MadSci Network: Biochemistry |
Hi, Daniel.
I may not be an eggspert, but I will do my best to eggsplain your questions. Alright, no more joking around, it's time to get serious and get cracking with your answer.
What defines perfection in terms of protein, refers to how much protein is bioavailable. If amino acids are wasted, the body can't store their nitrogen. The higher the amount of nitrogen retained, the higher the quality of the protein. By this standard, the egg is the most "perfect" protein food, and is the gold standard to which we compare other sources of protein.
Foods that provide all nine essential amino acids are known as 'complete proteins'. They're essential because the body cannot manufacture them. Both the white and the yolk contain complete protein, so it isn't that one's protein is more "perfect" than the other, it's just that they're different.
You see, amino acids combine in myriad of ways to form proteins. In the case of egg white, its main protein is albumen, whereas livetin and vitellin are specific to egg yolks. Another important difference between the white and the yolk, is that protein makes up almost all of the egg white, apart from its water content, while the yolk is about 70% lipids, again not accounting for water. So you get a lot more protein from egg white than you would from egg yolk. In other words, it's a question of quantity, not quality, of protein that sets them apart.
In addition, there are other nutritional components that favor the yolk, as it provides most of the thiamine vitamin, as well as the Vitamin A and iron of the egg. It even contains some Vitamin D and beneficial antioxidants, such as, lutein and zeaxanthin. These help fight dangerous free-radicals in our body.
So aside from the fact that egg white contains more protein, the yolk wins hands down nutritionally.
Hope that helps,
Peter Bosani.
References: On Food and Cooking - Harold McGee Nutrition Concepts and Controversies - Hamilton & Whitney
Try the links in the MadSci Library for more information on Biochemistry.