The cracks are practically invisible to naked eye but while boiling, they spew out all the yolk into the water creating a lot of mess. I'm wondering if I could test an egg before boiling it, so I could use the cracked ones for making omelettes, for example.
If I think an egg is cracked before I boil it, I make sure to add it to boiling water rather than letting it warm up with the water as it's heating. I give it a quick roll in the water with a spoon to make sure it is heated all round. This cooks the outside of the egg rapidly, sealing the crack immediately. no leakage. When you add a cracked egg to cool water, it has plenty of time to seep out uncooked.
The main reason eggs crack while boiling isn't because they are already cracked, but because of the sudden change in temperature from cold to hot. The same thing can happen to glass or metal, or practically any substance. (I've actually broken a glass coffee table top once by setting a pie pan out of the oven on top of it to cool... after a couple minutes, the top crashed to the ground, giving me quite a start!)
Some suggestions to prevent eggs from cracking while boiling can be found here. A summary is:
- Let the eggs warm to room temperature on the counter for 20 minutes before boiling.
- Put the eggs in the pan before heating the water, and raise the water temperature slowly (i.e. don't set the heat to 'High')
- Add vinegar or salt to the water to help heal any small cracks.
- Cook the eggs slowly.