Why does cutting onions make you cry?
If you’ve spent any time in a kitchen, preparing a meal, you’ve likely had to cut an onion. And, if you’ve cut an onion, you’ve almost surely experienced the curious ability of this food to induce tears.
Your experience is neither unique nor new. Onions are one of the oldest crop foods that we have in our diets today and have been grown and cultivated for thousands of years. Ancient Egyptians worshiped onions as their skin layers were thought to evoke eternity, and pharaohs were at times buried with onions, including Ramses IV who was entombed with onions in his eye sockets. Other ancient cultures even felt that onions had medicinal properties, including in ancient India where it was invoked as a diuretic, in ancient Greece where athletes used onions to try to boost performance, and in ancient Rome where onions were to felt be beneficial for health in numerous ways, including by inducing sleep, healing mouth sores, and improving vision.
Many assume that an irritating substance is released into the air when one cuts into an onion. This makes sense as cell walls are damaged during the dicing process. And it is true that a specific sulfur compound does get released into the air from these damaged cells: S-alkenyl cysteine sulfoxide. But this is not the molecule that leads to tears. The process is more complex and more fascinating.
Along with S-alkenyl cysteine sulfoxide, an enzyme called allinase is also released when an onion is cut. Allinase converts cysteine sulfoxide into sulfenic acids. And this process happens in the air around both the onion and us. Sulfenic acids are unstable molecules and are rapidly and spontaneously rearranged to form thiosulfinates. Thiosulfinates give onions their pungent aroma and flavor and for a long time were assumed to be the source of onion-induced crying, the supposed “lachrymatory factor”. But they are not the source.
Hunting for the lachrymatory factor two chemists isolated a substance from onion extract that reliably induced tears and crying when subjects were exposed to it. The lachrymatory factor had been found, and it wasn’t a thiosulfinate. Instead, they isolated a molecule called propanethial-S-oxide which is generated by the aptly named lachrymatory factor synthase from sulfenic acids. Thus, propanethial-S-oxide, is the illusory tear-inducing molecule.
To summarize, S-alkenyl cysteine sulfoxide and the enzymes allinase and lachrymatory factor synthase are released into the air when you cut into an onion. Allinase converts sulfoxides into sulfenic acids which then spontaneously convert to thiosulfunates. But, some of the sulfenic acids are acted upon by lachrymatory synthase to convert them to the actual lachrymatory factor, what we now know is propanethial-S-oxide. This cascade of required events may explain the delay between the initial cut and the tears which invariably follow.
As to how propanethial-S-oxide causes tears, the prevailing theory seems to be that it’s a direct corneal irritant.
For any home cook, the question then becomes how can this be prevented? There are many purported ways to help mitigate onion-induced crying, though none has been rigorously studied. One small study from 2004 looked at the use of contact lenses to block propanethial-S-oxide’s access to the cornea and reported a delayed sensation of eye irritation in those who wore contact lenses during exposure. Presumably goggles would offer an even more effective barrier function.
A chemist named Eric Block has advocated for chilling onions in the refrigerator, to reduce the volatility of the various factors that get released into the air when an onion is cut and sets off these chemical reactions. He also has suggested peeling the onion under running water, as propanethial-S-oxide is water-soluble and therefore theoretically will get washed away.
Plant biologists actually endeavored to genetically transform an onion strain to specifically lack lachrymatory factor synthase. Their findings were reported in 2008 in the journal Plant Physiology. Unfortunately, the authors performed an informal taste test and found that the modified onions lacked pungency when crushed raw, and were less flavorful than regular onions, though when cooked and sauteed they retained their normal, sweet flavor that all onions take on. So sort of a mixed bag.
One final puzzle: why don’t scallions (also known as green onions) and garlic also lead to tears when chopped? After all , both are cousins of onions from the genus allium. But they lack lachrymatory factor synthase and therefore cannot generate propanethiol-S-oxide. They mercifully do not make you cry.
Take Home Points
- There are multiple enzymatic reactions that occur in the air when an onion is cut that lead to corneal irritation and crying
- The first step involves the enzyme allinase converting S-alkenyl cysteine sulfoxide to sulfenic acids
- A second enzyme, lachrymatory factor synthase, converts these sulfenic acids to propanethial-S-oxide, which is the so-called lachrymatory factor. And that gives us all something to cry about… when cutting onions that is.
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Credits & Citation
◾️Episode written by Avi Cooper
◾️Show notes written by Avi Cooper and Tony Breu
◾️Audio edited by Clair Morgan of nodderly.com
Cooper AZ, Abrams HR, Breu AC. Something to cry about. The Curious Clinicians Podcast. April 27, 2022