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Separating eggs without breaking the yolk isn't one of those problems that keeps me awake at night. Nevertheless, there are occasions where I can't get a single damn yolk to hold together and other times when I can do anything short of play a round of tennis with 'em.

Could the freshness of the egg determine how likely the yolk is to break? Or maybe the temperature of the egg?

Generally the problem comes not when I crack the egg, but when I start to transfer the yolk from shell to shell.

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5 Answers

up vote 11 down vote accepted

Yes, the freshness is the factor. In the US eggs are sold in three grades: AA, A, and B (rare). The grading is based primarily on age. AA are the freshest, and B the oldest.

Here is a diagram depicting the internals of an egg:

egg

The characteristics of the freshest eggs are:

  • A large thick albumen (white)
  • A small thin albumen
  • A sturdy thick chalaze
  • A small air space
  • A sturdy round yolk when lying flat

As the egg ages the following things happen:

  • Thick albumen breaks down, getting smaller
  • Thin albumen gets larger
  • Chalazae degrades getting thinner and weaker
  • Air space increases
  • Yolk membrane weakens, when cracked it lies flatter
  • Embryo may become visible as a red speck

As a result of the weakening membrane the yolk is indeed easier to break. Michael's suggestion to use your fingers to separate the eggs is spot on. The edge of an egg shell is a little too risky for reliable separation of eggs.

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3  
If you want to check if old eggs in your fridge are still safe to eat you can place them in water and see if they float. If they float to the top, the air space is large and they are no good. If they stay on the bottom or lay flat on the bottom you're good to go. –  Rachael Wentworth Sep 6 '10 at 5:51
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I had to search around a bit to figure out why I had never seen anything other than a grade 'A' egg. In Europe it seems we only have two grades 'A' and 'B'. The relevant standards say '(i) Grade A eggs...should have a “normal, clean and undamaged” shell and cuticle; they will not be washed or cleaned before or after grading, and will be not chilled or treated for preservation.” (ii) Grade B eggs, i.e. eggs “which do not meet requirements applicable to eggs in grade A” may only be used by the food or non-food industries.' So that's a relief. –  Chris Steinbach Sep 8 '10 at 21:30
    
Regarding Rachael's advice, I tried this with one egg I bought today and another that has been in my fridge for over a week. They both sank like stones. The older egg was one from the batch where the yolks broke, so this isn't a good indication of how well the yolks are going to hold. –  Chris Steinbach Sep 8 '10 at 21:37
    
@Chris: I think she just intended it as a supplemental tidbit of safety info. –  hobodave Sep 8 '10 at 21:54
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Sometimes the sharp edge of the second shell can cause a break. Try using your hands instead of the shell to do the separation. Just pour the whole egg in your hand and then slowly open your fingers to let the white go through.

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I use my hands, too, but I put it in a bowl, and then reach in and grab the egg, and let the whites run back through my fingers. –  Joe Sep 6 '10 at 12:27
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As others have told already, the fresher the egg, the easier to manipulate.

European regulation calls for 28 days for the 'best before' date. If they are sold within nine days after laying, they are called 'Extra'. They cannot be sold after 21 days after laying.

So, look at the date on the box and choose the freshest eggs.

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As mentioned the freshness of an egg determines the strength of the yolk. Try buying local, as it doesn't need to travel as far to get to your grocer. Also a trick is to store your eggs pointy side down in the carton, this keeps the interior of the egg in good shape. As you see hobodave posted a diagram. Also stop storing your eggs in door of your fridge, the blast of warm air every time you open your fridge effects the eggs more than you'd assume.

Also, one thing I learned when handling food. Use your hands, just be clean about it. Don't bother with fumbling the yolk from shell to shell. The shell is hard and pointy and will easily break your yolk. I prepare french dishes often, and I know the importance of having no white in my yolk and no yolks in my whites. I simply use my hands and im able to separate the yolk and whites completely by letting the whites slip away between 2 fingers.

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Well, I have been raising chickens for a year and the age of the egg doesn't mean a thing. It is now winter (5 February) and almost all egg yolks break when cracked so I'm saying the cold does it. During the summer, yolks break very seldom.

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I won't up- or downvote, but while I disagree with the first sentence, the second one sounds plausible. Temperature really has irreversible effects on egg yolk (if you have ever had an egg freeze and tried to thaw and use it, you know what I'm talking about). It would be great if we could hear more info on this theory, maybe even rigorous data as opposed to the observation of one hen farmer. But I think it is a promising lead. –  rumtscho Feb 6 at 14:50
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