Why do onions make you cry?

cryThey cause our eyes to sting, noses to run, and usually leave us in tears. But spaghetti sauce, guacamole, salad, or hamburger without onions would be a culinary catastrophe. So much so, that Alice in Wonderland’s Queen of Hearts ordered the Seven of Spades beheaded for bringing the cook tulip bulbs in the place of sweet, juicy onions. According to the National Onion Association (yes there really is a National Onion Association) onions probably originated in Central Asia and have been grown for approximately 5000 years. They were one of the earliest cultivated crops as they adapted well to a variety of soils and climates, were easily stored, and were less perishable than other vegetables. Onions come in red, white, and yellow; each colour delivers different levels of sweetness and flavor, making them useful in different types of dishes. But onions bring more than just taste and colour to a dish, they bring nutritional value as well. They are rich in quercetin, which has antioxidant potential; are a source of vitamin C, B6, potassium, dietary fiber, and folic acid; they are low in sodium; contain no fat; and are cholesterol free. Yet before we can enjoy their taste and health benefits we have to go through a little pain and a few tears. But we can learn a little science as we shed those tears.

Oil is released when we cut into an onion. The oil contains a compound called 1-propenyl-L- cysteine sulphoxide, which reacts with enzymes to form a gaseous sulphur containing chemical known as the lachrymatory factor (propanethiol-s-oxide). The vapour is dispersed through the air and comes into contact with our eyes where it is perceived by the brain as a painful irritant. As a result a message is sent to the tear gland and tears are produced to rid the eyes of the painful substance. Many different solutions have been suggested for the crying problem. You can try chilling the onion before cutting it, keeping your head as far away from the cutting board as possible. Cutting the onion under water, wearing goggles, or my personal favorite, having someone else chop the onion, are also possible solutions. But wouldn’t it be great if an onion could be produced without the lachrymatory factor? This just may be possible through a bit of genetic engineering. A paper published in the scientific journal Nature described that for an onion to produce the lachrymatory factor three components are necessary: propanethiol-S-oxide, alliinase (the enzyme responsible for the onion’s taste), and, a previously undiscovered enzyme, lachrymatory-factor synthase (LFS). The researchers were able to show that if the LFS is removed from the mixture, no lachrymatory factor forms, and as a result, there are no tears. Therefore, a genetically engineered onion with an inactive LFS gene would produce a full flavored onion with all the health benefits, but not the tears. So in the future we may get to throw away those tissue boxes and chop onions with smiles instead of tears.

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