A lot of recipes that call for separating an egg, suggest that the egg be separated when cold rather than at room temperature even if it has to be brought to room temperature afterwards. I was just wondering what makes it easier to separate eggs when cold and if there is a chemical reason or the cells of the egg change under different temperatures? What's the science behind it?
It's a simple matter of viscosity, when eggs are cold the white is less runny. When eggs are warm the white is much more runny.
As for whether it is easier to separate eggs when cold or warm it depends on your method. If you use the shells to separate the white, by cracking them in half and then transferring the yolk back and forth between the halves, then I'd say cold is a bit better because once the white starts to drip out it tends to go as a whole. With warm eggs more white will stay in the shell with the yolk.
However, if you use your hands to separate the eggs, by letting the white drip between your fingers (or using an egg separator which does the same thing), warm eggs work much better because the runniness allows them to flow easier. This method works fine for cold eggs too, but warm is faster.
Of the two methods I prefer using my hands rather than the shell for a few reasons:
A handy trick if you are separating a lot of eggs is to separate each white into a separate bowl, then putting the white into a larger bowl with the rest of them. This is so if you get a bad egg, or a yolk breaks into the white you can discard the egg and keep the rest of them.