What exactly happens as an egg ages? Well you remember I mentioned that eggs have those tiny pores in their shells (the ones through which aromas and undesirable microbes can enter). Well, those holes are two-way streets. Stuff can get in, but stuff can also get out. Specifically carbon dioxide, which is initially stored in an egg as carbonic acid. That acid is what’s responsible for keeping the white of the egg firm, since a low (acidic) pH causes the proteins in the white to clump together. As the carbonic acid converts to gaseous CO2 and leaves the egg, the pH goes up, and that has all sorts of consequences for the egg’s texture.
All you egg aficionados out there have undoubtedly noticed that eggs contain two kinds of white (or “albumen”). There’s the “thick” albumen, which you find right around the yolk. The “thin” albumen forms the outside layer and is fairly runny no matter how fresh the egg is. Yet in a fresh egg the proportion of thick to thin albumen is about 60-40. As the egg ages that proportion changes, and can go as high as 40-60 in a very old egg.
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