Like a Scientist: Boiling Eggs

This week's Food Chemistry laboratory was directed at determining an optimal boiling condition for the everyday chicken egg. I learned several things during the course of this lab, and would impart that knowledge to you now.

First, the proper way to boil an egg in general, is by placing the egg in the cool/room temperature water before you begin to boil, as opposed to boiling water first, then dropping the eggs in. This reduces the rate of expansion of the proteins and fats inside the egg, thus reducing the cracking potential of the shell during boiling.

Second, reducing the boiling time to as short as possible yields less smelly eggs. As eggs boil, cystine residues on protein in the albumin (egg white) react with hydrogen to form hydrogen sulfide. Some of the hydrogen sulfide travels towards the yolk, mostly stopping at the yolk membrane and collecting as a greyish green substance observed after peeling the eggs and removing the egg white. Reducing the boiling time reduces the amount of gas produced, thus giving off less hydrogen sulfide gas, and less stink!

Third, boiling in salt water reduces streamers generated at cracks in the egg shell and reduces the amount of hydrogen sulfide that accumulates on the egg yolk. A plausible explanation for reduction of the string-like egg white that is sometimes generated at cracks in the egg shell is that the higher salt concentration causes the proteins to coagulate while still in the crack and sealing them if you will. In our lab, one technique to purify a protein is to salt it out, or precipitate it, by increasing the salt concentration. As to why the salt water reduces the amount of hydrogen sulfide on the surface of the egg yolk, I do not know. Perhaps this was a false positive result due to inconsistencies generated by the students in our lab, since two different groups prepared the eggs +/- salt.

Fourth, eggs have an air sac that expands with age, and this can be used to get a general idea of whether an egg is still good or not. The air sac at the fat end of the egg is what causes the flat surface at the bottom of a typical boiled egg. If you pin prick a hole into the shell on the fat end, this flat surface disappears as the air is dispersed by the expanding protein and fat coagulation.

The bottom line: Cook you eggs for the shortest time possible (we boiled for approximately 12 minutes on our shortest time point) in salt water (we used 42g/L, but this is likely a bit excessive), and remember not to boil the water first! Good eating-