Marlene Parrish - What Einstein Told His Cook 2

By the way, after you hard-cook eggs, keep them refrigerated. It’s easiest if you put them right back in the carton containing your uncooked eggs. But how do you tell the cooked ones from the raw? Easy. Put an egg on the counter and give it a spin. If it resists spinning, it’s uncooked, but if it spins like a top, it’s hard. The effect is most noticeable if you spin the eggs on their larger ends.

Or you could just pencil an H on the shell of each hard one, but where’s the fun in that?

Sidebar Science: Playing spin-the-egg

THE YOLKand white in an uncooked egg are liquid and free to slosh around slightly inside the shell. When you twist the egg fast in an attempt to spin it, the contents resist moving. That is, the contents have inertia , a desire to stay motionless until pushed by some force or other. That’s Newton’s First Law of Motion: an egg yolk at rest will remain at rest until shoved by something harder than raw egg white. (Those weren’t his exact words.) When you apply a twisting force to the outside of the egg, the force isn’t transmitted effectively through the egg white; it’s like trying to play pool with a liquid cue. The egg’s contents try to stay motionless and lag behind. In effect, some of your twisting force is wasted and the egg won’t spin as much as you might expect from how hard you twisted it. In a hard egg, on the other hand, the solid contents transmit your force to the whole egg mass, and the egg spins with the full amount of momentum you expect.

Want some more fun with egg physics? If you spin the egg on its wide end rather than on its side, it will spin faster. The reason is the same as for an ice skater who spins faster as soon as she pulls her arms and legs in closer to her body (closer to her spin axis ). Her body’s spin momentum (or angular momentum ) is proportional to both her spin speed (her angular velocity ) and the average distance of her body parts from the spin axis (her average spin radius ). Her total momentum must remain constant (that is, angular momentum is conserved ), so if she decreases her spin radius by pulling in her limbs, her spin velocity must go up. In the case of the egg, the spin radius is smaller when you spin it on its end rather than on its side, so it spins faster.

GREEN EGGS, NO HAM

I’ve always stayed clear of hard-boiled eggs with funny green or gray colors on the yolks. But I have a four-year-old boy and we’ve read Dr. Seuss’s Green Eggs and Ham many times. Sam-I-Am’s eggs are sunny-side-up with green yolks. Isn’t that a bad lesson to teach kids?

On the cover of my copy, both the egg yolks and the ham are green, so the good doctor must have intended the adjective green to modify both nouns, ham as well as eggs . But I’d strongly advise you to stay clear of green ham (but see chapter 7) and will limit my comments to the yolks.

What causes the greenish-black color of hard-cooked egg yolks? This is an oft-asked and oft-answered question—but I don’t mind tackling it again.

First of all, the color is harmless. Even if the colored substance were a toxic compound (it’s not), it is present in only trace amounts.

As an egg ages, some of the sulfur-containing protein in the egg white slowly decomposes, forming a small amount of evil-smelling hydrogen sulfide gas, H 2S. Heat greatly accelerates this decomposition; at the temperature of boiling water, hydrogen sulfide will be produced in an egg almost two hundred times as fast as at room temperature. The gas diffuses throughout the egg, and when it reaches the yolk, which contains a small amount of iron, it reacts with the iron to form iron sulfides, FeS and Fe 2S 3, known to chemists as ferrous sulfide and ferric sulfide, respectively. Ferrous sulfide happens to be black-brown, while ferric sulfide is yellow-green. Et voilà! A dirty-looking egg yolk. The exact color will depend on how much air is available inside the egg, because in the presence of air the blackish ferrous sulfide changes (it is oxidized ) to the greenish ferric oxide. Because older eggs contain more air, their yolks tend to turn greener.

The longer an egg is heated and the more hydrogen sulfide gas is produced, the more it migrates to the surface of the yolk, and the greener and darker the yolk becomes. If hard-“boiled” eggs are made without actually being boiled, but simply by being left in a covered pan of water below the boiling point, the slightly lower temperature of the water will make a big difference in slowing the production and diffusion of hydrogen sulfide. As soon as the eggs are done, you should stop these ugly chemical reactions dead in their tracks by cooling the eggs in cold running water.

If your little boy wants an egg that’s green enough to pass muster with Sam, try getting him a cassowary egg. A cassowary is a large, flightless Australian bird that lays eggs averaging about 3½ by 5½ inches and weighing about 1¼ pounds. The shells are quite green—although I confess I can’t vouch for the contents.

Sidebar Science: On rotten eggs

THE ALBUMEN,or white, of an egg is 88 percent water and 11 percent a mixture of about a dozen different proteins. But most (54 percent) of the protein is ovalbumin, each of whose huge molecules contains more than three thousand atoms, including only six atoms of sulfur.

Nevertheless, that tiny amount of sulfur can cause quite a stink when converted into hydrogen sulfide, H2S, a gas that is approximately as poisonous as the hydrogen cyanide gas, HCN, used in execution chambers. Fortunately, however, hydrogen sulfide is so incredibly smelly that people notice it long before it can build up to a harmful concentration in the air. It will empty a concert hall faster than a tone-deaf soprano.

Chemistry textbooks persist in telling us that hydrogen sulfide has the “familiar” smell of rotten eggs. But I’ll wager that not one in ten thousand people, excepting really bad vaudeville performers, has ever smelled a rotten egg. If you ever do, though, you’ll know that it’s hydrogen sulfide. In fact, extremely tiny amounts of hydrogen sulfide are responsible for much of the aroma and flavor of even a cooked fresh egg.

How do eggs turn rotten? Contrary to common belief, it doesn’t happen simply because an egg has attained geriatric status. Old eggs don’t rot. Like old soldiers, they just fade away.

To test this notion, I kept an unopened egg on my desk at room temperature for about six months, half expecting at any time to be driven from my office by the stench of hydrogen sulfide. But that never happened. When I finally opened the egg, its contents had shrunk to a gelatinous mass occupying only about one-fifth of the space inside the shell. The rest of the space was filled with perfectly fresh air, with not a trace of hydrogen sulfide odor. The egg may have died of old age, but it did not turn rotten.

The rotting of an egg is like the rotting of any organic matter: It results from the action of decay-producing bacteria. There were no breaks in the shell through which bacteria could enter, so my egg had not been contaminated and no putrefaction occurred.

Even though eggshells are porous and therefore ostensibly vulnerable to invasion by bacteria, eggs have several defense mechanisms. First, freshly laid eggs are coated with a protective film called the cuticle . The two membranes just inside the shell are a second line of defense. Third, the albumen actually has antibacterial properties; among other things it contains the enzyme lysozyme , which fights bacteria by dissolving their cell walls. (Lysozyme in our tears also fights infection in our eyes.)