DEAR HEALTHY PROFESSOR: Why does a greenish-black ring sometimes form around the yolk of hard-boiled eggs?
ANSWER: Preparing hard-cooked eggs may not be rocket science, but producing an aesthetically pleasing egg is a matter of science.
A chemical reaction occurs between hydrogen sulfide gas in the egg white and iron in the egg yolk. When an egg is boiled, the internal egg pressure increases and pushes the hydrogen sulfide gas from the white toward the yolk. The hydrogen sulfide combines with iron in the yolk, producing a greenish-black compound called ferrous sulfide along with the characteristic hard-cooked egg odor.
The discoloration is not harmful, just unappealing, and there are ways to prevent it. First, hard-cooked eggs should be simmered, not boiled. This prevents the eggs from becoming too rubbery and keeps the internal egg pressure to a minimum, preventing ferrous sulfide production. Cooking too long also contributes to the greenish-black ring, so simmering for 15 minutes should be enough. Consult a cookbook for different cooking methods.
After the eggs are done, immersing them in cold water will reduce the pressure inside the egg and will minimize the sulfur gas from migrating toward the yolk. Once you have a perfectly prepared egg, you can enjoy its nutritional goodness.
An average large egg contains only 75 calories and 7 grams of high-quality protein. Eggs are bountiful in vitamins and minerals. Egg yolks are one of the few foods that contain all the fat-soluble vitamins – A, D, E and K. The egg whites also have significant levels of B vitamins. Eggs are a good source of zinc and other important minerals.
Because of the relatively high cholesterol content in the yolks, the American Heart Association advises us to limit egg yolk consumption to no more than four yolks per week. However, many people healthfully enjoy fresh eggs more often by using mostly the egg whites with one yolk added for a more flavorful and colorful dish.
The best egg dishes are made with fresh eggs. Short of living on a farm, how can you be assured your eggs are fresh? Since eggs begin to deteriorate after they are laid, storage is important. Since egg shells are porous, eggs will lose moisture rather quickly. They should be stored in the refrigerator in their carton to retain moisture and to keep them from absorbing other food odors.
As an egg ages, moisture and carbon dioxide loss will cause the egg's air cell to enlarge. A fun way to check if your eggs are fresh is to submerge them in water. A fresh egg will sink. An older egg with a larger air cell will float. (This makes a great science fair project.)
A common question this time of year is if you can eat hard-cooked eggs that have been sitting out at room temperature. The simple answer is no.
With today's knowledge of food safety risks, I am obligated to advise you to eat hard-boiled eggs that have been safely refrigerated.
