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Offline Quantumcat

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Boiled eggs
« on: 23/05/2003 11:08:47 »
Why does a freshly boiled egg that has been allowed to cool over a long period of time develop a grey yolk, and the yolk of one that has been forced to cool very rapidly maintains a nice healthy yellow colour? I could make a few guesses but I don't really know ....


 

Offline NakedScientist

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Re: Boiled eggs
« Reply #1 on: 23/05/2003 23:44:58 »
An interesting observation and certainly one I've never experienced, not being a frequent egg boiler and cooler !

I'll be the first to admit that I don't know the answer. I'll hazard a guess that the grey discolouration is some kind of temperature-dependent reaction between the components of the yolk, or the sac which contains it, and the surrounding albumen (white).

Rapidly cooled eggs don't spend enough time at the required temperature for the reaction to occur and hence they are not grey.

Massive cop out, but the best I can do !

TNS
« Last Edit: 23/05/2003 23:46:19 by NakedScientist »
 

Offline Quantumcat

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Re: Boiled eggs
« Reply #2 on: 24/05/2003 03:27:41 »
Oh .... darn! oh well :-p
 

Offline Donnah

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Re: Boiled eggs
« Reply #3 on: 24/05/2003 17:19:35 »
I thought eggs turned grey when they were over cooked.  Maybe when they take a long time to cool, they get cooked enough to turn grey.
 

Offline Quantumcat

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Re: Boiled eggs
« Reply #4 on: 26/05/2003 05:13:20 »
I'm fairly sure that's not right, I'm quite sure that eggs put in the fridge or just left in open air to cool slowly will go grey and eggs run under water or put in a bowl of water with ice in it will stay yellow. Actually, come to think of it, I'll do an experiment tonight:D

I'll put two boiled eggs to cool at room temperature, one overcooked one cooked for normal time, two boiled eggs will be put in a bowl of ice water, one overcooked the other cooked normally!

I'll let you know of the results when I get them:)
 

Offline Donnah

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Re: Boiled eggs
« Reply #5 on: 26/05/2003 18:11:19 »
Great...then we'll know egg-xactly how to prevent going grey!
 

Offline cuso4

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Re: Boiled eggs
« Reply #6 on: 26/05/2003 20:52:14 »
Why preventing egg yolk going grey? Is this a bad thing?

Angel
 

Offline Donnah

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Re: Boiled eggs
« Reply #7 on: 26/05/2003 23:01:07 »
I've eaten a lot of grey eggs and I'm still alive.  Maybe it's important to good cooks and discerning palates.
 

Offline Quantumcat

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Re: Boiled eggs
« Reply #8 on: 27/05/2003 02:21:14 »
I don't think they taste any different, but it looks really gross, I think, as if it was rotten.
 

Offline chris

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Re: Boiled eggs
« Reply #9 on: 27/05/2003 17:38:58 »
So, Quantum, what was the result of the experiment ?

Chris

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Offline Quantumcat

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Re: Boiled eggs
« Reply #10 on: 27/05/2003 22:50:20 »
Fast-cooled normal cooked --> Yellow yolk, the white is a soft texture

Slow-cooled normal cooked --> Grey yolk, the white is a soft texture

Fast-cooled over-cooked --> Yellow yolk, rubbery hard texture of the white

Slow cooled over-cooked --> Grey yolk, rubbery hard texture of the white
 

Offline chris

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Re: Boiled eggs
« Reply #11 on: 27/05/2003 23:02:02 »
I presume that all of the eggs were similar and that grey yolks are not down to a certain type of egg ? Did you randomise the eggs before you started ?

That aside, looks pretty conclusive. Looks like we could have an international experiment on our hands here - Donnah, any chance you could repeat Quantum's experiment with some Canadian eggs ? Exodus can see what virgin (islands) eggs are like (almost certainly unfertilised) and I can see what happens with english eggs !

Quantum, can you spell out your experimental protocol so we can try it ?

Yours,

Chris

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Offline Quantumcat

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Re: Boiled eggs
« Reply #12 on: 28/05/2003 01:28:59 »
Okay, let's see ....

Well, I took out four eggs from their cardboard box. I put on a pot of water to boil. When it was boiling, I put the four eggs in. I let them cook for 20 minutes, I think it was. I took out two, left the other two in. I put one of the eggs on a plate on the kitchen bench, the other I put in a bowl and ran the tap over it for about five minutes, but it was still warm, so I filled the bowl with icecubes and water and let it sit on the bench. The other two eggs kept boiling for another half an hour, I think. After the half an hour I took them both out. I placed one on the plate with the other egg and marked a black cross on it so I knew which one was overcooked. I ran the other under the tap for five minutes like the other one. I dried it a little and marked a black cross on it too, then put it in the bowl of ice water with the other. I left them all sitting there overnight to make sure they were definitely completely cooled. I peeled them and sliced them all in half, recorded my results, and ate the normal cooked ones ... the other two I gave to my dog. He quite enjoyed them ... nice and chewy:)

Maybe I'll do it again to see if I get the same results ^_^
 

Offline chickadeee

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Re: Boiled eggs
« Reply #13 on: 31/05/2003 21:45:41 »
Dont know if this has anything to do with answering your question but its not often you get to share your boiled egg knowledge with people.

Eggs go hard by breaking and then reforming the non-covalent bonds between the amino acids which make up the protein.
When you heat an egg the energy gained breaks the bonds between the amino acids, the amino acids change structure and as the temperature increases enough energy is gained to form more covalent bonds with the other protein molecules. As these bonds are formed the water that surrounded each protein molecule is forced out which makes the egg go hard.

If you use a high heat to boil an egg a reaction occurs between the iron in the yolk and the hydrogen sulfide in the egg white. It forms a green iron sulfide film around the yolk.
 

Offline cuso4

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Re: Boiled eggs
« Reply #14 on: 01/06/2003 16:10:27 »
Welcome chickadeee! From what you said, the grey stuff is iron sulphide?

So if cooled rapidly then there isn't enough heat energy for the H2S to react with the iron, the egg yolk stays yellow.

Quantum, this might be the answer to your question!

Angel
 

Offline chris

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Re: Boiled eggs
« Reply #15 on: 01/06/2003 23:03:43 »
I don't agree - quantum tried boiling eggs for long and short periods with the same result. The critical difference was the duration of the cooling. So there must be something else going on here...

Chris

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Offline Quantumcat

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Re: Boiled eggs
« Reply #16 on: 02/06/2003 03:59:05 »
I think what Angel was saying is that if allowed to cool for a long time, the heat allows the H2S to react with the iron, but if cooled rapidly, the iron and H2S stay seperate!

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Offline nilmot

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Re: Boiled eggs
« Reply #17 on: 02/06/2003 08:15:55 »
But when you are cooking the eggs, weren't there a lot of energy for the hydrogen sulphide to react with iron?

Or is that a completely different thing I'm on about.

Tom
 

Offline Donnah

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Re: Boiled eggs
« Reply #18 on: 02/06/2003 18:19:54 »
Canadian results:

Fast-cooled normal cooked --> Yellow yolk with some grey, the white is a little rubbery

Slow-cooled normal cooked --> Yellow yolk with a touch of grey, the white is a little rubbery

Fast-cooled over-cooked --> Grey yolk, the white is a little rubbery

Slow cooled over-cooked --> Yellow yolk with considerable grey, rubbery hard texture of the white

Keep in mind that cooking is NOT one of my higher skills.
 

Offline nilmot

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Re: Boiled eggs
« Reply #19 on: 02/06/2003 21:59:39 »
Err.. they seem to be all greyish, I'm sure there is nothing wrong with your cooking skills Donnah.

I don't know if the age of the egg matters. I want to try it but we need the egg for the sandwiches we bring to school and of course dinner.

Tom
« Last Edit: 02/06/2003 22:34:00 by nilmot »
 

Offline Donnah

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Re: Boiled eggs
« Reply #20 on: 03/06/2003 19:17:38 »
I wonder if the grey is related to the temperature the eggs are cooked at.
 

Offline Quantumcat

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Re: Boiled eggs
« Reply #21 on: 04/06/2003 01:33:03 »
How about we specify exactly the temperature they are boiled at, and how long for, the conditions of cooling and the fact that the eggs should be placed in only when the water is fully boiling away. Otherwise our results will always be different. Also I guess I generalised my own results a bit, I probably shouldn't have done that, but if we can get the results with recorded definite variables the experiment will be more valid ...

also what Tom said we should consider, why doesn't it react when it's boiling? Why does it only react when it's cooling (assuming what Angel said was correct, as seems to be) also I guess the white goes rubbery because the heat makes the water evaporate out of pores in the shell.

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« Last Edit: 04/06/2003 01:35:39 by Quantumcat »
 

Offline ToFishTeacher

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Re: Boiled eggs
« Reply #22 on: 08/07/2003 09:44:37 »
I was just wondering about what makes some boiled egg yolks grey and others stay nice and yellow, so I typed "grey yolk" into Google and here I am!  

I had two boiled eggs today, both were from the same box of raw eggs, put in the same water at the same time, boiled at the same heat for the same length, and removed and cooled together (I don't recall if it was fast or slow cooling).  They were stored together in the refrigerator and then moved to my lunch where they continued to experience the same environment as one another.  

One of the eggs didn't have even the slightest hint of greying, but in the other one, the yolk was grey -- not just on the yolk surface but nearly through and through!  While I bet both tasted the same, I definitely had a grossed-out feeling while eating the grey one. Let's put it this way: it was grey-gross enough for me to look up "grey yolk" on the internet!(It was first opened, so I was surprised when I opened the second one and found it so beautifully perfect yellow.)  

When I make a nice cobb salad for guests, I definitely prefer for the yolk of the boiled eggs to be yellow.  I didn't even think I might have control of the situation, though.  If anybody comes up with a good explanation, I'd really love to learn it!
 

Offline Donnah

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Re: Boiled eggs
« Reply #23 on: 09/07/2003 00:32:53 »
Maybe it's a variable we haven't considered yet; the conditions where the egg was laid.  How long is the egg under the hen, in the collection container, in the egg washer, sitting out to be refrigerated, in the fridge...and what did the hen eat?
« Last Edit: 09/07/2003 00:33:35 by Donnah »
 

Offline bezoar

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Re: Boiled eggs
« Reply #24 on: 14/07/2003 15:02:36 »
I think the age of the egg might really matter.  It seems I have always known that an older egg makes a better hard boiled egg -- doesn't stick to the shell when you try to peel it. To get a control on the age of the eggs, I guess you'd have to find someone who has chickens.  How long had the Canadian eggs been sitting in the refrigerator?  Also, coming out of the chicken, the eggs have an oily coating called the "bloom".  If the bloom isn't washed off, the eggs will stay fresh at room temperature for up to two weeks.  That's why you see these pictures of country kitchens with eggs hanging in the kitchen in a basket.  Anyway, the bloom, which inevitable has some small fragments of feces in it, is washed off before being sold commercially.  Do you think the amount of bloom left on the egg could affect whether or not the yolk turns grey?

Bezoar
 

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Re: Boiled eggs
« Reply #24 on: 14/07/2003 15:02:36 »

 

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