Onions make you cry because they release a chemical irritant when you cut, chop, or slice them. This irritant is known as syn-propanethial-S-oxide. Here’s how it works:
Onion cells contain various compounds, including amino acid sulfoxides and enzymes. When you cut an onion, you rupture its cell walls.
Enzymes present in the onion cells, specifically alliinase enzymes, come into contact with the amino acid sulfoxides, converting them into sulfenic acids.
The sulfenic acids produced are unstable and rapidly rearrange themselves into syn-propanethial-S-oxide, which is a volatile, sulfur-containing compound.
When syn-propanethial-S-oxide is released into the air, it can reach your eyes. It is a powerful irritant to the eyes, triggering a reaction to protect them from potential harm.
Your eyes respond by producing tears to flush out and dilute the irritant. This results in a stinging or burning sensation and the characteristic tearing when cutting onions.
There are a few methods to reduce the tears when cutting onions:
Chill the Onion: Refrigerating or freezing the onion before cutting can slow down the release of the irritant.
Use a Sharp Knife: A sharp knife reduces the cell damage when you cut, potentially reducing the release of irritants.
Cut Underwater: Cutting onions under running water or submerged in a bowl of water can help contain the irritants and reduce their contact with your eyes.
Wear Goggles: Wearing protective goggles or glasses while cutting onions can shield your eyes from the irritants.
Ventilation: Ensure good ventilation in your kitchen, as it can help dissipate the irritants.
While crying when cutting onions is an annoyance, it’s a temporary and harmless reaction caused by the release of these irritants. Once the onion is cooked or the irritants are washed away, the tears usually stop.