OK, so I can’t fry an egg without breaking the yolk. So why does the yolk harden so quickly once broken, but stay runny if unbroken?
More surface area exposed to heat when broken.
Most likely because once the yolk sac is broken the yolk can spread out and greatly increase it’s surface-to-volume ratio. compact shapes, like an intact yolk take longer to heat through, because their surface area is small with respect to the volume.
It’s not difficult to crack an egg without breaking the yolk. Tap the side of it in the edge of your pan just hard enough to crack the shell, then finish breaking the shell apart over the pan. With practice you can do this one-handed and not get any bits of shell in too.
No, I don’t think so. Same happens when I gently prod it with a fork adn it doesn’t run all over the pan.
So you’re saying that piercing the yolk sac seems to cause the yolk to harden more quickly than not piercing it? I’ve never heard of this before and I find the suggestion somewhat dubious; have you observed this phenomenon in one of several eggs in the same pan (where the poked egg hardened but the rest remained liquid), or do you cook your eggs one at a time?
Egg yolks are about 50% water, with the other 50% being mostly water soluble fats and proteins.
Remove the water and the fats and proteins will react to form a solid. This will happen even without heat (you can make a pretty good paint from egg yolks).
The yolk sac retards, but does not completely prevent, the evaporation of the water in the yolk. Breaking the sac or poking holes in it allows the water to evaporate more quickly.
Thank you, Zigaretten!