Tear drops on my cutting board

Wash your hands, remove your makeup, and put on the safety goggles from first-year chemistry. You grab the cutting board and knife and start to chew gum.

No matter how much we prepare when cutting onions, we still end up crying. The good news is everyone cries when chopping onions; however, the bad news is there isn’t much we can do to avoid it.


Getting ready to cut onions. Source: Flickr


Recently, a group of chemists at various university in the United States discovered what causes us to cry when we cut onions. Onions have a defence mechanism for when they are cut open. They release a chemical close to tear gas that is supposed to deter the predator. The chemical is called lachrymatory factor (LF) causing “inconvenient eye irritation.” LF is formed when there is a reaction with the enzyme lachrymatory factor synthase (LFS) and sulfenic acid which only happens when we cut onions.

They explained the structure of the enzyme by comparing it to an acid receptor commonly found in plants and discovered how the enzyme and acid can coexist without reacting all the time. The two chemicals are near each other in an onion, but they only react when the cell walls break (aka when an onion is cut).

Only two other types of plants (Petiveria alliacea and Allium siculum) have this defence mechanism, but we do not consume them. Many animals deter from eating these plants, but we can’t stop.

Allium siculum plant. Pretty but has a nasty defense mechanism. Source: flickr

You may be wondering, is there a way to get rid of the side effect onions give us? Technically yes. Researchers from Japan, genetically modified (GM) the onion by removing LFS. These GM onions were not only tearless, but they were also less pungent because the amount of LF in the onion was much less. This can be a deal breaker for some chefs and foodies because many of us like the pungent taste of onions. Also, with a public backlash on GM products, it will be very difficult for the GM onions to enter the market in most western countries.

There are other ways to cut onions that require no chemistry equipment and crying. No, it does not involve chewing gum, lighting a candle, or breathing through your mouth because none of these “tricks” work. What does work is modifying the reaction yourself by slowing it down. When an onion is placed in a cold space, aka the fridge, the reduced temperature causes the reaction to slow down. This, however, makes the taste less pungent as well.

We see this same occurrence with other produce. Chilling tomatoes in the fridge alters the DNA and removes some of the flavour.

This research was groundbreaking in the science community because how the reaction works was never explained. Scientists can now visualise the process using computer models and fully understand the mechanism responsible for what makes us cry when cutting onions.

For the real science engaged chefs out there, with or without your chemistry goggles, we are conducting a chemistry experiment when we cut onions. Now go grab that lab coat… erh I mean apron.

2 Responses to “Tear drops on my cutting board”

  1. Debbie says:

    Is there a reason why some people are more sensitive to onion tears than others? I could be standing in an adjacent room and still end up with tears streaming down my face, yet when I walk into the kitchen, my partner is perfectly fine!

  2. scripps says:

    That’s so cool I never knew it was a defence mechanism! I agree cooked onions are so good though I wouldn’t want to remove the stinging sensation at the cost of reducing the flavour.