This was supposed to be posted similar time with the post on how to make perfect hard boiled eggs, but I never got to finishing this, till now.
I attempt to answer some mind boggling questions about chicken eggs. But not chicken or egg first.
White or brown?
The two most common colours of eggs we see in supermarkets are white and brown. In Singapore, brown eggs are the most common; we hardly see white eggs, except at atas (premium) supermarkets. I used to think that white eggs are better because they usually cost more, and partly because I always see them using white eggs in angmoh cooking shows.
However, there is actually absolutely no difference between brown and white eggs in terms of nutritional values, taste, texture etc.. They are identical except for their colour. Nutritional value of the egg is influenced by the feed the hens are fed with.
(Photo taken from http://hiporganicmama.blogspot.com/)
Colour of eggshell ranges from pink to green to blue. The pigmentation is genetically determined and deposited on the shell during egg formation in the oviduct.
Why does the egg yolk turns greenish gray when overcooked?
(Refer to my earlier post on How to make the perfect hard boiled eggs)
The short version:
The sulphur in egg white reacts with the iron in egg yolk when the egg is heated. They react with each other and *BOOM*, they form ferrous sulphide, which is a greenish black precipitate. Hence we observe the gray coloration on the egg yolk.
The (very) long version:
I actually found a research article written in 1920 on “The Formation of Ferrous Sulphide in Eggs during Cooking” (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1258902/), and I was very excited when I found this article (sorry, it is part of my occupation). The experiments conducted by the researchers were so simple, and the results explained the strange phenomenon. This is why I always love classic research papers. They demonstrate how science can be so simple and elegant.
Almost a hundred years ago, scientists have noticed this strange phenomenon and it puzzled them. Quoting the part on what would know be known today as the ‘introduction’ of the article:
It is a matter of common experience that on prolonged cooking of an egg in its shell a greenish black coloration is produced on the surface of the yolk.
It is also well known that if a so-called “hard-boiled egg” be immersed in cold water immediately after cooking, the green color is either not apparent at all, or is much less marked than is the case when the egg is allowed to cool slowly.
In this article, they identified the culprit that gives the greenish black coloration – ferrous sulphide. Through a series of experiments, they discovered that some sulphur compound in the egg white decomposes when heated, producing hydrogen sulphide gas. This gas then reacts with the iron in egg yolk to form ferrous sulphide.
Interestingly, although both the egg white and yolk contain sulphur in similar amounts, only the egg white produces hydrogen sulphide.
They also found that soaking the egg in cold water immediately after cooking simply prevents the formation of ferrous sulphide, by stopping the decomposition of sulphur compound in the egg white. If your egg was overcooked to begin with, soaking in cold water is not going to help.
Minikin's Lair 