The biology and chemistry of eggs

Egg yolk is special. Normally, things become food because humans decide to eat them. Egg yolk, however, is designed by nature to be food. It’s food for the chicken embryo. Egg yolk accounts for one-third of the egg’s weight but carries three-quarters of the calories. It’s rich in fat and protein. When the proteins are heated, they denature and then coagulate, forming an interconnected network that solidifies the yolk.

The yolk sets somewhere between 60⁰C and 65⁰C. If the temperature stays above 65⁰C for too long, the yolk will turn solid. On the other hand, if the yolk’s temperature is not high enough for long enough, salmonella in the yolk will survive. To be safe and quick, I would hold the yolk at 63⁰C for 5 minutes. It’s not straightforward to measure the temperature of the yolk, and even harder to control it precisely. We will tackle this problem later in this article.

Meanwhile, 90% of the egg white is water. There are trace minerals and a little bit of glucose. It’s too little for the egg to taste sweet, but it enables the Maillard reactions that turn the egg white in the Chinese thousand-year eggs (皮蛋) brown.

The rest of the egg white consists mostly of proteins. As designed by nature, they are not food but protection for the embryo. One protein inhibits viruses. Another kills bacteria. Multiple proteins block digestive enzymes. Contrary to the teachings of a master in an old Kungfu movie (where all boys get their nutritional advice, naturally), eating raw eggs actually causes lab animals to lose weight. It takes high temperatures to deactivate the proteins’ biological functions and turn them into nutrients.

Different proteins in egg white set at different temperatures. The most abundant protein in egg white, ovalbumin, doesn’t coagulate until about 80⁰C. It seems contradictory to our experiences, but that’s why the Japanese Onsen-tamago (温泉蛋) gets its semi-solid yolk and watery white. In the hot spring, the egg is held in a water bath at a temperature where the yolk starts to set, but some of the proteins in the white have not coagulated.

We have only scratched the surface of the biology and chemistry of eggs. But we know what we want from the perspective of cooking: in addition to the right texture, we want the proteins in the egg white to experience high enough temperatures to denature so they don’t fight our digestive enzymes, and we want the egg yolk to be held within a narrow range of temperatures. How do we achieve that with heat?