An onion looks peaceful. Inside, it is a loaded chemical trap, and the trigger is your knife. For over a century chemists thought they understood why cutting one makes your eyes burn. Then, in 2002, a research team in Japan found a second enzyme nobody had accounted for, and the story changed. This is the three-minute version of what actually happens between the cutting board and your tear ducts.
The chemistry weapon hidden in every onion
The bulb stores odorless sulfur compounds called S-alk(en)yl cysteine sulfoxides, the most important of which is isoalliin. In separate compartments, the plant keeps two enzymes that never touch those compounds while the cells are intact. When the knife breaks the cell walls, the ingredients finally meet.
The first enzyme, alliinase, chops isoalliin into a reactive intermediate in milliseconds. For decades, textbooks assumed that intermediate rearranged on its own into the irritant. In 2002, Shinsuke Imai and colleagues at the House Foods research lab showed that a second enzyme, which they named lachrymatory-factor synthase (LFS), actively reshapes the intermediate into the final tear molecule: syn-propanethial-S-oxide.
Think of two kitchen drawers, one holding vinegar and one holding baking soda. The moment the knife crosses both, they fizz. Here, what fizzes is a tiny volatile gas built for your eyes.
Why your eyes burn
Syn-propanethial-S-oxide is volatile, so it rises straight from the cutting board to your face. On the wet surface of the eye it reacts with water and leaves traces of sulfenic and sulfuric acid. Your cornea carries sensory neurons tuned to exactly this kind of chemical threat, through a receptor called TRPA1, mapped in detail by David Julius’s lab, work that contributed to his 2021 Nobel Prize.
TRPA1 fires an alarm. The brain orders the tear glands to flush the irritant out. That is the sting, the blinking and the tears. It is not sadness, weakness or an allergy; it is a reflex that evolved to wash a chemical off the eye as fast as possible.
The numbers back it up. Imai’s team showed that without LFS, onions produce far less lachrymatory factor, and plants engineered to silence the enzyme barely make anyone cry. In 2008, a New Zealand and Japanese group announced a tearless onion built exactly that way.
A defense that backfired on the plant
Why would a plant evolve a weapon it only fires when wounded? The leading explanation is defense. Onions, garlic and their relatives invest heavily in sulfur chemistry to deter insects, fungi and hungry mammals. The lachrymatory factor and its cousins are toxic or repellent to many small herbivores, and the two-enzyme design means the plant pays no cost until something bites it. Humans simply happen to be large, curious mammals who kept cutting anyway.
The practical tricks follow directly from the mechanism:
- Chill the onion. Cold slows both enzymes, so less gas is produced per cut.
- Use a sharp knife. A sharp blade crushes fewer cells and releases less substrate.
- Cut under running water or near a fan. The molecule travels through air; move the air and it never reaches your eyes.
- Goggles work, gum and bread do not. The target is the cornea, not your mouth.
So the tears are a 2002-confirmed enzymatic reaction, a plant defense older than agriculture, firing in your kitchen in under a second.
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If scientists can silence LFS and sell tearless onions, would you buy one, or does cooking without the sting feel wrong? Tell us under the video. Next time we travel to 1676, to a Dutch draper who looked through a single bead of glass and saw something no human had ever seen.
Prefer to watch it instead? The full story, with the diagrams and the sources, is on the channel: Why Onions Make You Cry: The Enzyme Behind the Tears. Subscribe for one real science question, answered in three minutes.
Sources: Imai et al., Nature 419, 685 (2002); Julius lab, TRPA1 and chemical nociception (Nobel Prize in Physiology or Medicine 2021); Eady et al., Plant Physiology (2008), tearless onion via LFS silencing. This article accompanies an AI-assisted video; the script and visuals were produced with AI tools and reviewed before publication.
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