From a chemical point of view, the yolk contains 50% water, a third of lipids, such as lecithin and cholesterol, and 15% protein. For its part, the egg white is essentially water with a tenth of protein. In short, and to a first approximation, the egg is nothing more than water and proteins , macromolecules similar to long threads folded and refolded and built from smaller ones, amino acids, like the links of a chain. The interesting thing is that each one has its behavior when it comes to cooking. Understanding the changes in the proteins in eggs when they are cooked, whisked, or mixed with other ingredients is essential to understanding the role eggs play in cooking.
What happens if we want to make some hard-boiled eggs, or soft-boiled? As the temperature rises, the water in the egg heats up until it begins to boil. Meanwhile, proteins unfold as they collide with surrounding water molecules, breaking the weak bonds that held them folded. These “unkempt” proteins collide with one another, binding together to form a stringy network . If we cook the egg long enough, we will see that the white goes from translucent to opaque: it is the protein network that is visible to the naked eye. If the temperature jumps over a hundred degrees, the water evaporates and the egg hardens.
cook eggs
Of course cooking is not an easy thing. One of the problems is that the yolk coagulates 8 degrees above the temperature at which the white does. When 60 degrees are reached, the proteins in the egg white absorb the energy communicated by the fire and prevent the temperature from rising and the yolk from doing the same. It is the same thing that happens when boiling water: the temperature does not exceed 100º as long as there is liquid in the saucepan. Thus, the famous 3 minutes of soft-boiled eggs correspond to the time in which the white protects the yolk from cooking: after 4 minutes in boiling water, the temperature has risen the 8 degrees it needs to coagulate.
Now, if we want to make hard-boiled eggs, we cannot go too far when cooking them. Because if we do , the proteins in the egg white that contain sulfur atoms end up releasing hydrogen sulfide , which dyes the yolk green. And let’s remember that this molecule is responsible for the well-known smell of rotten eggs…
Meringue or soufflé?
The thing changes if we want to make a soufflé or a meringue. In this case, the beating action is the mechanism we use to incorporate air into the water and protein solution that we have converted the egg into (it can increase its volume up to eight times!). By introducing air bubbles we achieve the same as heating it: making the proteins unfold . But what happens next is different. To understand it we must take into account a basic fact about the amino acids that make up proteins.
In the same way that there are people who like football and there are others who hate it, the same thing happens to amino acids with water: some love it, they are hydrophilic, and others hate it, they are hydrophobic. When the protein is folded, the hydrophilic amino acids are on the outside, near the water, while the hydrophobic ones are packed in, away from the water. The appearance of bubbles in the beaten egg -and where we have the uncoiled proteins- causes the hydrophilic part of the protein to remain immersed in the water while the hydrophobic part puts its head in the air of the bubble. And, just like with hard-boiled eggs, the proteins begin to form a tight network, this time around the bubble . When we put our soufflé in the oven, the heat causes the expansion of the gas locked inside. If we do it right, the protein network solidifies under the effect of heat and the structure does not collapse when the bubble bursts. If not, the soufflé sinks.
Reference:
This, H. (2010) Kitchen Mysteries, Columbia University Press
