Cutting an onion makes many people cry because it turns a quiet plant tissue into a tiny chemistry lab. When the knife breaks onion cells, chemicals that were stored in separate parts of the cells mix together. The result is a volatile sulfur compound that travels through the air and irritates the surface of your eyes.
Understanding this process connects food science, chemistry, biology, and everyday health.
The main tear-causing molecule is called syn-propanethial-S-oxide, also known as the lachrymatory factor. It forms through enzyme reactions that begin when onion tissue is damaged by slicing, chopping, or crushing. When the vapor reaches the eye, it dissolves in the tear film and triggers sensory nerves, which signal the lacrimal glands to produce more tears.
Cooling onions, using a sharp knife, or increasing ventilation can reduce crying by slowing reactions or moving vapors away from the eyes.
Key Facts
- Cutting onion cells releases the enzyme alliinase, which reacts with sulfur-containing compounds inside the onion.
- Lachrymatory factor synthase helps form syn-propanethial-S-oxide, the main vapor that makes eyes water.
- C = n/V describes concentration, so adding more tear fluid lowers the concentration of irritant on the eye.
- Rate of reaction generally increases with temperature, so chilled onions usually release tear-causing vapors more slowly.
- Diffusion time can be estimated by t ≈ x^2/(2D), where x is distance and D is the diffusion coefficient.
- Onions also contain nutrients and plant chemicals such as vitamin C, fiber, fructans, and quercetin.
Vocabulary
- Lachrymatory factor
- A volatile chemical that stimulates tear production when it reaches the eyes.
- Alliinase
- An enzyme in onions that starts sulfur chemistry when onion cells are damaged.
- Enzyme
- A protein that speeds up a chemical reaction without being used up in the reaction.
- Volatile compound
- A substance that easily evaporates and spreads through the air as a vapor.
- Lacrimal gland
- A gland near the eye that produces tears to clean, protect, and moisten the eye surface.
Common Mistakes to Avoid
- Thinking the onion juice itself must splash into your eye, which is wrong because the main irritant is a vapor that can travel through air.
- Using a dull knife, which is a mistake because it crushes more cells and allows more enzyme and sulfur compounds to mix.
- Assuming all onions cause the same amount of tearing, which is wrong because onion variety, freshness, storage, and sulfur content can change the amount of vapor produced.
- Believing tears mean the onion is harmful to eat, which is wrong because the eye response is protective and onions can still be part of a healthy diet.
Practice Questions
- 1 A chopped onion releases vapor that travels 0.50 m to your eyes in 2.0 s. What is the average speed of the vapor movement in m/s using v = d/t?
- 2 An eye has 0.010 micromoles of irritant dissolved in 0.020 mL of tear fluid. After tearing, the tear volume rises to 0.100 mL while the amount of irritant stays the same. What are the initial and final concentrations using C = n/V?
- 3 Explain why chilling an onion and using a sharp knife can both reduce crying, but for different scientific reasons.