Today’s post will be focused on the biochemistry of cooking, AKA molecular gastronomy. I never realized how much research exists behind cooking. Check out Gastronomica: the journal of food and culture for some interesting food articles. Another great source I found was the book “On Food and Cooking: The Science and Lore of the Kitchen” by Harld McGee. Lastly, the Bible of biochemistry, “Fundamentals of Biochemistry: Life at the Molecular Level” by Voet, Voet and Pratt will be referenced.
There is a lot about eggs to study: Why do chicken eggs have hard shells? What makes eggs different colors? How do the egg yolk and white differ in terms of nutritional content? What happens when you cook an egg? We will focus on this last question today, specifically, the cooking of egg whites. Here’s a fun fact: a search on PubMed today for “egg white” resulted in 5,280 articles!
The basic principle behind the cooking of egg whites is the denaturation, or unfolding, of egg white proteins. When the proteins are properly folded, egg whites look clear, like this:
Proteins fold due to the properties of protein subcomponents called amino acids. Amino acids are the building blocks of proteins. Some amino acids do not like water; this is called hydrophobicity. Other amino acids are attracted to water; that is, they are hydrophilic. The technical term defining a protein’s hydrophobic and hydrophilic tendencies is called hydropathy. When in water, the protein will conform itself so that the amino acids with hydrophobic, or “water fearing” characteristics are tucked inside the protein. Meanwhile, the amino acids with hydrophilic, or “water loving” tendencies will orient towards the water. Other factors that determine protein stability are electrostatic forces and disulfide bonds. We will talk about disulfide bonds in a little bit.
When egg white proteins are heated, they start to unfold. Next, the unfolded proteins start to clump together, forming a network of protein. This is what coagulates, or solidifies, the egg white, making it look something like this:
A variety of forces can cause proteins to denature, including heat, pH, detergents, and chaotropic agents. We denature our egg white proteins by heat (For a cool video on the denaturation of egg whites using acid, click here. I suppose it would taste pretty normal once you rinsed off the acid, but I wonder about salmonella…). Egg whites contain a variety of proteins, the top three being ovalbumin at 54%, ovatransferrin at 12%, and ovomucoid at 11%. Each of these proteins denatures at a different temperature: ovalbumin at 84.5 degrees Celsius (about 184 degrees Fahrenheit), ovatransferrin at 60 degrees (140 degrees Fahrenheit), and ovomucoid at 70 degrees (158 degrees Fahrenheit).
As I said earlier, once egg white protein denatures, the proteins aggregate together, changing the egg white from a clear, viscous liquid to a white solid. I would assume that the proteins are driven to each other because the hydrophobic amino acids are exposed to water and don’t like it, but I could be wrong. The type of bond that the proteins form with each other are disulfide bonds, as determined in this paper by Mine et al. Disulfide bonds are formed between the thiol groups of cysteine residues, a component of certain amino acids.
If you heat your eggs to 84.5 degrees, the melting point of ovalbumin, the egg whites will solidify and become rubbery. So apparently, we don’t like all of our ovalbumin denatured in our eggs. The recommended temperature to cook egg whites is about 70 degrees, which would denature some, but not all of the egg white protein. Mathematician Douglas Baldwin has done a lot of studying behind the science of cooking an egg perfectly. In fact, he figured out the math behind cooking an egg to have slightly firm egg white while keeping the yolk liquid, which you can read here if you so desire.
J. Kenji Lopez-Alt also has a great post at seriouseats.com in which a bunch of experiments were performed with eggs. This picture taken from the post shows what happens to an egg when you lower it into boiling water:
I’d like to leave you with a photo of my absolute favorite breakfast food: Eggs Benedict 🙂
Thank you for stopping by my blog, and have a wonderful day!