Eggs sometimes crack when lowered into boiling water, but thermal shock is not the cause.
Thermal Shock Does not Cause Cracking
Many people hard-cook eggs by putting them into already-boiling water. The temperature shock, it is claimed, causes the shell to expand quickly and crack, resulting in stringy and unsightly egg white leaking out. But while this theory seems to make sense, it is not true.
First of all, an expanding shell is not likely to crack because it is getting bigger and relieving pressure on the contents. Yes the contents expand also, but the shell, being on the outside, heats and expands first. A shrinking shell is more likely to crack because of increased pressure on the contents. In this case, an egg could crack when it is done cooking and is transferred from the boiling water to the cold water bath. Of course, by that time, cracks do not matter.
Why do Eggs Crack?
So, why do some eggs crack when you boil them? One reason is that some eggs already have invisible cracks in them from rough handling during production and shipping—or if you dropped them into the pan instead of gently lowering. Existing cracks will indeed open up in hot water and release egg white. In some cases, if the water is actively boiling, the eggs will be bounced around and cause new cracks. Thus, if you must boil eggs, use gently simmering water.
A Demonstration
I once saw Julia Child demonstrate this on her TV show. She took a bunch of raw eggs from an ice water bath and put them directly into simmering water, and not one of them cracked. Pretty convincing! This has also been my experience, although like many other cooks, I prefer the steaming method these days. Please refer to this page for steaming instructions.
Best Practices to Prevent Cracking
Bringing your eggs to room temperature ahead of time will not prevent cracking, although it does no harm. In my experience, starting the eggs in cold water does not make any difference in cracking. Likewise, adding salt or vinegar to the cooking water will not prevent cracks, although it may reduce the escape of egg white if a crack does form. The best things to do are to inspect your eggs for cracks ahead of time, be gentle when lowering them into the pan, and cook at a simmer and not an active boil. Many cooks, myself included, think that the steaming method gives fewer cracked eggs.
If you need perfect eggs, for example if you're making deviled eggs, the best thing to do is cook a few extra. If a few crack you can always use them for egg salad!
Some More "Eggy" Myths for You!
Brown eggs and nutrition.
All eggs are not nutritionally equal!
