Have you noticed that the outer shell of boiled yolk sometimes changes color - and the color can vary from greenish to different shades of purple? However, the color is perceived by each person individually, and here it is easier to say that the yolk simply changes color, without specifying it.
Everyone probably knows - this happens if you digest an egg.
But what's interesting - sometimes the yolks change color, and sometimes they don't, and at the same time the cooking conditions - that is, the temperature regime and duration - are the same? And sometimes the yolk gets a strange smell.
Such a smell seems to me bad (but this is my perception), therefore sometimes there are suspicions about the freshness of the eggs ...
But this, it turns out, is not so!
Both the color change and the appearance of smell are quite natural chemical process during cooking. Oh how. Did you know?
During cooking under the influence of high temperature, the reaction of hydrogen and sulfur in the egg white begins. The speed of this reaction depends not only on temperature and time, but also on the percentage of sulfur - therefore, if the egg is not very rich in it, then the reaction will go slower.
Sulfur and hydrogen will produce hydrogen sulfide, a gas known to smell bad. Hence the smell may appear.
And hydrogen sulfide, in turn, reacts with iron in the yolk - and already forms iron sulfides - it is they that change the color of the yolk shell at the junction with the protein.
All this is in minimal quantities, therefore it does not affect the edibility of the egg, however, the type of product can be seriously spoiled.
By the way, the best way not to spoil the egg is not to boil the eggs directly from the refrigerator, but to send eggs at room temperature into water at room temperature.
And do not put them in a bunch in a pan, but lay them only with the bottom layer so that the water completely covers them and is only a couple of centimeters higher.
And cook - only until boiling, then turn off the heating and leave the eggs "to rise in hot water." Thus, the temperature inside the egg will not reach the point at which a violent reaction of hydrogen with sulfur will take place.
It is also advised so that the color of the yolk does not change, immediately after boiling, send the eggs into cold water, but I noticed that this method does not always work.