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.
Decreasing acidity also has consequences for the yolk, which begins to absorb water from the white as a result of changes in osmotic pressure. The yolk swells and becomes diluted, which is why old eggs tend to have pale yolks. The increasing size also causes the yolk membrane to stretch and become weaker. If you’ve ever wondered why the yolks of old eggs break so easily when you try to separate them, that’s the reason.
The last thing that happens as the egg speeds along the downhill slope toward spoilage is that water begins to escape. This has the effect of making the air cell in the egg larger and larger, to the point that a very, very old egg will float when you put it into a bowl of water (throw that one away, laddie).
So what does all this mean for the baker? Unless the eggs are truly spoiled, not a whole lot. Older eggs, though they can have a slightly “eggier” flavor due to an increase in hydrogen sulfide, perform about the same in batters as fresh eggs. And as I’ve written before, old eggs are preferable for foam-making. They’re also better for hard boiling, since the alkalinity of old eggs causes the white just inside the shell to break down, and that makes them a lot easier to peel.
All of which goes to show when it comes to anything other than poached or fried eggs — where nice firm, high-standing whites both look and taste better, farm freshness is vastly overrated.