If you've ever chopped raw onions to add a bite to pico de gallo or crunch to a hamburger, chances are, you've found yourself tearing up over the cutting board. But why is this single vegetable, only in raw form, solely responsible for a great deal of weeping?
Along with chives, garlic, and leeks, onions fall under the allium genus. These plants are known to absorb sulfur from soil, which aids in the formation of volatile molecules called amino acid sulfoxides. When an onion is cut, it releases enzymes that break these sulfoxides down. Then a second type of enzymes converts into a gas that irritates the eyes. When the onion's vapors reach your face, the cornea of the eye signals gland cells to wash the irritant away with tears.
While green onions, shallots, leeks, and garlic produce the same gases, these vegetables produce much less of them, and won't make you cry. When you cook an onion, you're actually rendering certain enzymes inactive, so vapors don't burn your eyes. Although a tearless onion has been developed, if you don't have one on hand, simply remember to chop onions last.
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