When we open a hard eggcan happen to find the yolk covered by a strange green patinano fear, it is a natural process that takes place when we boil an egg without lending attention to the time and cooking methods. AND harmless to healthbut it can alter taste and consistency. During the boiling, the heat causes the denaturation Proteins: their three -dimensional structure opens, in a nutshell it “unroll”, and reorganized into an insoluble network, transforming the egg from solid liquid. However, If the cooking is too longa particular chemical reaction can be triggered: from the denatured proteins of theegg whitecan be released sulfur in the form of sulphide acid (H₂S), a gas that spreads towards the yolk. There he reacts with the iron present, forming iron sulphide (FES), the responsible substance of the gray-green color that surrounds the edge of the yolk. To avoid it, it is recommended Do not exceed 10 minutes of cooking And immediately cool the eggs in cold water.
Because the yolk becomes green: what happens
The main guilty of the color change of yolk is the cysteinean amino acid containing sulfur. During boiling, egg proteins go against a process called denaturation. Under normal conditions, these proteins have one three -dimensional structure well defined, folded on itself spread, where the parties hydrophobic (i.e. not soluble in water) are hidden inside the macromolecule, while the parts hydrophile (soluble in water) are exposed to the outside. When the egg is heated, the thermal energy breaks the weak ties that keep this structure together, causing the opening and loss of the original form: it is as if the protein is “unroll“.
When the album proteins are denied, from cysteine, sulfur is freed in the form of sulphide acid (H2S), a gaseous molecule with a classic pungent smell, in short, the typical egg smell. This gas spreading (i.e. moving) towards the yolk, comes into contact and reacts with the iron which is naturally present inside: from this reaction it is formed I knowiron lfuro (FES), a gray-green substance that gives life to the famous greenish halo that we can see near the yolk, once the boiled egg was cut. This summary reaction is endothermictherefore favored by prolonged color, and can be schematized with the following chemical equation:
H2Sort (sulphide acid) + Fe (iron) + heat → Fes (iron sulphide) + H2 (molecular hydrogen)
The iron sulphide that comes to form it is not toxic or in any case dangerous for human health, but can slightly change the taste of the egg and give it one more pharynous consistency or dry in the affected area.
How to avoid it (and obtain perfect hard eggs)
To avoid the green halo, just a little attention in times and in cooking methods. According to what reported by the BBC, You don’t have to exceed 10 minutes of boiling. Later, it is useful cool eggs immediately in the water, to block the formation of H2S, which is formed through an endothermic process, that is, favored by high temperatures: If we cool the eggs, lower the temperature and avoid the formation of this gas. In this way you should get a yellow yolk and without unwelcome greenish shades. So no, if the yolk becomes green it is not a blow of the egg that has “gone bad”, but it is the fault of the chemistry … and of a somewhat too long cooking.
