Question
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.
Answer
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:
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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