Eggs are an excellent source of nutrients. Lipids, proteins, lipids, vitamins, and minerals are found in eggs. Eggs have been associated with a variety of functionalities due to their high protein and lipid content, and are widely used in the food processing industry. It can also be cooked in a variety of ways. Sunny side up, poached, scrambled and hard boiled eggs are popular options. Hard boiled eggs, when cooked properly has firm and tender white, while the yolk is bright yellow. But this is not always the case. Sometimes, you may end up a yolk with a gray to green discoloration that surrounds that yolk. For many, it quite unsightly. But the question is, is green yolk of hard boiled egg safe?
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GREEN YOLK IS SAFE TO CONSUME
Well, the short answer is yes, green yolk on hard boiled egg is totally safe to eat. That green discoloration is caused by ferrous sulfide (FeS), a harmless compound of iron and sulfur. The sulfur is from the egg white, whereas the iron is from the yolk. This is why the discoloration only occurs at the interface of the white and yolk. But in some instances, the discoloration affects the whole yolk. It is inevitable to cook boiled egg with green yolk, especially if the egg is relatively old.
The alkalinity of the egg plays an important role in the reaction that leads to the discoloration. The American Egg Board says fresh egg’s pH (acidity or alkalinity) is near alkali level, with values as low as 7.6. But as the egg ages, the alkalinity increases to around 9.6 due to loss of carbon dioxide. The egg white also becomes thinner.
Because of this, chemical reactions are able to take place much easier. One of this is the formation of ferrous sulfide that’s responsible for the ring of green discoloration around the yolk. Boiling eggs for 15 minutes is a certain way to produce it.
Eggs are packed with proteins. These proteins are denatured (unfolding) when exposed to elevated temperature (boiling water). When the protein has unfolded, the cysteines in the egg white albumen protein react more readily to produce hydrogen sulfide (H2S). H2S from the egg white is the source of sulfur of FeS. (According to this study, more H2S is produced with increasing temperature and alkaline pH values).
During boiling of eggs in water, H2S migrates from the white and reacts with iron in the yolk, forming FeS and the green-gray discoloration of hard boiled eggs.
HOW TO PREVENT IT?
As earlier mentioned, chemical reactions are much easier to take place when the egg has already aged. To minimize the occurrence of green yolk on hard boiled eggs, use fresh eggs and boil them as quick as possible.
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Green discoloration on yolk starts forming if the egg has been boiled for 15 minutes. For large eggs, 12 minutes is the maximum time you should allow. Most cookbooks say 10 to 12 minutes is the ideal duration for large eggs, and 8 to 10 minutes for medium size ones.
Green discoloration also appears on omelets and scrambled eggs in the form of green patches. This is the result of the same reaction that happens in hard boiled eggs. Acid (typically lemon juice or vinegar) can be added to eggs about half a teaspoon per egg to slow down this phenomenon without altering the flavor.
In breakfast buffets, omelets and scrambled eggs are placed and kept hot under a heat lamp or in chafing dishes.
