If you’ve been following my cooking and chemistry blog series, I hope you’ve enjoyed recaps of interesting lessons I learned from HarvardX’s course. For my last post in this series, I’m digging deeper into the science behind cooking with eggs, one of the most beloved and versatile foods.
In the course, we covered how at different temperatures, different proteins in the egg denature, which means the proteins experience modifications to their molecular structure. To imagine protein denaturation, think ramen noodles. They come tangled in a bunch, and when you put them in hot water, they start to uncoil. The same happens when proteins are exposed to heat.
For eggs, this creates those different and yummy consistencies desired in the kitchen. I wanted to confirm the protein denaturation in eggs with my own eyes by putting it “under the microscope” — in this case, by actually seeing the proteins change. To do this, I used Tycho, which I’ll explain more below.
Why eggs are fascinating
Before I talk about my Tycho experiment, let me share why eggs are fascinating. The egg is a complex biological structure. Chicken eggs are considered one of the most nutritious and versatile human foods. Eggs are a rich source of protein, lipids, vitamins, and minerals. They also have many functional properties, such as foaming, emulsifying, and a unique color and flavor, which make them important for cooking different products. An average egg weighs approximately 57 grams (2 ounces) and consists of approximately 10% shell, 60% white (tight and loose, which are only differentiated by their water composition), and 30% yolk.
The temperature scale for perfect eggs
Eggs also act like a thermometer. In a very small window of temperatures, between 135 °F and 158 °F, the egg transforms from raw to totally cooked. Poached, Benedict, boiled, and other textures are produced when the egg reaches different temperatures along that temperature scale and, therefore, a specific set of proteins are denatured.
At 135 °F, the egg is completely raw, however, it’s completely pasteurized and safe to use for meringue, mayonnaise or other recipes that use pasteurized eggs. At 140 °F, it’s just barely set, like a poached egg. At 145 °F, you get the perfect egg for Eggs Benedict. The color has changed on the white, it holds up, but it’s still custardy and soft. At 150 °F, you have a fully set egg yolk, but still extremely soft and custardy, perfect for toast. At 155 °F, the yolk is soft and moldable, and at 160 °F, the yolk is more like marzipan. Finally, at 165 °F, the yolk is like a hard-boiled egg but doesn’t have any sulfurous aroma, and there’s no greening. Yum!
Slow cooking confirms optimal temperatures for cooking eggs
In the course I took, eggs were cooked at the various temperatures mentioned above using the sous vide method. Sous vide is a low-temperature cooking technique that uses a very precise temperature-maintained water bath in which food is vacuum sealed in a bag and kept in the bath for longer periods. That way, you can ensure a specific temperature will give you the egg consistency you want.
Keeping an egg in a sous vide bath for 40-60 min allows the whole egg to reach the same temperature evenly. When we cook eggs on a pan, the parts that are in contact with the heat will be at a higher temperature than the parts that aren’t.
How to confirm proteins have denatured
Okay, cooking eggs at different temperatures is all well and delicious, but, as I mentioned, I wanted to confirm the proteins were denatured. I used the NanoTemper Tycho instrument to measure the protein integrity of the different parts of an egg.
For my three samples, I used the egg yolk, the egg white, and a boiled egg white. I then mixed each of the separate egg components to make them more liquified. Finally, I loaded 2 capillaries per component (six capillaries in total), and I placed them in the Tycho instrument.
Tycho measures the protein integrity by applying a fast temperature ramp rate and measuring the intrinsic fluorescence of the amino acids that compose the proteins. Depending on the chemical environment of these amino acids, the fluorescence will be different. When a protein has gone through stress (by heat, changes in the acidity of the solution, degradation by other proteins, etc.) it will unfold. Simply, Tycho identifies the unfolding by comparing the stressed to the non-stressed protein.
And, in as little as 3 minutes, you obtain the results!
What the results show
Looking from left to right, the graph shows you what’s happening as a protein is going from folded to unfolded. Along the x-axis, you can see the thermal ramp applied. The y-axis shows you changes in intrinsic fluorescence during the defined temperature treatment. Finally, the temperature at which a transition occurs is called the inflection temperature (Ti), which are the vertical lines you see in the graph.
As the results show, even though there are many proteins in the egg, most of the proteins begin to denature after 60 °C (or 140 °F). That’s your poached egg!
You can also confirm that there are different proteins in the egg white and the yolk. Each peak on the graph, or the increase in the wavelength ratio, corresponds to a domain unfolding. Most of the proteins are composed by one domain, which you can see with each peak. When you have a mix of proteins in a sample, you’ll see a peak for each protein that is in the mix.
Also, with the boiled egg sample (“boile”), you can confirm the proteins have already been denatured since there are no inflection points. When a protein is folded, you can measure the transition between the folded and unfolded one, but when the protein (or mix) has already been under a lot of stress and has been denatured (as in the case of a boiled egg white), there is no transition.
Why you should test your proteins (in or out of the kitchen)
I love when lab instruments can be used in the kitchen and vice versa. After all, the kitchen is also a lab where many of us experiment. Although I looked at proteins in eggs, Tycho can give you information about any protein’s functionality, purity, concentration, and similarity. Before having a Tycho, I used to do many experiments that failed because of poor protein quality. After long purifications and processing samples, it was frustrating to see that there were no differences between my samples. After Tycho, I can test my protein during purification, saving me many headaches later.
Thank you for following my cooking and science adventures! I’m always whipping up something in the kitchen (and the lab), so you may hear more from me.
E. D. N. S. Abeyrathne, H. Y. Lee, D. U. Ahn, “Egg white proteins and their potential use in food processing or as nutraceutical and pharmaceutical agents—A review”. Poultry Science, 92 :3292–3299 (2013) http://dx.doi.org/ 10.3382/ps.2013-03391