Few kitchen tasks are as reliably tearful as chopping an onion. The reason is a defence system the onion uses against animals that damage it, and it involves a small but impressive bit of chemistry.
Key Takeaways
- Cutting an onion breaks cells and mixes sulfur compounds with enzymes.
- The reaction produces a volatile gas, syn-propanethial S-oxide, that irritates the eyes.
- Your eyes produce tears to flush the irritant away.
- Chilling the onion, using a sharp knife and cutting near ventilation all help.
What happens when you cut
Onions absorb sulfur from the soil and store it as compounds in their cells. Enzymes are kept separately. When a knife breaks the cells, they mix.
An enzyme called alliinase converts sulfur compounds into sulfenic acids. Another enzyme, discovered and described by Japanese researchers in 2002 and called lachrymatory-factor synthase, converts those acids into syn-propanethial S-oxide, the tear-causing gas.
Why your eyes react
The gas drifts up and reaches your eyes, where it stimulates sensory nerves in the cornea. Your body responds by producing tears to wash away the irritant. The sensation peaks about 30 seconds after cutting.
Why some onions are worse
Onions grown in sulfur-rich soils and some varieties produce more of the compounds. Sweet onions generally have less sulfur and are milder. In 2008, New Zealand scientists described an experimental tearless onion developed by silencing the enzyme, and conventionally bred low-tear onions have since appeared in some markets.
Do the tricks work?
Several commonly suggested methods have a basis in chemistry:
- Chill the onion. Colder temperatures slow the enzyme reactions and make the gas less volatile.
- Use a sharp knife. A sharp blade crushes fewer cells than a dull one.
- Cut near ventilation or a fan to move the gas away.
- Wear goggles that seal around the eyes.
- Cut the root end last. Some cooks find this helps, as the root end may have more of the compounds.
Holding bread in your mouth or chewing gum have weaker evidence.
Why cooked onions taste sweet
Heat deactivates the enzymes and breaks down the sulfur compounds. Cooking also develops sweetness through caramelisation and the Maillard reaction, so cooked onions lose their bite.
Garlic is different
Garlic uses a related chemical system to produce allicin, which gives garlic its pungent smell. It lacks the specific enzyme that makes the tear-causing gas, which is why chopping garlic does not usually make you cry.
Frequently Asked Questions
Is the gas harmful? It irritates the eyes but is not considered dangerous at kitchen levels.
Do contact lenses help? Some people find lenses offer some protection, but goggles work better.
Why do my eyes sting more over time? Gas builds up in a small kitchen. Ventilation helps.
The bottom line
Onion tears are a chemical alarm. Chilling the onion, cutting with a sharp knife and moving the air are the best ways to keep it quiet.
