Read What Einstein Kept Under His Hat: Secrets of Science in the Kitchen Online
Authors: Robert L. Wolke
| | A YOKE OF YOLKS | |
How do double-yolk eggs form? Do the yolks replace the space that would have been taken by the white? Or is it a bigger egg?
....
I
n general, the yolks are smaller but the eggs overall are bigger.
About 3 to 5 percent of hens’ eggs have two yolks. Some hens, driven by their genes and the conformation of their oviducts, seem to specialize in producing these twins. When you find a double-yolker in an otherwise normal dozen, it’s a mistake—not of the hen, but of the inspectors at the egg farm, because all eggs are candled before being sold, and candling will show up any double-yolk eggs, which are put aside for special uses.
There’s nothing wrong with eating a double-yolk egg when you come across one. In fact, double-yolkers are in demand for their novelty, and the supply can’t keep up. When you do come across one, fry it sunny sides up.
| | A CRACK ATTACK | |
I recently read somewhere that adding salt to the water before boiling eggs prevents the eggshells from cracking. It has worked for me the three times I’ve done so. What is the chemical explanation for this effect?
....
T
here is no chemical explanation because it’s not true. Salt (sodium chloride) has no effect on eggshells (calcium carbonate), either chemically or physically.
An eggshell can crack when there is a sudden temperature difference between a cold shell and hot water. The heat makes the shell expand quickly but unevenly, because it has different thicknesses in different places. The uneven heating causes stresses that can fracture the shell at its weakest points. (Hold an egg up to a very bright light and you’ll often see fine cracks in shells that look perfectly intact from the outside.) An egg may also crack if the air space inside the large end expands too quickly, before it can seep out slowly through the porous shell. A good precaution is to puncture the large end with a pushpin to let the air bubble out harmlessly as the egg heats.
To cook crack-free eggs, avoid plunging cold eggs into hot water. Instead, put them into a pot of cold water and then heat the pot until the water begins to boil. As soon as you see a few bubbles, turn down the heat to a simmer. Depending on how long you simmer them, the eggs will cook through the entire spectrum of doneness, from coddled in 1 or 2 minutes to hard-“boiled” in 12 to 15 minutes.
So how do we explain your experience? With all due respect, three trials do not a scientific experiment make. How do you know that the eggs would have cracked without the salt? Please repeat your experiment with six dozen eggs, half of them cooked with salt and half without. Record the number of cracks in each case and turn in your lab report to me by next Thursday.
But wait! There is indeed some wisdom in salting the egg water. Read on.
| | DUTCH BOY IN A BOTTLE | |
Several people have told me that adding a little vinegar to the water when boiling eggs prevents the shell from cracking. Does it? And how?
....
I
t doesn’t. And that gets me off the hook for your second question.
The acetic acid in vinegar will coagulate the protein in any albumen (egg white) leaking out of a shell that already has a crack in it. That’s how acids in general act on proteins in general. But salt can have the same effect. Thus, adding vinegar, lemon juice (citric acid), or salt to the water is insurance for pessimists who expect their eggs to crack and want to stem the exodus of albumen. Acids and salt are leak stoppers, not leak preventers; they are fingers in the dike, so to speak.
| | THE EGG-SPINNING OLYMPICS | |
When I hard-boil eggs, why do the shells sometimes peel off quite easily and at other times they stick like crazy to the whites?
....
W
hen the eggs are done, cool them down quickly in cold running water. That shrinks the albumen away from the shell somewhat and makes an egg easier to peel. (It also keeps the yolks from turning green; see the following question.) Very fresh eggs are harder to peel because the albumen tends to stick to the membrane on the inside of the shell. But after several days the membrane retreats and the sticking tendency diminishes, so don’t hard-cook your freshest ones.
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 YOLK
and 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?
....
O
n 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 p. 281) 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
2
S. 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
2
S
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 3
1
/
2
by 5
1
/
2
inches and weighing about 1
1
/
4
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.)
As soon as an egg is broken open, though, it’s fiesta time for every microorganism in the neighborhood (can’t you hear those tiny mariachis?) and it’s all downhill from there.
One more non-rotten-egg story: Recently, I had the dubious privilege of examining a four-year-old hard-boiled and dyed Easter egg. It seems that for some cockamamie reason my dear wife, Marlene, had promised to save it for her four-year-old grandson, Oscar, until he went to college. He’s eight now. Unbeknownst to me, the egg had been in our refrigerator all along. When Oscar visited us recently, he asked about his egg and Marlene produced it. Although I wouldn’t want to eat it, it was still quite inoffensive, demonstrating once again that without bacteria and conditions conducive to their growth, an egg will not turn putrid. But we’ll check it again during Oscar’s freshman orientation program for the class of 2018.
| | ADORABLE EGGS | |
Why do some peeled hard-cooked eggs have a big dimple in one end?
....
A
hen’s egg contains a small air pocket just inside the shell at the broad end, the purpose of which is to let the chick breathe while pecking its way out on its birthday. Because the shells are porous and permeable to gases, outside air can diffuse in and exchange with the air inside. As an egg ages and its contents shrink away from the shell, more air comes in to fill the space and the air pocket grows larger. A really fresh egg will sink in water, but as it ages and the air pocket grows bigger, it will tend to turn its broad end up. Eventually, a stale egg will float entirely on the surface.