By Harold McGee
Designed as it was to protect itself for the duration of the chick’s development, the egg is unique among our raw animal foods in its ability to remain edible for weeks, as long as it’s kept intact and cool. Even so, the moment the egg leaves the hen, it begins to deteriorate in important ways. There is a fundamental chemical change: both the yolk and the white get more alkaline (less acidic) with time. This is because the egg contains carbon dioxide, which takes the form of carbonic acid when it’s dissolved in the white and yolk, but is slowly lost in its gaseous form through the pores in the shell. The pH scale provides a measure of acidity and alkalinity. On the pH scale, the yolk rises from a slightly acidic pH of 6.0 to a nearly neutral 6.6, while the albumen goes from a somewhat alkaline 7.7 to a very alkaline 9.2 and sometimes higher.