“It is so beautiful that I have to cry”, exclaimed the actress in the movie looking out the train window at the snow-covered mountain peaks of the Swiss Alps. And large tears rolled down her cheeks to prove her statement. But the segment was filmed in a studio against a blue screen and to produce the tears an onion was used.
The chemical reaction causing tears to flow
Tears from chopping onions (Photo: angrywayne)
Onions, which are cultivated and used around the world, produce a pungent aroma when chopped that irritates the eyes. We all know this. But you probably don’t know the complexity of the process you have just started that happens in quick succession.
As a first step when you chop the onion you damage its cells. This will release enzymes called alliinases that attack amino acid sulfoxides in the onion. This is causing the formation of several sulfenic acids. So far so good. But you also released lachrymatory factor synthase by cutting through the onion.
As a second step this further enzyme acts on one of the sulfenic acids called 1-propenesulfenic acid. There is no stopping your tears when the volatile gas called syn-propanethial-S-oxide, or onion lachrymatory factor, is released. This gas diffuses through the air and soon reaches your eyes. Here it activates sensory neurons, creating a stinging sensation. And tear glands produce tears in order to dilute and flush out the irritant.
Eating onions not all bad
But onions are not all bad. Let’s separate fact from fiction.
In ancient Greece, athletes ate large quantities of onion because it was believed to lighten the balance of the blood. Roman gladiators were rubbed down with onions to firm up their muscles. In the Middle Ages, onions were such an important food that people would pay their rent with onions, and even give them as gifts. Doctors were known to prescribe onions to facilitate bowel movements and erections, and to relieve headaches, coughs, snakebite and hair loss.
Although this was probably only wishful thinking, science has later identified clear benefits from onion consumption. Onions are believed to benefit health in that they contain phenolics and flavonoids that have potential anti-inflammatory, anti-cholesterol, anticancer and antioxidant properties. These include quercetin and its glycosides quercetin 3,4′-diglucoside and quercetin-4′-glucoside. Since there are considerable differences between different onion varieties in antioxidant content, it might be good to know that shallots have the highest levels.
Sensitive people and pets
Enjoy your onion soup but don’t feed it to pets (Photo: jeffreyw)
Not all people should handle onions since it can produce allergic reactions in a small sensitive sub-population. It can cause symptoms like contact dermatitis, intense itching, rhinoconjunctivitis, blurred vision, bronchial asthma, sweating and anaphylaxis. Despite such reactions from handling onions, there seems be no allergic reaction when they consume onions, particularly when cooked since this would denature potentially damaging proteins.
Also be aware that onions can be deadly for dogs, cats, guinea pigs, monkeys and other animals. The sulfoxides mentioned above as the starting material for the formation of the human crying gas can be toxic to pets. They are present in raw as well as cooked onions, although cooking destroys the enzymes that induce the gas formation in raw onions. Many animals are unable to digest sulfoxides which results in anaemia caused by the distortion and rupture of red blood cells.
So although you can safely enjoy a delicious onion soup, don’t feed left-overs to your pet.
(This blog was written in response to the WordPress Daily Prompt: Moved to tears)
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Focus on food safety 
This is really fascinating. It’s good to know I cry chopping onions because of science, not because I’m a wimp (that remains true, but I suppose I’ll have to find another excuse.) Thank you for including my post in your Related Articles section.
Interesting… I have a severe neurological response to onions when they are being cut. Immediate severe pain. From 0-8 on the pain scale in an instant. Instant vertigo, nausea, dry heaves(sp?), slurred speech. As soon as I remove myself from the onions the worst goes away but the nausea remains. I can’t eat raw onions. No anaphylaxis. Just disgusts me. Makes me gag. But if onions are well cooked and transparent, they are delicious and I have zero negative symptoms. I’ve tried buying and cooking frozen onions but while the reaction is not nearly severe, it still bothers me. This used to be just white yellow and purple. I could still eat chives raw . But now even the greenest chives bother me.