Onions are one food of truly time-tested value. Onions have been cultivated for some 4,000 years, probably originating near present-day Afghanistan. Today, this hardy member of the lily family is among the most widely traded vegetables in the world. From soufflés to sour cream dip—versatile onions go in and with dishes throughout the menu and across cuisines. Recent findings on their cancer-fighting qualities—they are rich in antioxidants and flavonoids—promise to make this humble bulb even more popular.
Just the same, onions are misunderstood, blamed for everything from bad breath to stale odors. As with people, an onion’s qualities arise from a combination of heredity and environment. With understanding, you can defuse the onion’s challenging personality to make use of all it has to offer.
The Two Sides of Sulfur
An onion’s puzzling nature is literally rooted in its upbringing. Onions typically grow in sulfur-rich soils. Through chemical reactions, sulfur becomes part of a plant protein. From a cook’s point of view, this process is important for leading to the formation of flavor precursors. A precursor is a substance that, under the right conditions, can be changed into another substance.
The right conditions are created when the onion is cut and the cells are ruptured. The sulfur-based precursors meet with an enzyme, alliinase, which is contained in pockets or pits inside the cells. The reaction produces the flavor compounds that give onions their “bite” and pungent aroma. Different onion varieties contain different combinations of precursors, depending on their ability to extract sulfur from soil, resulting in different flavors. Some are tart, resembling cabbage. Others are mild, more like chives. Flavor compounds are most concentrated in the center of the bulb. They grow more potent with age.
This reaction produces something else—sulfenic acid, which rises as a gas. This acid reacts with water—like the moisture inside your eyes and nose—to form a mild but irritating sulfuric acid. Scientists believe this is how onions protect themselves from predators. It’s also why cutting onions can leave you teary-eyed and sniffling. Rubbing your eyes only makes matters worse, since the compounds are on your hands too.
Cooks have various methods to deal with the problem. Chilling onions just before preparing them slows the enzymatic reaction. Placing the onions under a thin stream of running water while cutting on a cutting board keeps the gas away from your face. A sharp knife is particularly useful for onions. A clean cut does less damage to cells.
Food scientists are investigating ways to “turn off” the gene that makes an enzyme responsible for another irritating chemical. This enzyme is not involved in flavor production, so disabling it should reduce the onion’s sting without diminishing its bite.
A Sweeter Disposition
Fans of milder flavors are not out of luck. Onions grown in low-sulphur soils and those with lower sulphur “uptake” through the roots are less pungent. Some are even called “sweet onions.” A popular technique to maximize their sweetness is to caramelize them.
Chemically speaking, caramelization is something of a mystery, even to scientists. It occurs when sugars reach temperatures between 200ºF and 325ºF, depending on the type of sugar. The heat drives hydrogen and oxygen—that is, water—from the sugar molecules. The remaining, “drier” fragments combine in larger molecules that are high in carbon, which gives a rich brown color. They can also be quite sweet, up to 50 times sweeter than table sugar.
Caramelization takes slow cooking and regular stirring to help ensure an even temperature. Sugars in this state are easily burned, giving a bitter taste.
Caramelization brings out the flavor and sweetness in other vegetables and fruits as well as onions. It’s the first step in making peppermints and peanut brittle—but not in making caramels. Caramel candy is the product of a related process, the Maillard reaction, in which sugar molecules react with amino acids in protein. This reaction takes place at temperatures that are more apt to blacken onions than to brown them—between 300ºF and 500ºF.
In caramel recipes, the sugar is sucrose. Cream supplies the protein. Given the vast number of possible sugar-amino acid combinations in foods, however, the Maillard reaction can create countless flavor, aroma, and color compounds. It’s responsible for the sweet, tender, golden-brown crust on angel food cake, and the dark, “meaty” flavored crust on a seared steak. If you can control both the Maillard reaction and caramelization, you can make that traditional dish appreciated by many—liver and onions.
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