Now, those who know me well will agree with me when I say that there are two things that I really enjoy in life – chemistry and cooking! (As an aside, the name for this blog stems from one of two nicknames that my dad used to give me when I was a bit younger, the other being Culinary Chris). Recently I’ve been combining the two and learning more about the chemistry of food, although in this post I’m just restricting it to looking at eggs. I do intend to do some more posts on this area in future, as there’s just so many places I could go with this that it was hard to choose! Here’s just some of the fascinating things I’ve discovered (from here and here).
The science of cooking eggs
The white of an egg (known as the albumen) contains a large amount of protein, specifically a type called globular proteins, so-called because they have a roughly spherical (globe-like) shape. As we apply heat to the egg (like when frying an egg or making an omelette), the extra energy causes these proteins to move around more vigorously, colliding with each other and weakening the bonds between the different protein strands. As these bonds become weaker and weaker, their globe-like structure starts to unfold and the protein becomes “denatured”. These strands of proteins are then free to make these bonds with each other, causing it to coagulate (which is why the egg whites go from transparent to opaque as the egg is cooked). Overcooking creates too many of these bonds, so the proteins join together too tightly and you end up with rubbery eggs.
Whipping egg whites
A similar thing happens when egg whites are whipped. The mechanical action of incorporating air into the watery protein mixture of the egg white also causes the proteins to denature and unfold. To be able to explain this in a little more detail, we have to understand that some parts of a protein are water-loving (hydrophilic) and other parts are water-fearing (hydrophobic). Normally the hydrophobic parts of the globular protein stay tucked inside the structure and the hydrophilic parts are pushed to the outside, so that it can dissolve well in water. However, as we incorporate air bubbles into the mix, when the strands of protein hit the boundary between the air bubble and the watery mixture, the hydrophilic parts will turn themselves around to stay dissolved in the water and the hydrophobic parts will move into the air bubble. As the protein strands unfold, they bond to one another in this new arrangement and the air bubbles become trapped within their structure. The more that the whites get whipped, the more air is incorporated into the mixture and so the proteins can bond with each other more strongly, giving ‘stiff’ egg whites.
Copper bowls and egg whites
If you’ve ever heard the old chestnut that it’s better to whip egg whites in a copper bowl, there’s actually some science behind it! This is because copper ions in the metal bowl migrate into the egg white mixture and combine with one of the proteins called conalbumin. This combination of copper and conalbumin is more stable than the protein by itself, so it is less likely to denature from over-whipping. The whipped egg whites are therefore more stable and they’ll be less likely to deflate (for more detailed information, have a look here). For those like me who don’t have a copper bowl, it’s nice to know that it’s possible to achieve the same effect by using an acid (such as cream of tartar or vinegar), as is used in creating meringue and pavlova.