What Should I Know?
When an onion is cut, the alliinase enzyme comes into contact with air and produces a volatile compound called syn-propanethial S-oxide.
When this compound reaches the eyes, it causes a burning sensation and tearing.
Onions developed this mechanism as a way to defend themselves against microorganisms and animals.
The root end contains the highest concentration of these compounds.
Refrigerating the onion, providing ventilation, or chopping it under water can help reduce the effect.
Why Does It Matter?
Onions are a staple ingredient in millions of kitchens worldwide. Yet, the burning and tearing experienced while chopping them is nearly universal. Understanding this biochemical mechanism not only makes kitchen life more comfortable, but also helps us appreciate the defense strategies plants have evolved in nature.
What Do the Rules and Science Say?
When an onion (Allium cepa) is cut, its cell structures are damaged, activating the alliinase enzyme. This enzyme breaks down certain compounds in the cells to form 1-propenesulfenic acid. The unstable compound is then converted by another enzyme, lachrymatory factor synthase, into the volatile gas syn-propanethial S-oxide.
Once airborne, this gas reaches the eyes and triggers burning and tearing. Our eyes respond with reflex tearing to wash away the irritant. This chemical chain reaction is essentially a natural survival mechanism that onions developed for protection.
How Can It Be Reduced?
There are several methods to reduce tearing while chopping onions:
Leave the root end until last.
Use a fan or ventilation to disperse the gas.
Wear swimming goggles or other physical barriers.
Chop under running water or chill the onion before cutting.
These methods either limit the release of the volatile compound into the air or slow down the enzyme activity.
Why Does It Taste or Feel This Way?
The onion’s characteristic sharp taste and smell come from the same volatile compounds and sulfur-containing molecules responsible for eye irritation. In other words, the mechanism that makes us tear up is also part of what gives onions their unique aroma.
What Are the Common Misconceptions?
Eye-watering is not caused by “just gas,” but by the enzymatic reaction that produces it.
Not all onions have the same effect; sweeter varieties produce less of the compound.
Soaking an onion in water before cutting does not prevent eye irritation; the critical moment is when compounds are released during chopping.
A spoiled onion smells bad for different reasons and has nothing to do with eye irritation.
Why Are We Sharing This?
The phenomenon of “crying onions” is a small but universal kitchen experience. Explaining the science behind it not only improves everyday comfort for consumers, but also deepens our awareness of the fascinating chemical defense systems plants have developed.