Tutorial: How to cut onions without crying

When it comes to cooking, it's not just recipes. There's some things you can learn which can make cooking a more enjoyable experience.

[Science lesson of the day] Onions contain an enzyme called alliinase which react with sulfur compounds in the onions. When the cells are broken, the compounds are free to mix together - its similar to when you snap a glow stick and the 2 liquids are free to mix and it starts giving off light. The reaction between the enzyme and onion results in gases that are released into the air and dissolve in your eyes, forming sulfuric acid. The acid stings and irritates the tissues and causes you to tear up to dilute the acid.

How can you prevent this?

  1. If you cut an onion underwater or under running water in a sink, the gases dissolve into the water instead of your eyes.
  2. If you chill the onion before cutting, it will have less free energy for the enzyme to react. If you remember from CHEM 1A, rate of reactions double every 10C. That means a chilled onion will be 4 times slower enzymatically than a room temperature one and should produce less tear inducing gas.

If all else fails, you can wear goggles to shield your eyes.....or ask one of your friends to cut the onion for you :)


Anonymous said…
Genius. But, if you cut underwater, then your onions are wet =( and with the lost visibility you run a higher risk of cutting yourself. Also, contact wearers have extra shielding from fumes, woohoo!
tomkim said…
It doesn't have to be fully submerged. If you place a cutting board on a baking pan and fill it partially with water, it should help.
It's true that contacts can help, but it just blocks the acid from reaching your eyes. I wonder what the effect of sulfuric acid is on contacts.
Thanks for your comments.
Unknown said…
I heard another one-
You cut the onion but leave the root intact until the last two cuts. (Therefore you are slicing the onion, but it still holds together, until it all falls apart on the last cuts).
Jenn Chen said…
I usually soak the onion (after peeling it) in a bowl of salt water (a couple teaspoons of salt works fine).

This works great but I have no idea why...I slept through most of my Chem 1A classes =P
tomkim said…
it might be because the salt kills the enzyme and the reaction never takes place. that would prevent the release of sulfur gases

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