Why do onions make you cry? I’ve heard it has something to do with sulfur compounds, but can you explain the specific chemical reaction that occurs when you cut into an onion that leads to the release of these compounds? What exactly is the lachrymatory factor, and how does it irritate our eyes? Furthermore, is there any scientific evidence to suggest that certain types of onions are more likely to cause tearing than others? And finally, are there any proven methods or techniques to minimize the tearing effect when cutting onions? Are things like refrigerating the onion, using a sharp knife, or holding water in your mouth actually effective, and if so, why?

Answer

When you cut an onion, you break open its cells, releasing enzymes called alliinases and sulfenic acids. Specifically, one of these sulfenic acids is 1-propenyl-L-cysteine sulfoxide (also known as PRENCSO).

Alliinases catalyze the breakdown of PRENCSO into several different sulfur-containing compounds. One of these compounds is propanethial S-oxide, a volatile sulfur compound.

Propanethial S-oxide drifts up into the air and when it comes into contact with the surface of your eye, it reacts with the water there to form sulfuric acid.

The sulfuric acid irritates the sensory neurons at the cornea, which signals the brain. The brain then activates the lacrimal glands, which produce tears to dilute and wash away the irritant. This is a reflex response designed to protect the eye.

Essentially, the onion’s defense mechanism against being eaten involves creating a mild irritant that causes tears and discourages consumption.