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In this case, tears are the price we pay for flavor and nutritional benefits. The rowdy onion joins the aristocratic shallot, gentle leek, herbaceous chive, sharp scallion and assertive garlic among the 500 species of the genus Allium. Allium cepa is an ancient vegetable, known to Alexander the Great and eaten by the Israelites during their Egyptian bondage. Indeed, his charges chastened Moses for leading them away from the onions and other flavorful foods that they had come to relish while in captivity. And with good reason: onion is a rich source of nutrients (such as vitamins B, C and G), protein, starch and other essential compounds. The chemicals in onions are effective agents against fungal and bacterial growth; they protect against stomach, colon and skin cancers; they have anti-inflammatory, antiallergenic, antiasthmatic and antidiabetic properties; they treat causes of cardiovascular disorders, including hypertension, hyperglycemia and hyperlipidemia; and they inhibit platelet aggregation.
The tears come from the volatile oils that help to give Allium vegetables their distinctive flavors and that contain a class of organic molecules known as amino acid sulfoxides. Slicing an onion's tissue releases enzymes called allinases, which convert these molecules to sulfenic acids. These acids, in turn, rearrange to form syn-propanethial-S-oxide, which triggers the tears. They also condense to form thiosulfinates, the cause of the pungent odor associated with chopping onions-and often mistakenly blamed for the weepy eye. The formation of syn-propanethial-S-oxide peaks about 30 seconds after mechanical damage to the onion and completes its cycle of chemical evolution over about five minutes.