There are two basic parts to an egg: yolk and white.
The yolk is the nutritive source for the developing
embryo, and it accounts for about 75 percent of the calories
in an egg. Yolks may appear rich and fatty, but, in fact, they
are essentially sacks of water that contain dissolved proteins,
along with larger masses of protein and fat linked together
with lecithin, an emulsifying molecule that allows fat and
water molecules get along together harmoniously. We’ll get
back to that in a moment.
The white is also mostly water, along with a few proteins
—the most important being ovalbumin, ovomucin, and
ovotransferrin, which give it its unique capacity to both set
when cooked and be whipped into stiff, shaving cream–like
peaks.
Because the proteins in eggs are already dissolved and
spread out in a liquid, it is very easy to incorporate them
into other foods—much more than, say, meat proteins,
which are relatively firmly set in place in relation to one
another. (Have you ever tried whipping a steak? I have. It
doesn’t work.) Additionally, the fact that eggs contain such
a wide variety of proteins, each of which behaves in a
slightly different way when heat or mechanical action is
applied, means that as a cook, you have great control over
the final texture of your finished dish. Eggs cooked to
140°F, for example, will be soft and custard-like, while
those cooked to 180°F will be bouncy and firm.
Labeling: Size and Quality
Q: Eggs come in a few different sizes at the supermarket.
Which ones should I be reaching for?