“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.
