Why do onions make us cry when we cook them?


Fans of pissaladière, onion soup, or even ratatouille know this well… THE onions stings my eyes! But where exactly does this property come from? And what are the best techniques for peeling and cutting a fresh onion without shedding a tear on the cutting board?

Onions make you cry: a question of chemistry!

Like shallots, onions can indeed make us cry… As Dr Jean du Breuillac, general practitioner and deputy secretary general of the College of General Medicine (CMG), explains to us, it is a completely natural and almost inevitable chemical reaction.

Propanesulfonic acid, a merciless irritant

Onions contain many sulfur compounds. When cut, aliinase enzymes are released which mix with sulfur compounds to better form sulfenic acid. Too unstable, the latter spontaneously reorganizes into propanethia oxide (syn-propanethial-S-oxide), a tear gas that disperses in the surrounding air and usually eventually reaches our cornea.

This is where things get tricky. As Dr du Breuillac reminds us, our cornea is constantly hydrated by basal tears which aim to oxygenate it but also to lubricate and nourish the cells of the eye. However, at the slightest contact with humidity, propanediol oxide transforms into propane sulfonic acid liquid as irritating as sulfuric acid: hence the intense burning sensation in the eyes!

In this case, our tear glands activate and begin to produce as many tears as possible to protect our eyes and quickly eliminate the irritating acid. Concretely, these so-called “reflex” tears allow us to rinse our eyes and reduce unwanted symptoms. Dr Jean du Breuillac, general practitioner.

Good to know: this chemical reaction is a natural adaptation of the onion to protect itself from predators and pathogens. In practice, these sulfur compounds prevent infections and help extend the life of onions by inhibiting the growth of bacteria and fungi. That said, they no longer take effect when the onions are cooked!

Eye health: is onion good for the eyes?

As you probably know, it is absolutely not recommended to eat onions. Given their richness in antioxidants, their consumption is rather encouraged as part of a varied and balanced diet.

Contrary to what you might think, onions are not dangerous for our eye health. Tearing is temporary and irritation subsides once exposure stops. That said, it may last longer if onion juice comes into direct contact with your eyes…

How to avoid crying while peeling or cutting an onion?

Although impossible to completely eliminate the irritating effect certain tips help limit the production of tears…

Peel an onion without crying

In theory, peeling onions poses few risks. But we are never safe from a bad surprise! To avoid tears, you can:

  • peel the onion under water;
  • peel the onion as quickly as possible;
  • peel the onion under an extractor hood;
  • direct a fan towards your work surface;
  • or refrigerate the onion for at least 30 minutes before peeling.

Cutting an onion safely

  • Before cutting it, place the onion in the refrigerator for 30 minutes (or in the freezer for about 15 minutes).
  • Use a sharp knife which will break down the onion cells less and reduce gas formation.
  • Cut the onion under a stream of cold water or install a water source near your work surface.
  • Stick your tongue out for the duration of the cut, so your saliva will capture the irritating gas and your eyes will be spared. Some people even advise to chew gum to increase saliva production!
  • Cook in a well-ventilated space to promote the dispersion of the irritating gas (open your windows or use an extractor hood).
  • Wear protective glasses ideally airtight swimming goggles, to prevent irritating vapors from coming into contact with your eyes.

Everyone has their own method! Anyway, once the onion is cut, Think of wash your hands well or even rinse them with lemon juice or vinegar before running them under cold water.

“Sunions”, finally a variety that doesn’t sting the eyes!

Can’t bear to shed a tear every time you cut an onion? At the risk of surprising you, there is a revolutionary variety of onions: unions. Although they are identical in every way to traditional yellow onions, they still have the particularity of not making us cry. FYI, these onions are not the result of genetic manipulation but of natural crossing! “It took thirty years of research in conventional breeding to develop this onion,” explains David Corré, head of the seeds sector at BASF France, recently interviewed by 20 Minutes (source 1).

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