How to center a yolk and get the cooking time just right.
COOKBOOKS SAY THAT TO OBTAIN a hard-boiled egg with a centered yolk, the egg must be cooked in water that has already been brought to a boil. Experience often demonstrates the soundness of this advice, but sometimes one follows it and the yolk ends up being off center. Other times the yolk comes out in the center when the egg has initially been placed in cold water. What good is advice if it isn’t always good?
First things first. Why is the yolk sometimes decentered? Because it has changed position inside the egg. Why has it changed position? Because it is subject to the forces of gravity and buoyancy. Do these forces push the yolk upward or downward? It is often supposed that the yolk is more dense than the egg white. Let’s find out whether this is true by doing an experiment. In a tall, slender glass—a transparent “shell” that will permit us to see the respective positions of the yolk and the white—place a yolk and, on top of this, four or five whites. The yolk slowly rises, which is easily enough explained by the fact that it contains lipids, or fats, which are less dense than the water that makes up most of the egg white.
Does the fact that the yolk comes to float atop the whites account for its decentering in the hard-boiled egg? We can test this hypothesis by another experiment. First, place an egg on its side in a saucepan filled with water. Then bring the water to a boil and let the egg cook for ten minutes or so, making sure that it remains motionless throughout. On peeling off the shell of the hard-boiled egg we find that the yolk has moved toward the top. Let’s repeat the experiment with an egg that has been left in a vertical position long enough for the yolk to rise. If we cook the egg in the same position we discover that the yolk is decentered once again toward the top. These experiments confirm that the difference in density between the yolk and the white is responsible for the position observed.
“Yes, but what about the membranes that center the yellow in the egg?” those who know of their existence will ask. The experiment with the glass removes these membranes and so eliminates their effect, it is true. But the cooking experiment demonstrates that they are not sufficient to hold the yolk in place.
How, then, can we reliably obtain a cooked egg with the yolk in the center? The answer may be deduced from the preceding experiments: We must prevent the yolk from rising in the shell. How? By closing off the vertical axis of movement, and with it the yolk’s ability to float. In practice this means manipulating the egg while it cooks. Put one in boiling water, rolling it around in the saucepan for about ten minutes, and then remove the shell. You will find that the yolk is centered.
The same experiment, only starting with cold water, produces a centered yolk as long as one rolls the egg around for a longer time, which becomes tedious after a while. The cookbooks are partially correct, then, but they offer no insight into the forces actually at work. The key to success, it turns out, lies in not allowing the yolk to stay still.
Hamine Eggs
What is the recipe for a perfectly cooked egg? The question may seem odd because tastes vary so greatly. Some people like the dark green ring on the surface of the yolk, for example, and others detest the sulfurous odor that accompanies it. Cookbooks say to cook the egg for only ten minutes in boiling water, without any further explanation. Let’s look first at the problem of cooking time.
Why ten minutes rather than, say, five or fifteen? Because after five minutes the egg has not yet hardened, and after fifteen minutes it is assumed that the egg white will be rubbery and the yolk sandy. Yet the latter result is not universally observed throughout the world. Hamine eggs prepared in Jewish communities in Greece and elsewhere are famous for their tenderness, although they are cooked for several hours. How do these cooks avoid the sulfurous smell of overcooked eggs? And why does the yolk in their eggs not take on the greenish color found in eggs cooked longer than ten minutes in France?
These questions give rise to others. How do eggs cook in the first place? The white consists of about 10% proteins (amino acid chains folded upon themselves in the shape of a ball) and 90% water. During cooking the proteins partially unfold (they are said to be “denatured”) and bind with each other, forming a lattice that traps water—in other words, a gel.
The tenderness of the cooked egg white depends on the quantity of water trapped (the loss of a part of this water is what makes overcooked fried eggs rubbery and overcooked yolks sandy) and on the number of proteins making up its lattices (more lattices mean that more water is trapped, rigidifying the entire system).
Experiments in which eggs are cooked at different temperatures provide a clue to the mystery of hamine eggs. When an egg is cooked in boiling water, at a temperature of 100°C (212°F), not only does its mass progressively diminish as water is eliminated from the gel that forms, but many kinds of protein coagulate as well. By contrast, when an egg is cooked at a temperature just a bit higher than the temperature at which its proteins have all coagulated—about 68°C (154°F)—it thickens (thanks to the coagulation of only a few proteins) while retaining its water, a guarantee of tenderness and smoothness.
Hamine eggs traditionally are cooked in embers, with a temperature range of only 50–90°c (122–194°F). There is therefore no paradox, only a good empirical understanding of the coagulation of the egg’s proteins during cooking. Cooking eggs in boiling water nonetheless has an advantage: Because the temperature is constant, one obtains a constant result by fixing the cooking time. However, this temperature does not take into account the nature of the egg.
Isn’t it time that we avail ourselves of the benefits of modern technology? High-quality thermometers now make it possible to cook eggs at temperatures closer to those at which their proteins are denatured: at 62°C (144°F) one of the proteins in the white (ovotransferrin) is cooked, but the yolk remains liquid because the proteins that coagulate first in this part of the egg require a temperature of 68°C (154°F). Obviously this would mean longer cooking times, but the result is a perfectly cooked egg.