That’s a very pretty, upstanding yolk, no? It’s the mark of a fresh egg. If you’re ever in a position where you need to evaluate the relative age of an egg, a yolk is a good place to start. If it’s fairly roundish and bright yellow, and is sitting high atop a slightly milky-looking mound of almost gel-like egg white, your egg is extremely fresh. Make cake, cookies or muffins out of it. Or better still cook it up, because fresh eggs make mighty good eatin’.
Old eggs are very different in their appearance just out of the shell. The white becomes very clear and watery due to a progressive change in the pH of the egg, which begins as soon as the egg is laid. That change in pH causes the proteins in the white to drift away from each other, dispersing the large aggregations that formerly made the egg white jelly-thick and whitish. As that happens the membrane around the yolk starts to weaken. More water from the white enters the yolk, diluting its pigments and giving it a pale appearance. As that happens the yolk membrane stretches out, causing the yolk to lie almost flat. If you’ve ever tried to separate an old egg, you know just how weak that membrane is after many weeks of sitting. If the egg is sitting at room temperature, the pH change happens many times faster.
But let’s talk yolk, shall we? Yolks are the food source for the chicken embryo (the little white bud that sits next to it). As such they contain large amounts of nutrients. Indeed the yolk of an egg contains 75% of the egg’s calories, 50% of the protein and all of the fat. For all that, the vast majority of the yolk is composed of water. A myth about egg yolks is that they’re very fatty. That’s not the case, as eggs are quite lean. A yolk contains just five grams of fat, only a fraction of which is saturated (the so-called “bad fat”).
True, egg yolks have gotten a bad rap in the past as a result of the relatively high amount of low density lipoproteins (LDL’s) they contain, and those structures have cholesterol within them. Lately the reputation of egg yolks has been rehabilitated, however, as more studies are showing that high cholesterol levels are more a factor of the way our bodies work than the food we eat.
But then if it isn’t the fat in egg yolks that makes them such great tenderizers of baked goods, what is it? Ah, glad you asked. The answer is: emulsifiers. Yolks, you see, abound with tiny structures, and structures within structures, and structures within structures with…well you get the idea. These tiny globules, which are all suspended in a watery medium, contain all the things an embryo needs to develop: proteins, fat, cholesterol and vitamins. The walls of these structures — which are designed specifically to keep water and fat separated from one another — are made up in large part of a compound called lecithin.
Take that lecithin out of the egg yolk and put it into say, a cake batter, and it performs a very similar job of keeping fats and water droplets separated from each other and/or combining into large masses. This has the effect of dispersing fat very evenly through the batter. And when fat is very evenly dispersed the result is a tender cake, as fat disrupts the gluten networks that make cake tough or chewy. An irony to this story is that very tender, highly emulsified cakes are also stronger than cakes that aren’t as well emulsified, since the crumb had a very fine, self-reinforcing structure (think of a lattice of beams holding up a domed roof instead of a big, central pillar). They have give, but tend not to fall apart and crumble, if that makes sense.
Yolks of course also impart a lovely color to baked goods because of the pigments they contain. The brightness of these pigments are largely a result of what the chicken eats. Alfalfa feed creates nice bright yolks, as does corn. If neither of those is on-hand many farmers add marigold petals to a chicken’s diet, which accomplishes pretty much the same thing.