Important Science


I am disappointed by America’s Test Kitchen Podcast. Not deeply, but slightly.

Listening to a few back-episodes there was a brief mention that the problem of making hard-boiled eggs that peel perfectly “is not understood.” These are the people who do experiments on all kinds of stuff – but perhaps the experiments they do in the test kitchen are all just a cover-project for eating lots of cake and spare ribs, or something.

After tackling this problem using science I wrote ATK a quick explanation, but I’m sure they don’t care. Because, if they did, they would have done some thinking and figured it out themselves. For the record: I don’t care either. I eat a lot of hard-boiled eggs, so it was an easy experiment to perform.

Good science would entail establishing a theoretical framework that matches everything we observe without any contradictions, then an experiment is devised that adds additional potentially contradicting evidence. We present the evidence and our theory, then conclude by eating breakfast.

Observe tiny bubbles

Observe tiny bubbles

We observe:

  • When eggs are put in boiling water as it’s coming to a boil, very small bubbles often come from the egg. These are not the result of steam (“rolling boil”) in the pan, they originate apparently on the egg itself.
  • The surface of eggs appears to be slightly porous.
  • Inside the egg there is a thick sort of rind layer, then the white and yolk. The rind can sometimes hold together a slightly cracked egg.
  • In many eggs I have successfully and easily peeled, the rind is the dividing-point between the white and the shell/rind; i.e.: it peels cleanly with the rind.
  • Baby chicks grow in eggs. There’s probably some way for them to get air.
  • The America’s Test Kitchen people said that older eggs hard boil better. (  <- this is apocryphal information not evidence)
  • Peeled eggs, when placed in water, do not dissolve – they remain hard and retain their shape.

We hypothesize that:

  • The shell of an egg is porous. The white of a cooked egg is not porous. The rind must be porous or the chick would die in the egg.
  • That air gets driven out of the egg means that it expands when heated (of course) and the contents of the egg may also expand slightly, driving out the air. That explains the little bubbles.
  • As the egg cooks, the air gets forced out. As it cools, stuff gets sucked back in. If that stuff is air, it’s easier for the rind to stick to the cooked proteins of the white. If that stuff is water, it gets sucked back in and keeps the rind and the protein from binding thanks to a thin layer of water between the now-cooked sticky proteins of the white and the rind.
  • If older eggs have dehydrated slightly (since the shell is porous) there will be more air in the egg, so older eggs will suck back in more water when they cool; they may peel more easily.
cool water with green food dye, standing by

cool water with green food dye, standing by

Experiment:

  • Boil an egg 5 minutes. Observe tiny bubbles
  • Leaving eggs completely immersed in water, flush the pan with cool water containing food dye
  • Allow the eggs to cool, with an additional flush of dyed water
  • After 3 minutes, rinse outside of eggs with clear water
  • Rinse hands, pan, etc, with clear water
  • Place eggs on white paper towel (for visual contrast)
  • Test peeling the eggs – do they peel easily Y/N?
  • Observe whether there is food colorant inside the egg’s shell as it is peeled
eggs in pan with green dye, cooling

eggs in pan with green dye, cooling

How to make easy-peel hard boiled eggs:

  1. Cover eggs in cool water
  2. On low/mid heat and bring slowly to a boil
  3. Boil gently for 5 minutes
  4. Without dumping the water – keeping the eggs completely covered with water at all times – place the pan under tap and run cold water until the pan and water the eggs are in is cold
  5. Let sit 4 minutes
  6. Peel the eggs
  7. Eat
rinsed egg in shell and slightly great easy-peel egg

rinsed egg in shell and slightly green easy-peel egg

Our hypothesis appears to be confirmed, at least to the degree that: there was green dye inside the egg’s shell and on the white, which indicates that the dyed water went into the shell before it was broken and that the shell and membrane are porous.

I believe this also explains why eggs that have slightly cracked shells are sometimes easy and sometimes very hard to peel: the easy ones are the ones where water gets into the rind and the eggs cook with the water separating the rind from the white, the hard ones are the ones where the water doesn’t get sucked back in as the eggs cool, because the shell is compromised.

This batch of eggs peeled easily. They tasted great.

I cannot claim that this applies to all eggs by induction, but I have been making all my hard boiled eggs this way for about a year, now, and I have only had trouble on the occasions where I am too impatient to let the eggs cool sufficiently under the water.

divider2

I never got to do school science fairs. Can you tell?

Comments

  1. says

    An important question: Does refrigeration make a difference, cold eggs or room temperature? (Well, not to me, I don’t eat boiled eggs. ^_^) Warm or cold makes a difference in boiling water or how well popcorn kernels will pop.

    Side note: In many Asian countries, eggs at supermarkets are not kept refrigerated, they’re stored in cartons at the store’s ambient temperature (usually, with air conditioning). Chickens lay and nest their eggs without cooling, so it obviously doesn’t do them any harm. It’s only recently (the last four months) that supermarkets in Taiwan have started refrigerating eggs.

  2. Dunc says

    I believe that rapidly cooling the eggs after cooking also prevents overcooking and minimises the discolouration of the yoke.

    According to this, the ease of peeling is determined mostly by the pH of the egg white, which increases with age, making older eggs easier to peel than fresh eggs. This certainly matches my experience.

  3. Dunc says

    Side note: In many Asian countries, eggs at supermarkets are not kept refrigerated

    Most European countries too… They refrigerate eggs in the US because they wash them, which removes the cuticle. It’s all to do with salmonella

  4. jacobletoile says

    I’m not sure if you listened to the first or second one they did on hard boiled eggs. In the first one they focused on how to get a perfect (as they define it) egg every time, in the second they focus on getting them to peel perfectly. I really like their method of getting them to peel perfectly and since using it I have not had any issues. Even for soft boiled eggs. The second method was steaming them, then shocking them in cold water.

  5. says

    left0ver1under@#1:
    An important question: Does refrigeration make a difference, cold eggs or room temperature?

    It oughtn’t. After all, birds sit on eggs to protect them and keep them warm, outdoors.

    My guess is that they refrigerate them so that if an egg is slightly broken (and becomes food for bacteria!) it won’t smell bad. Then the store would have a problem: imagine a stack of boxes of eggs, and a bad smell – which carton of eggs has the broken one in it? You’d probably break more trying to locate the bad egg. Keeping them cool and air conditioned is going to keep the customers from realizing that a significant number of eggs on any given shelf are broken and rotting.

    That’s a guess, though!! Although it’s based on my theory that you can explain almost anything stores do solely in terms of profit/loss.

  6. says

    Dunc@#2:
    I believe that rapidly cooling the eggs after cooking also prevents overcooking and minimises the discolouration of the yoke.

    I have no idea! But that’s an interesting possibility. I’ll note that in this experiment discoloration wasn’t an issue since I was cooling the eggs in green food dye. A nice green yolk would have matched!

    According to this, the ease of peeling is determined mostly by the pH of the egg white, which increases with age, making older eggs easier to peel than fresh eggs. This certainly matches my experience.

    Unfortunately, your link was dead, I’m assuming it’s something like this. I am unconvinced. How is the lower-ph water getting into the egg? Also, the link I found shows inconclusive results. It quotes Harold McGee:

    Difficult peeling is characteristic of fresh eggs with a relatively low albumen pH, which somehow causes the albumen to adhere to the inner shell membrane more strongly than it coheres to itself.

    C’mon McGee, “which somehow causes…” is not very sciency! We’re doing sciency here!

  7. says

    Caine@#4:
    you’ve never made tea eggs, then?

    No!

    Lemme bust one of my scrolls of Google…
    Hmm, they appear to be immersing the eggs, which I take as support for my theory!

  8. says

    jacobletoile@#5:
    I’m not sure if you listened to the first or second one they did on hard boiled eggs. In the first one they focused on how to get a perfect (as they define it) egg every time, in the second they focus on getting them to peel perfectly.

    I must have heard the first one, not the second. I wonder if the second is regarding the 6 page illustrated explanation I sent them! Probably not…

    The second method was steaming them, then shocking them in cold water.

    I will have to try to find that episode; now I am curious. “shocking them in cold water” – I wonder what their theory of operation is for that. Depending on how long they are left in the cold water, the “suck water back in” theory I present above still applies.

  9. Dunc says

    Unfortunately, your link was dead, I’m assuming it’s something like this. I am unconvinced. How is the lower-ph water getting into the egg?

    Oops, sorry about that, let’s try it again: Why Are Some Boiled Eggs Easier To Peel Than Others?. It’s nothing to do with the pH of the water, it’s the pH of the albumen itself, which changes as the egg ages. The precise mechanism by which “the proteins in the egg white bind tightly to the keratin in the membrane during the cooking process” is not described, but detailed biochemistry is a bit heavy-duty for these sorts of articles…

    I certainly believe that older eggs are easier to peel based on my own experiences, but I don’t have any robust data to back that up.

    My guess is that they refrigerate them so that …

    No need to guess, the reasons are detailed in the link in my comment at #3. It’s basically only the US that refrigerates eggs, AFAIK.

  10. felicis says

    My wife found a paper article, “Easy-Peel Hard Cooked Eggs” in a magazine called “Cooks Illustrated (March and April 2016 pp 12-3).

    Their method is to steam the eggs – get water boiling, use a vegetable steamer and place the eggs in the steam for 13 minutes (I’ve found this time to not be terribly critical – 12-15 minutes works); cool in ice-water for 15 minutes (again – not critical, in a rush I’ve gone as low as 8 minutes with no peeling problem later).

    The big idea is to prevent the albumin and other proteins of the white from binding to the protein of the membrane – the faster the whites are denatured, the less binding there will be, so into boiling water or into steam (they also used a pressure cooker to good effect, since its temperature rises very quickly). Starting from cold water or baking in the oven both led to poor results.

    This article preferred steaming to boiling since adding the eggs to the boiling water reduced its temperature in an unknown amount – making the cooking time unpredictable. Steaming keeps the temps around the egg high(-er, I am not so sure it makes that big of a difference for hard boiled eggs – the steam method was part of an earlier experiment with soft-boiled eggs where the timing is more critical).

    I have been using this method now for a few months and find the eggs (from our chickens, so all very fresh, rarely more than 2 weeks old, often only a couple of days) easy to peel with no binding between the membrane and white.

    A note on your observations –

    You see bubbles coming off the eggs –
    (a) air from inside the egg
    (b) nucleation sites for the boiling water on the shell
    (c) a little of both

    I would expect (b) to dominate – this is born out by the fact that the air within an egg is usually at one end, but the bubbles come from all over. Water does not appear to be sucked into this air space.

    The passage of water during the cooling process was clearly demonstrated – but I don’t think it was ‘sucked in’ because of the absence of air – the dye could easily enough have passed through the membrane by osmosis.

    Previously we cooked eggs by essentially your process (start in cold water, boil, hold, rinse in cold water until cool), but our eggs regularly failed to peel well.

    Now it’s time for breakfast!

  11. says

    felicis@#11:
    Cooks Illustrated is the magazine version of ATK – it’s all Christopher Kimball’s operation. FYI. (I love Cooks…)

    The big idea is to prevent the albumin and other proteins of the white from binding to the protein of the membrane – the faster the whites are denatured, the less binding there will be, so into boiling water or into steam (they also used a pressure cooker to good effect, since its temperature rises very quickly). Starting from cold water or baking in the oven both led to poor results.

    I am amazed by this; I’ve had eggs explode outright if you put them into water that’s already boiling hot.

    You see bubbles coming off the eggs –
    (a) air from inside the egg
    (b) nucleation sites for the boiling water on the shell
    (c) a little of both

    That’s probably a flaw from my illustration (which I just put there for local color) – you’ll see the bubbles rising from the eggs long before the water reaches boiling temperature. So it’s a) – and by the time the water reaches boiling, the eggs will usually have stopped “outgassing” through the shell and you’ll see the boiling begin where the egg contacts the pan, at the bottom.

    I would expect (b) to dominate – this is born out by the fact that the air within an egg is usually at one end, but the bubbles come from all over. Water does not appear to be sucked into this air space.

    The age old conflict between theory and practice arises, yet again. If water’s not getting sucked into the eggs through the shell, why was the white of my egg green after I cooled it in green dye?

    the dye could easily enough have passed through the membrane by osmosis

    Sure. I guess I’m going to have to boil an egg in green dye and cool it in clear water, now.

    I didn’t say all the air. It just needs enough pressure to move some water in. As far as whether the membrane and the proteins keep from binding because of the rapid cooling, or whatever – I don’t buy that, for the simple reason that if the proteins and the membrane bind because of heat, then they’d bind while the egg was cooking.

  12. felicis says

    “I don’t buy that, for the simple reason that if the proteins and the membrane bind because of heat, then they’d bind while the egg was cooking.”

    Well – maybe – except protein-protein interactions are pretty complicated. I can easily see a differential rate of binding based on temperature, and that the binding might be less at a higher temperature (where both proteins are binding to themselves instead of each other).

    I did not realize that ATK and CI were the same group (I had not heard of either). I can say positively that my egg-cooking results are best with the steam method they recommend, and poor with starting from cool water as you recommend. Since you have good results with your method, there must be some other differentiator that we are both missing, or the differences in how we each use the two methods include something un-noticed that changes each to reverse the outcomes…

    I think we can both agree that eggs are delicious.

  13. says

    Dunc@#10:
    It’s nothing to do with the pH of the water, it’s the pH of the albumen itself, which changes as the egg ages.

    Interesting. So they think that changing the PH of the water the egg is cooking in … somehow it goes in and something somethings the albumen? That seems really unlikely to me.

    Or the baking soda changes the PH of the membrane so it doesn’t stick to the albumen? That’s more believable, except that we’d have to assume it’d take some time for the baking soda’d water to get through the shell (how? using the mechanism I identified?) and change it – after it had already started cooking.

  14. says

    felicis@#13:
    except protein-protein interactions are pretty complicated

    That’s for sure!!!

    I can say positively that my egg-cooking results are best with the steam method they recommend, and poor with starting from cool water as you recommend.

    The tricky part of this is that ATK/CI’s process overlaps my hypothesis: I think that when they “shock the egg in cold water” they are doing the important part: it’s sucking the water into the egg as the egg cools, and keeping the membrane from binding. I don’t think the steam/sudden heat has anything to do with it, it’s the cold water-bath at the end – and keeping the eggs covered under water the whole time – that matters.

    there must be some other differentiator that we are both missing, or the differences in how we each use the two methods include something un-noticed that changes each to reverse the outcomes…

    I haven’t got the CI version, but: how long are they saying to leave the eggs in the cold water after they have been “shocked”? If they’re leaving it in there long enough for the eggs to cool enough to suck cold water back in, then they’ve independently hit on the same solution I have, they just don’t understand what’s happening.

    I think we can both agree that eggs are delicious.

    Yes! I am prepared to experiment lots!

  15. says

    The CI articles are behind a paywall, but what I can see here is:

    Boiled eggs that start in cold water are hard to peel because the proteins in the egg white set slowly, which gives them time to fuse to the surrounding membrane. When you try to remove the shell, parts of the white cling to the membrane, and the surface of the egg is unattractively pockmarked. Instead of a cold-water start, we place cold eggs directly into hot steam, which rapidly denatures the outermost egg white proteins, causing them to form a solid gel that shrinks and pulls away from the membrane. The shell slips off easily to reveal smooth, unblemished hard-cooked eggs.

    So: I have been starting my eggs in cold water for years and always thought that adding eggs to boiling water(steam is hotter) can cause them to crack. How interesting!

    Their assertion that a cold water start makes eggs unattractively pockmarked and harder to peel does not match my many many many experiments over the course of years.

    I believe that they are incorrect and are focused on the front-end of their process and don’t realize that the back-end is where the important stuff is happening. But I am not sure how to experimentally determine that.

    I wonder if I quick steam-cook the eggs, under their theory, the albumen will have denatured and not stuck to the membrane, and if I let the eggs slow-cool instead of “shocking” them, they still ought to peel just fine.

  16. jacobletoile says

    They shocked the eggs that were cooked in cold water and brought to a boil, the eggs that were started in boiling water and the eggs that were steamed. the cold water is to rapidly stop the cooking process so your yoke don’t develop a green ring indicative of over cooking, as well as to reliably achieve that just set yoke they are defining as perfect. I happen to agree with them on that. There is quite a buffer between a perfectly cooked yolk and a green ring.

  17. blf says

    I admit I haven’t read either the OP or the comments in full — mostly because I dislike hard-cooked eggs — but the go-to source on food / kitchen science, Harold McGee’s On Food & Cooking, discusses the problem. It’s a bit long so I won’t quote it in full. The important point seems to be (this is from the 2nd Edition) “Difficult peeling is characteristic of fresh eggs with a relatively low albumen pH, which somehow causes the albumen to adhere to inner shell membrane more strongly than it coheres to itself.” Note that he says “somehow causes”, indicating that the precise mechanism is unknown. He also suggests hard-cooking old eggs as one method to avoid peeling problems, but those apparently have a tendency to crack when hard-cooked (however, he doesn’t say explicitly they are more likely to crack then fresh eggs, so that may be a misreading on my part).

  18. says

    jacobletoile@#18:

    I understand why they shocked the eggs in cold water. The question is whether they understand that shocking the eggs in cold water may be the reason they peel well; they may be completely right about their theory of keeping the yolks from turning green, but completely wrong about their theory of peeling well. There are 2 different problems here and they may have solved both of them with one solution, and the other solution may be irrelevant.

  19. says

    blf@#19:
    Note that he says “somehow causes”, indicating that the precise mechanism is unknown

    If the mechanism is unknown, how does he know that PH is the reason the albumen sticks to the membrane?

    I did this posting as a miniature room for discussing the scientific method, so I am not going to accept an authority who just says “which somehow causes the albumen to adhere” …

    I have a PH tester over in my lab. I guess I am going to have to start recording the PH of eggs. For science.

    He also suggests hard-cooking old eggs

    Ones that have dehydrated more, perhaps. My theory explains that. It doesn’t explain the PH question, but neither – apparently – does McGee. Does he include PH charts that show the correlation?

  20. says

    I believe someone is trying to punk me with this “put eggs in water that’s already boiling” (steam would be hotter) Because, in this one test-run, the eggs blew up immediately.

    Maybe the ATK people get extra super hard-shelled eggs from Whole Foods or someplace – eggs that can withstand being put from the fridge into boiling water.

    If I die from eating too many eggs, it was for science.

  21. Dunc says

    Marcus@#14: My link doesn’t say anything about the pH of the water, nor does it mention baking soda.

    And @#22: you don’t need to know the details of the mechanism to know an effect is real. For example, we don’t actually know how most forms of anaesthesia work in detail, but we know they work.

  22. Pierce R. Butler says

    Purely for the advocacy of deviltry: The chemical composition of eggs obviously varies by breed of hen (= color of shell) and diet (= color of yolk). Sorting out the variation in results from different kitchens using (supposedly) similar techniques may require more consistent inputs – ideally, perhaps, outputs from the same hen.

  23. jacobletoile says

    Marcus @ 21 except they found different levels of pealability with different cooking methods, while shocking them all. If the shocking was what was causing them to peel easily they wouldn’t see a difference in peelability across different cooking methods. Next time you boil water for eggs put a glass rod in with them, compare when the bubbles form on the eggs to when they form on glass. I suspect they will be very close to the same.

  24. jacobletoile says

    Sorry to double post, I have never used already boiling water, but I have used the steam method several times and never exploded an egg.

  25. says

    So, on the hypothesis that adding baking soda to the water (Per Dunc@#2) I make the following observations:

    Egg boiled 13 minutes in blue dyed water, then rinsed clear in clean water:

    Experimental setup:

    Peeled, it reveals no blue

    ONE possibility is that boiling the eggs with baking soda encourages water to perfuse out of the egg because the baking soda’d water is denser and there’s a diffusion gradient. That would amplify the tendency (proved with my 1st green dye experiment) for eggs to suck water back in through the shell while they cool. I do not consider this proof that I’m right, but I’ll note here that it is consistent with my theoretical framework for egg-boiling.

    I don’t consider this a disproof of the PH theory, but since we observe that the dye doesn’t go into the eggs we can probably assume the sodium bicarbonate doesn’t either. So, sodium bicarbonate would presumably be a no-op except for as noted above.

    Boiling the eggs in strong brine would probably have a similar effect if diffusion is what’s going on – and it’d be easier to taste minute quantities in the eggs (though the dye is very fine and quite obvious, see my green eggs experiment)

    By the way, putting food dye in a pan of cool water and bringing up the temperature looks pretty darned neat. I will have to try it with a glass beaker!

  26. says

    One of these was boiled from cold water to boiling for 10 minutes.
    The other was lowered into boiling water with a spoon for 10 minutes.

    I don’t see a difference in the surface texture of the eggs. Obviously, this is N=1.

  27. says

    Dunc@#31:
    You’re right. I got confused between comments about PH and another posting I read when I was researching the ATK bit.
    I’m sorry! I really screwed that up.

    I guess I could boil an egg in borax… I have some downstairs. YUM!

    Edit: This is the posting that confused me. It also references McGee, which is probably how I wound up conflating everything. Again, I’m sorry.

    No, wait! Does this mean I’ve been “peer reviewed”?!

  28. says

    jacobletoile@#28:
    Sorry to double post, I have never used already boiling water, but I have used the steam method several times and never exploded an egg.

    A couple possibilities:
    One is that when you put an egg in a container full of steam, the temperature is going to drop very rapidly.
    Two, it hasn’t got the thermal mass of a pan full of boiling water. So maybe what they’re doing is a rapid rise in the egg’s inner temperature that’s not enough to make it explode.
    Three, my eggs may have cheap thin shells and I should be using farm raised hand polished organic eggs like I am sure the folks at ATK use.
    Four, the two eggs I used may have had unusually weak shells.
    Five, I may have placed them in the pan too hard (I lowered them with a spoon but maybe I suck)

    PS- double postings are entirely acceptable under this blog’s commenting policy.

  29. says

    jacobletoile@#27:
    Next time you boil water for eggs put a glass rod in with them, compare when the bubbles form on the eggs to when they form on glass. I suspect they will be very close to the same.

    I have one of those! Good suggestion; I will give it a shot. Next time I cook an egg.
    Which won’t be for a while since I’ve got some travelling coming up.

  30. says

    For the record, I ‘hard boil’ eggs in my steamer, and I’d never, ever go back to water again. Put the eggs in my steamer basket, turn it on, pull the eggs out when done, cool them, and they peel beautifully, every time, regardless of age.

  31. says

    Caine@#35:
    Put the eggs in my steamer basket, turn it on, pull the eggs out when done, cool them, and they peel beautifully, every time, regardless of age.

    Cool them how?

  32. RationalismRules says

    @#23 To avoid eggs ‘blowing up’ when placed into boiling water you prick them at the less-pointy end with a pin (I thought this was standard practice until I encountered this thread). The explanation I’ve heard is there’s a pocket of air at that end, and the pinhole allows it to escape, avoiding explosions!

    [Apologies if this has already been covered, I only skim-read the thread – I like both eggs and science, but not that much…]

  33. kestrel says

    I had no idea just how vitally important this particular science was!

    Never before tried to get all sciency about hard-boiled eggs. It has always been my experience that if I try and hard boil eggs that are just a few days old most of them won’t peel well no matter what magical method I use to cook them; but if the eggs are at least 10 days old, more and more of them peel very easily and the number goes up the older they are. I typically use 2-week-old eggs for hard boiling. If people buy eggs from the store, it would be difficult to impossible to know what day the egg was laid. I’m just lucky enough to have my own hens, so the date the egg was laid is something I can know about.

    I’ll have to try these various methods. Nice having them all on one page like this!