What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (15 page)

I’ve read so many hints about how to cut onions without crying, but none of them seems to work. Is there some trick that I can use, short of always buying sweet or “no tears” onions, which don’t have the same pizzazz as ordinary ones?

....

W
hat makes
me
cry is all the misinformation going around about onions, primarily about what causes the eye irritation and how sweet onions differ from ordinary ones.

The tear-producing chemical (the
lachrymator
) in onions is not pyruvic acid, as has too often been stated in print. Nor does pyruvic acid “come from sulfur in the soil,” as has also been frequently asserted, because pyruvic acid contains no sulfur. Nor does the lachrymator consist of sulfonic or sulfurous acid or any of the other often-blamed chemicals. Indeed, it is not an acid at all. It is a sulfur-containing compound called thiopropanal sulfoxide, a.k.a. thiopropionaldehyde-s-oxide, which I shall henceforth refer to as
compound T
, for tear gas.

While knowledge of that fact isn’t going to change your life, the record has long been in need of being set straight. So there.

Many people apparently think that there is a single chemical in onions that both irritates our eyes and gives us the flavor effect of pungency, a hot, sharp, and stinging sensation in the mouth. But these two effects are largely due to different compounds. Compound T is not primarily responsible for the onion’s pungency, as is commonly stated in the popular food literature. Nor are the most pungent onions necessarily the most notorious tearjerkers.

Here is an oversimplified account of the complex chemical reactions that take place when you cut an onion.

Neither the tear gas compound T nor the pungency compounds exist as such in the uncut onion. They are formed when the cells are broken open by cutting or chewing, at which time the enzyme alliinase (A) and a group of compounds known as S-alk(en)yl cysteine sulfoxides (S), which until then had been isolated from each other in different parts of the onion’s cells, are liberated. They then react with each other to form the tear gas: A
1
S
T.

A different set of alliinase-activated reactions produces a mixture of ammonia, pyruvic acid, and unstable sulfenic acids. The sulfenic acids react further to form a number of flavor and pungency compounds, primarily alkylthiosulfinates.

The amount of pyruvic acid formed in the last two reactions is generally used as an index of pungency, but only because it is stable and easy to measure in the laboratory.
It is not itself responsible for the pungency of onions.

And now for a few of what Dave Letterman might call “stupid onion tricks,” measures that are often claimed to prevent tears while cutting onions. After reading my parenthetical comments, you can decide how much sense the tricks make.

•
 Cut the stem end off before the root end. (Any onion with half an IQ will remember the cutting order and behave accordingly.)

•
 Let a stream of cold water run in the sink while you do the cutting. (When the onion vapors see the water they will rush toward the sink to drown, even if it’s halfway across the room.)

•
 Clamp a wooden match between your teeth. (You’ll never notice your eyes stinging if you bite on the match head.)

•
 Keep a piece of bread in your mouth. (And make sure to chew it ostentatiously, so the onion will know it’s there.)

•
 Wear contact lenses to protect your corneas.

•
 If you wear contact lenses, remove them because the irritating vapors can get behind them, preventing the tears from washing them out.

•
 Cut or chop your onions under water. You may either fill the sink with water and do your cutting beneath the surface, or put on your scuba gear and take the job to the swimming pool. (These methods should work, except for the little problem of the onion pieces floating away before you can collect them.)

But seriously, folks, here’s the most practical and effective method of all (fanfare, please, Maestro): Chill the onions in the refrigerator for a couple of hours before cutting. This slows down the chemical reaction that produces the tear gas and lowers its vapor pressure (its tendency to float around).

Best of all, just learn how to dice an onion as quickly and efficiently as the chefs do, and there won’t be time for the irritating vapors to bother your eyes very much. Several cookbooks illustrate the technique. And remember that using a very sharp knife will break fewer cells and produce less tear gas.

An effective, inexpensive knife sharpener made by Fiskars of Finland. The ingeniously angled abrasive wheels remove a minimal amount of metal. Available by the name “ASPEKT” at IKEA.

And speaking of sharp knives, everyone swears by his or her pet sharpening device, sometimes to the point that swearing can actually occur when two chefs defend their favorite methods. But I have found that a fancy, expensive sharpener isn’t at all necessary. The inexpensive device shown in the illustration on the preceding page does an excellent job, especially if you follow it with a few swipes on a “sharpening steel,” which straightens out any microscopically wavy edge left by the sharpener.

Why are Vidalia onions so much sweeter than other onions?

....

T
erroir
.

Permit me to explain.

Vidalias aren’t unique in their mildness. There are several brands of mild onions grown in other parts of the country, including the Maui, Walla Walla, Texas 1015, and OSO. Note that I have called them “mild,” not “sweet.” They don’t necessarily contain more sugar than other onions; they simply contain lesser amounts of both the pungent-tasting and tear-producing compounds.

In the case of Vidalia, Georgia, a town on the state’s southern coastal plain, the mildness of the onions has been attributed to a dearth of sulfur in the area’s sandy loam soils. Because both the pungency chemicals and the tear-producing chemicals are compounds of sulfur (see p. 125), sulfur-deprived plants are presumably able to manufacture less of them.

Regardless of soil, weather, and cultivation conditions, any plant is the species and variety it is because the genes in its seeds tell it to manufacture precisely the proper proteins, enzymes, and hormones, and nothing a farmer can do will change that. Vidalia onions, for example, are all of the variety called yellow granex.

But that’s heredity, and as everyone knows, an organism’s characteristics are shaped by both heredity and environment. A multitude of environmental factors, such as the amounts of various nutrients in the soil; the soil’s texture and drainage; its micro-flora and -fauna; its proportions of sand, rock, and clay; the land’s slope; the growing temperatures; the amounts of rain, wind, and sun—in sum, a plant’s entire micro-milieu, short of the phase of the moon at planting time—can lead to subtle differences in the ultimate fruit or vegetable. (That’s no joke about the phase of the moon. Vintners who practice so-called biodynamic winemaking reportedly wait until the moon has waned before removing the sediment from their wines. They believe that inasmuch as the full moon draws up the tides, it would obviously draw the sediment upward and keep it from settling. What could be more logical?)

French winemakers have lumped all of these variables and imponderables (usually excluding the phases of the moon), along with a healthy measure of Gallic shrug and a
soupçon
of mysticism, into the concept of
terroir,
which is now modishly being applied to virtually all fruits and vegetables. But there is nothing profound about
terroir
, which literally refers to an agricultural region or territory. It is merely a summation of all the peculiarities of a particular local growing environment. And anyone who has traveled in France knows that on the other side of every hill or around every bend may be lurking a very different microclimate.

Let us return now to Vidalia, Georgia, where the Vidalia Onion is a registered trademark of the Georgia Department of Agriculture and where a state law decrees the characteristics the onions must have in order to wear that jealously guarded label. A vigorous marketing campaign encouraging us to “eat ’em like an apple” (but why would we want to, may I ask?) undoubtedly plays a role in gilding the onion, so to speak. And no one can deny that economics and politics are central ingredients in the onion’s reputation. Quite a few lawsuits have been fought along the lines of “My onion is more Vidalia than your onion.”

Clearly, not every acre of farmland within the officially sanctioned and virtually sanctified twenty-county Vidalia Onion region of southeastern Georgia can contain exactly the same amount of sulfur in the soil. Thus, sulfur cannot be the only factor. Lacking credible scientific evidence, I’m willing to chalk it all up to a
je ne sais quoi
factor. In other words, I believe in
terroirism
.

At times, I find a cucumber that is bitter. Why? Does the compound that causes the bitterness have any health implications? Could it be dangerous?

....

C
ucumbers have been cultivated for thousands of years, and like many food plants, they have been improved by cross-breeding to accentuate the better and eliminate the bitter.

Old recipes often include a de-bittering step, such as soaking the slices in salt water. (I doubt that that works anyway.) But modern varieties are rarely bitter except in the skin, which can be peeled off.

Part of the flavor of cucumbers is due to slightly bitter compounds called cucurbitacins. But when a cucumber has had a hard life back on the plant, such as a long spell of hot, dry weather or a battle with insects or disease, the amount of cucurbitacins builds up defensively in the flesh as well as in the skin. The bitterness is Nature’s way of saying, “Don’t eat me or you’ll be sorry.” Alkaloids, for example, a class of mostly toxic chemicals found in plants, all have a bitter taste. But the amount of cucurbitacin you’re likely to find in a cucumber certainly won’t kill you. If you come upon a bitter cuke, chalk it up to the luck of the draw and move on to some others that may have had a less stressful youth back on the farm.

Today’s rarely bitter cucumbers are often sliced and salted, not to remove bitterness but to crisp them up. Sprinkle salt on sliced cucumbers in a bowl, top them with a layer of ice cubes, and put them in the refrigerator for an hour or so. The salt will draw water from between the fruit’s cells, firming up their structure. Wash the excess salt off before using.

While a coating of solid salt will draw water out of cucumber slices and crisp them, soaking them in salt water has the opposite effect: they will soak up water and become softer, or wilted. That’s because osmosis draws water from a less salty environment into a saltier one. When the cucumber cells are in contact with solid salt, some of its water will be drawn out. But when they are in contact with a rather dilute salt solution, some of the solution’s water will be drawn into the cells. (For further details, see “Osmosis,” p. 188.)

Cucumber skins aren’t completely impermeable to moisture, so the fruit will eventually dry out and shrivel if not protected by a moisture-proof coating. Cuke moguls therefore spray their product with an FDA-approved, edible wax to prolong their produce-counter lifetimes. The smaller, warty-skinned pickling cucumbers are not waxed because in pickling it is essential that the pickling liquor penetrate the vegetable. So-called English cucumbers, being long and thin with consequently large surface areas, must be protected by more than a wax coating and are usually wrapped in plastic film.