BOOKS - The Chemical Weapons of Onions and Garlic

By Harold McGee What do garlic and onions have in common with gunpowder? A lot. They’re incendiary. They can do harm and they delight. Sulfur is central to their powers. And they helped inspire the work of a chemist who has just published a welcome treatise on the smelly yet indispensable allium family. Eric Block became hooked on chemistry by way of basement and driveway pyrotechnics while growing up in Forest Hills, Queens. By high school he had become the science nerd while his schoolmates Paul Simon and Art Garfunkel were the epitome of cool. He found his calling in allium chemistry as a new Ph.D., and over four decades has worked out many of its details at the State University at Albany. Dr. Block’s book “Garlic and Other Alliums: The Lore and the Science” was published earlier this year by the Royal Society of Chemistry. The chemical details are tough for a nonspecialist to follow, but much of the text is in happily clear English. It includes a wide range of cultural references and beautifully reproduced images, among them excerpts from Sumerian cuneiform tablets and “Dracula” and pictures of the firework-like flower heads of ornamental alliums, the onion domes of Russian churches and Antonio Gaudí’s garlic-topped Barcelona apartment house. Dr. Block also carefully evaluates the mixed evidence for allium efficacy in folk and modern medicine, and explicates the chemistry and treatment of garlic breath. (It can emanate from deep within for a day and more; raw kiwi, eggplant, mushrooms or parsley can help.) Most helpfully for the cook, he sorts out the different kinds of allium flavors and how they evolve on the cutting board and stove. And he gives an intriguing preview of new alliums just over the horizon. “It’s still astounding to me what happens when you cut or bite into an onion or a garlic clove,” Dr. Block told me in a telephone conversation last month. “These plants originated in a very tough neighborhood, in Central Asia north of Afghanistan, and they evolved some serious chemical weapons to defend themselves.” Their sulfur-based defense systems give the alliums their distinctive flavors. The plants deploy them when their tissues are breached by biting, crushing or cutting. The chemicals are highly irritating, and discourage most creatures from coming back for seconds. They kill microbes and repel insects, and they damage the red blood cells of dogs and cats. Never feed a pet onions or garlic in any form. Any cook knows that chopping alliums releases chemicals that sting. Garlic can get into the eyes and mouth even if a clove is just rubbed on the foot, a body length away. Its active ingredient passes right through the skin and into the blood. Prolonged contact with garlic will blister and burn the skin, as some of the book’s less pleasant photos document. Dr. Block explains that different alliums stockpile different sulfur chemicals to make their weapons, and this accounts for their varying flavors. The stockpiles themselves are inert, but when the plant’s tissues are damaged, enzymes in the tissues quickly convert the sulfur compounds into reactive, stinging molecules. Garlic cloves produce a chemical called allicin, which is responsible for their strong pungency and aroma. It’s a relatively large molecule and acts mainly on direct contact with the eater, the plant world’s version of hand-to-hand combat. The flat-leafed allium known as Chinese or garlic chives produces a small amount of garlicky allicin, but much more of a different weapon that has a milder, cabbage-like aroma. Onions, shallots, scallions and leeks share a special stockpiled chemical and a second defensive enzyme. They produce a sulfur molecule that’s small and light enough to launch itself from the damaged tissue, fly through the air and attack our eyes and nasal passages. This long-distance weapon is called the lachrymatory factor because it makes people’s eyes water. “The lachrymatory factor is extremely potent,” Dr. Block said. “Only tiny amounts get anywhere near your face when you cut onions, but it’s still enough to make you tear up. When I smelled the pure compound it was overwhelmingly painful, like being punched in the eye socket.” Other familiar alliums, like elephant garlic, ordinary chives, wild ramps and ramson, generate variable mixtures of the garlic, Chinese chive and onion weapons, and have a blend of their flavors. The same reactivity that makes the allium sulfur compounds such potent weapons also makes them short-lived. They immediately begin to react with other molecules in the plant tissue and gradually generate a flavor that is less pungent but also less fresh-smelling, more harshly sulfurous. The heat of cooking speeds these and other reactions, largely eliminates the pungency, and allows the sweetness of the alliums to emerge and blend with the sulfurous aromas. Heat also knocks out the tissue enzymes, so they can’t produce any more pungency. This basic chemistry leads to some general guidelines for cooking. If you’re using onions or garlic or chives raw, in a dressing or salsa, either chop them just before serving or rinse the chopped pieces thoroughly. Water removes the harsh aging sulfur compounds from the cut surfaces, so you’ll taste only the fresh ones. If you’re heating garlic or onions or their relatives, then cooking whole or coarsely chopped bulbs will moderate their flavor. Crushing or grating will intensify it. Crushing can also diversify the flavors that alliums contribute to cooked dishes. They’re valuable ingredients in part because their sulfur chemistry suggests and reinforces savory meat flavors. Last year a German study of meat stews found that by far the strongest contributor to the overall “gravy” aroma was an unusual sulfur compound that came not from the meat, but from the onions and leeks. And that compound appears only after these vegetables have been cut up. So if you’re counting on alliums to give depth to stews or braises or stocks, then chop them finely or crush or purée them. Heat will eliminate the bite and develop the aroma. Dr. Block’s book may be the definitive word on the alliums for the moment, but as it and he make clear, there are new flavors to look forward to. Researchers in New Zealand and Japan recently developed an experimental onion that lacks the lachrymatory-factor enzyme. So it’s tear-free, but unlike very mild onions like Vidalias, it still has its full stockpile of sulfur materials. It ends up boosting the levels of trace compounds that Dr. Block discovered and named zwiebelanes. He describes them as having “a wonderful, fresh, sweet onion aroma.” So this tear-free onion promises to be intensely oniony, but in a new way. Dr. Block and some of his colleagues are also beginning to study the several hundred allium species that still grow wild in Central and Southwest Asia, a number of which are harvested locally and eaten in large amounts, and have very different defensive chemistries. “I want to see what other surprises nature has in store for us,” he said. But flavor exploration begins at home. “There’s a lot going on under your nose while you chop and cook,” he told me. “Use your nose, follow the changes, and you’ll discover new and delicious things.” "The New York Times Online", June 7, 2010