Adding vinegar softens eggshells, according to Juan Silva, a professor of food and science technology at Mississippi State University. “The eggshell is made of calcium carbonate … and adding vinegar to the water will dissolve some of the calcium carbonate,” Silva says. The result is a softer shell and easier peeling.
A number of factors can affect the peeling process, including how the egg has been stored and its age. For instance, older eggs peel more easily. “As time goes by, the albumin [protein] starts to break down and thus upon boiling the shell will come off easier,” says Silva. Another method he recommends is to add sodium carbonate, also known as washing soda, to the water, which will increase the acidity of the eggs. This helps break down the protein that holds the egg to the inside of the shell.
You can also cook eggs in boiling water, then rapidly dunk them in cold water (or a bowl of ice water) and peel. The cooling process allows the inside of the egg to shrink faster than the shell (which is less prone to expansion and contraction) and therefore the egg will not be as tightly adhered. The ice-water step must be done quickly, because the passage of time will allow the egg’s insides to once again expand and reattach to the shell.