Onions

From Chempedia

David Martin
Karin Baty
Ronneil Aviles
Erica Jennings

Why do onions make you cry?

Cooking with onions can be a problem. Strong fumes from a cut onion can cause the cook’s eyes to sting and water. If multiple onions are needed for a recipe, the stinging may become intense enough to make the cook have to leave the room in order to recover. Several chemical reactions are involved in this phenomenon, occurring in both the onion and the cook’s eyes. The most direct cause of these tears is a gas emitted by the onion, known as syn-propanethial-S-oxide, or ethyl sulfine. This gas is generated when an onion is peeled, cut, or crushed, releasing two enzymes; allinase and lachrymatory-factor synthase, which convert sulfur molecules in the onion to sulfenic acids. These acids interact with one another and form (spontaneously) ethyl sulfine, which is the ultimate trigger for the tears. The pungent smell of onions is due to other sulfur compounds known as thiosulfinates, which are also products of sulfenic acids interactions, but while the smell may be most noticeable during the chopping it is not the source of tears. The production of syn-propanethial-S-oxide is over after about five minutes (with peak gas formation happening roughly 30 seconds after cutting has begun).^[1]

Syn-propanethial-S-oxide is both gaseous at room temperature and water soluble. As the gas leaves the onion and begins to mix with the air in the kitchen, some of it comes into contact with the cook’s eyes and dissolves in the watery film covering them. This produces sulfuric acid, which irritates the nerve-endings found in the cornea. This prompts the eye to produce tears to both dilute this acid and wash it away.^{[2] [3]}

The unique taste and smell of an onion, as well as that of its relatives, can be useful in preventing attacks by predators both large and small. Any disturbance caused by a microbe or an insect would result in the formation of these sulfenic acids, helping to stop the intruder. This property of onions has lead to its use as both a healing agent and a natural pesticide. In one study done in the Soviet Union during World War Two, significant improvement was reported after infected amputee wounds were exposed to onion vapor.^[4]

Including onions in one’s diet also seems to provide health benefits. Numerous medicinal properties attributed to onions include cures for coughs, colds, joint pains and bacterial infections.^[5] Recent studies of populations with different eating habits indicate that onion-eaters have decreased rates of stomach cancer, reduced blood-clot formation and reduced heart disease.^[6]

Avoiding the painful sting and tears of an onion can be accomplished in several ways, with differing degrees of success. Wearing swimming goggles is reported to stop ethyl sulfine from coming into contact with one’s eyes, thus preventing sulfuric acid from forming and tears flowing. Washing the onion with cold water may help to get rid of much of the water soluble gas (just make sure your knife does not slip on the wet onion). Also, holding a piece of bread or a slice of lemon in your mouth may help absorb some of the gas as it travels up from the cutting board and into your eyes. More effective and practical techniques involve storing the onion in the fridge, which slows down the chemical reactions, and cutting the onion using a sharp knife, leaving more onion cells intact and lessening enzyme-substrate interactions which lessens gas production.^[7]

Modifying onions to produce less sulfur-rich gas is also an option. Onions grown on sulfur deficient soils generate less ethyl sulfine, but the low sulfur content also reduces the desired flavor, which is also due to sulfur compounds. Genetically modified varieties of onions may soon appear that retain the distinct onion flavor while reducing the level of gas produced. Researchers have found that the taste of onions is not affected by the second enzyme, lachrymatory-factor synthase. Sulfur-rich onions modified to lack this second enzyme would retain much of their flavor, but with reduced tear causing gas, though some gas would still slowly form spontaneously without the enzyme.^[8] While there may be some choppers out in the world that empathize with the innocent onion causing tears to shed, the majority of people become misty eyed due to the interaction of sulfur crystalline structures and the water in their eyes. Onions have valuable health properties, so either put up with the tears or try some techniques from this report to reduce the water lost due to the chore of cutting onions, and find one that seems to work for you, or create an apparatus to diminish the tears running down your cheeks.

Footnotes

1. ^ How Stuff Works. http://science.howstuffworks.com/question539.htm (accessed Oct 2005).
2. ^ Kohman, Edward F, “The Chemical Components of Onion Vapors Responsible for Wound-Healing Qualities”, Science, 1947. pp. 625-627.
3. ^ Craig J. Winston, “Onions: a Global Health Benefit”, Vibrant Life, http://www.vibrantlife.com, May-June, 2005.
4. ^ Hodgkin E. Georgia, “Onions, Garlic & Leeks: the Sweet Scent of Better Health”, Vibrant Life, http://www.vibrantlife.com, January 2000.
5. ^ Wolke, Robert L, What Einstein told his Cook 2, W.W. Norton & Company Ltd, London, 2005, pp. 125-128.
6. ^ From Science News, Vol. 162, No. 16, October 20, 2002, p. 244, Copyright © 2002, Science Service, as published under license in AccessScience@McGraw-Hill, © The McGraw-Hill Companies 2000-2003.
7. Rose. Patrick. Scientific American. (accessed Oct 2005). " What is the chemical process that causes my eyes to tear when I peel an onion?." Scientific American. Oct 1999. Oct 2005 < http://sciam.com/askexpert_question.cfm?articleID=000C5FF7-5DB5-1C72-9EB7809EC588F2D7 >.