Onion cells store sulfur compounds and a separate enzyme called alliinase in different compartments, kept safely apart while the onion is intact. Slicing through the cell walls breaks that separation, letting the enzyme react with the sulfur compounds and produce unstable molecules called sulfenic acids within seconds.

A second enzyme, discovered only in 2002 by researchers in New Zealand, converts those sulfenic acids into a volatile gas called syn-propanethial-S-oxide. It's light enough to drift straight up into the air and reach your eyes before you've even finished cutting.

The Gas Released the Moment You Cut

Why Your Eyes React, and What Actually Helps

Once the gas touches the watery tear film covering your eyes, it reacts to form a very dilute sulfuric acid. That irritates the cornea and triggers the trigeminal nerve, which signals the brain to flush the eyes with reflex tears, a purely defensive reaction rather than an emotional one.

Chilling onions for about 15 minutes before cutting slows the enzyme reaction, and cutting near a running extraction fan or under cold water carries the gas away before it reaches your face. A sharper knife also helps, since it crushes fewer cells than a dull one, releasing less gas overall.

Sources

  1. American Chemical Society β€” the chemistry of onion tear-inducing compounds
  2. National Onion Association β€” practical tips for reducing onion tears
  3. Nature β€” discovery of the lachrymatory factor synthase enzyme

FAQ

Does refrigerating onions really reduce tears?

Yes, cold temperatures slow down the enzyme reactions that produce the irritating gas, which is why chilling an onion for 15 minutes before cutting genuinely helps.

Do all onions cause the same amount of tearing?

No, sweeter varieties like Vidalia or Walla Walla onions contain fewer sulfur compounds and typically cause noticeably less irritation than pungent storage onions.

Is wearing goggles while cutting onions actually useful?

Yes, sealed goggles physically block the gas from reaching your eyes, which is why some people genuinely use swim goggles in the kitchen.

About the Author

We reference American Chemical Society, National Onion Association, Nature to explain the background and current understanding of this topic.

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