formed and opaque solid, though it is still quite soft and moist (see the white of the 5-minute egg).
  • At 180 degrees: The main protein in egg whites—ovalbumen—will cross-link and solidify, giving you a totally firm egg white (see the whites of the 7 and 9 minute eggs). This is very similar to the gunk that seeps out of the surface of overcooked salmon.
  • 180 degrees-plus: The hotter you get the egg, the tighter these proteins bond, and the firmer, drier, and rubbier the egg white becomes (the 11-15 minute eggs). Hydrogen Sulfide, or "rotten-egg" aromas, begin to develop. Ick.

Egg yolks, on the other hand, follow a different set of temperatures:

  • At 145 degrees: They begin to thicken and set up.
  • At 158 degrees: They become totally firm, but are still bright orange and shiny.
  • At 170 degrees: They become pale yellow and start to turn crumbly.
  • 170 degrees-plus: They dry out and turn chalky. The sulfur in the whites rapidly reacts with the iron in the yolks, creating ferrous sulfide, and tinging the yolks.

Thus, for perfect hard-cooked eggs, you want whites that don't cook much beyond 180°F, and yolks that have just hit 170°F throughout. Cooking relatively gently allows for this, but easy peeling requires a full 212°F blast of heat.

So how does one cook easy-peeling eggs that also have relatively tender whites? There are a couple of options. If you're boiling, you can plunge your eggs into boiling water, let them boil for 30 seconds or so just to set the exterior of the whites, then drop the temperature and finish them off at around 180 to 190°F (a very low simmer). You end up with eggs that are easy to peel, with tender whites throughout.

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Steaming eggs cooks them more gently than boiling, making them less likely to crack or explode, and less likely to turn rubbery.

With steam, there's no need to lower the heat—steam's relatively lower density means that steamed eggs actually cook a little more gently than boiled eggs. Your whites will be ever-so-slightly tougher than with simmered eggs, but not enough to be off-putting. The gentleness of steam and the ability to load up a steamer insert and lower it gently into a pot also has the advantage of reducing your chances of shells cracking and whites leaking out.

Finally, steaming your eggs has the advantage of being the fastest method around: instead of waiting for a pot of water to come to a boil, all you need to do it boil a half inch of water. Throw your steamer insert in, cover the pot, set your timer, and you've got perfectly cooked eggs.

What About the Pressure Cooker?

Yeah, what about it?

For a while now, I've been reading about cooking eggs in a pressure cooker. Those articles promise you a magic bullet: cook your eggs at low pressure in a standard pressure cooker, and they'll practically jump out of their shells.

But here's the thing: I couldn't find a single source that actually tested that hypothesis in a controlled, scientific environment, and all the anecdotal evidence in the world is not worth the pixels it's written with when confronted with strong scientific evidence.

So I tested the pressure cooker method, multiple times with multiple eggs in each batch, using different pressure cookers, different timings, and different cooling methods. Dozens and dozens of eggs in total. Not only that, but I tested it side-by-side with boiled and steamed eggs, and to get really scientific about it, I went double-blind, with one test administrator (who did not know which eggs had been cooked by which method) handing cooked eggs in random order to a third party who peeled them all (also with no knowledge as to the cooking method for the eggs he was peeling). All of the eggs were rated on ease of peeling (1 = easiest to peel, 10 = most difficult), and were observed for surface damage after peeling.

Guess what? There is absolutely zero correlation between cooking eggs in a pressure cooker and ease of peeling. In fact, depending on how long it took to bring the pressure cooker to pressure, eggs cooked in one actually showed a slight negative correlation!

There's an even more nefarious problem: Even when timed perfectly, the pressure cooker produces slightly rubberier whites than other methods.

Here's the thing: Pressure cookers cook hotter than steaming or boiling—even at low pressure they hit temperatures in excess of 220°F. And the hotter something cooks, the greater the temperature differential between the center and the edges.

Pressure-cook your eggs for six minutes to get perfectly hard-cooked yolks, and the outer layers of your whites end up tough. At least, tougher than equivalently steamed or simmered eggs.

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Pressure cooked for five, six, and seven minutes.

The bigger issue is that pressure-cooker eggs have a smaller window of perfection. With a stovetop simmered or steamed egg, the difference in end results between 10 minutes of simmering versus 11 minutes is not that great. With a pressure-cooked egg and its rapid cooking, however, the difference a minute makes is huge. At five minutes, the yolks are still translucent yellow. At six minutes, the yolks are tender and just set. At seven minutes, you've already started to develop the dreaded green tinge.

My advice? Save the pressure cooker for things that it actually improves or makes faster. Eggs ain't one of 'em.

Out of the Pot and Into the Oven

The other method that seems to be getting a fair bit of play recently is cooking eggs in the oven. Now, nobody wants to heat up the oven just to make a couple of eggs for breakfast, but what if you're making deviled eggs or breakfast for a crowd? Heating an oven to cook a few dozen eggs seems like it might be a swell idea.

Alton Brown (a generally trustworthy guy) recommends placing eggs on a moistened towel directly on an oven rack in a cold oven, then setting the oven to 320°F and coming back 30 minutes later.

Even before I tested the method, alarms were going off in my head. Recipes that call for cooking in an oven while its temperature is in flux (either starting cold or turning off in the middle) are never reliable. Ovens on their own are bad enough—most home ovens swing their temperatures by as much as 50°F above and below their set temp (assuming they were calibrated well to begin with). To add the confounding variable of how efficiently your specific oven heats or cools on top of that is asking for trouble.

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What may work perfectly in one oven probably won't in another. The method certainly didn't work in mine. One batch heated too quickly and came out with dark spots on the shell and a pale brown color to the white. For a second batch, I lowered the cooking time and managed to get eggs that looked ok when split open, but they were a bitch-and-a-half to peel, which makes sense. Remember: cold start = harder to peel eggs. It doesn't matter if it's in the oven or in a pot of water.

Ovens are inherently unreliable because there is no physics-based indicator or limiter of heat. A pot of simmering or steaming water, on the other hand, is reliably at the same temperature (assuming constant atmospheric pressure), which means that no matter whose kitchen I'm in and no matter how powerful their stovetop burner, so long as I can boil some water, my eggs will cook reliably time after time.

When cooking eggs in large numbers, I'll stick with the steamer and cook my eggs in batches if I have to rather than play Russian roulette with the oven.

A Simple Cure for the Dimple

Does anyone else get bothered by the dimple created by the air space in the fat end of the egg? I sure do. The problem is that the older the egg is, the bigger that air space gets. This means that the eggs that are most suitable for boiling also happen to come out the ugliest,* and nobody wants to be that guy who serves the deformed deviled eggs.

*I'm sure there's some parallel to be drawn here between the kinds of people who are having babies and the kinds of people most suited to raising babies, but that's another subject for another time.

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A pin-prick in the fat end can help remove the air bubble... when it works. But we have a better way.

If you're to trust Jacques Pepín (and I usually do!), solving this problem is as simple as pricking the fat end of the egg with a pin. This allows the air inside to be pressed out rapidly as the egg cooks, leaving you with a completely smooth egg. Usually.

But occasionally, you end up with this:

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This is what can happen with pricked eggs.

Instead of creating a prettier egg, things can go sour when boiling water enters your egg, creating these strange, deep craters. Wouldn't it be great if there were a more foolproof method of getting rid of those dimples?

As it turns out, there is, and it was one I discovered completely accidentally.

See, I came in early one morning to start boiling eggs before the office filled up with coworkers and distractions. I'd gotten through three dozen test runs, working in six-egg batches and testing various combinations of cooking temperatures, cooking methods, and cooling methods. When I started shelling those eggs, I noticed that despite the eggs all coming from the same origin, some batches of six had perfectly round bottoms, while others had dimples. What was the correlation?

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Shocking in ice water can help remove the divot in a boiled egg.

Here it is: The eggs that I had immediately shocked in ice water after cooking came out full-figured and smooth, regardless of the initial cooking method, whereas those that I allowed to cool naturally had dimples.

Shocking!

My theory? When you pull the hot egg out from the pot, the yolk and white have yet to firm up completely (you can feel this when you try to peel a hot-from-the-pot egg compared to an egg that has rested or cooled a bit). By shocking it, you very rapidly cause the steam that has built up inside that air pocket to convert to water, instantly dropping to about .5% of its original volume. The still-malleable boiled egg moves in to fill its place.

Let the egg cool slowly, on the other hand, and by the time that steam has cooled sufficiently, the egg is already basically set in its shape. Instead of the egg moving in to fill that space, air is drawn in through the egg shell.

Further testing is needed to prove the theory, but it sounds pretty legit to me, and the practical effects on appearances are undeniable. Shocked eggs also have a slightly higher success rate for peeling.

Time to Peel!

One last factor that improves your chances of a successful peel, and it's a major one: make sure to chill your eggs completely. I mean let them sit in that ice bath for at least 15 minutes, or better yet, let them sit overnight in the fridge. The cooler the egg is, the firmer and tighter its structure will be, and the less likely is it to develop craters when you pry off the shell.

Chilling eggs overnight reduced poorly-peeled-egg count by a further 50% compared to those peeled after a few minutes of shocking (or even worse, straight out of the pot).

Sure, sometimes you want to serve your hard boiled eggs hot. My advice for those times? Serve them in the shell so that if they don't peel well, it's your guests who will feel inadequate. This is a good strategy for instilling a bit of insecurity and fear in them before a brunch-time debate over politics or religion.

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Crack your egg all over before starting to peel.

Before I start to peel my eggs, I crack them gently all over their surface, starting at the fat end and gently rotating and tapping all around. I don't use the roll-it-against-the-counter method, as I've accidentally squashed too many eggs with my ham-hands that way.

The many cracks makes it easier to gently peel away the shell under running water.

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Peeling under running water is the way to go, and yes, I forgot to shock this one.

When I've got a bunch of eggs to peel, I'll place either a fine mesh strainer or the insert of a salad spinner in the sink under the tap and peel the eggs under the flow, letting the shells drop down into the strainer below for easy cleanup afterwards.

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And that's about all there is to it. Start with old eggs, cook them hot (but skip the pressure cooker), chill them rapidly and completely, crack all over, and peel under running water.

Another Food Lab article, another 5,000+ words on a subject that probably only really needs 10, but what are humans if not voraciously curious and insatiably wasteful? At least this time around there were only two egg puns. But it was an egg-ceptional one. D'oh!

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So what does one do with dozen and dozens of boiled eggs? How about these nine in-your-face deviled egg variations?

P.S. For the record, baking soda still doesn't make any detectable difference.

About the author: J. Kenji Lopez-Alt is the Chief Creative Officer of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.

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