This probably happened to all of you. You decide to chop that freshly peeled onion and out of sudden you start crying and can’t stop. Why does this happen? Why do we shed tears when cutting onions? Do we feel sorry for them?

Today I would like to explain this phenomena to you. And give you a solution to avoid onion-caused outburst of tears.

Please do not chop onions while reading this article.

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Introduction to onions

Latin name for onions is Allium cepa - cepa is Latin for onion. Onions are close relatives of garlic, challots or chives. They are cultivated and used all around the world and are part of many cooked dishes. You probably ate one in the last month. I bet you. Fortunately eating one is usually not as unpleasant as chopping one. While chopping onions most of us experience eye irritation in the form of excessive production of tears. But. What if I told you that we cry all the time?

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Lacrimal gland

Yes, we cry almost all the time. This process is called basal tear secretion and is under the control of parasympathetic nervous pathways. Both of our eyes have these almond-shaped glands termed lacrimal glands that secrete thin layer of aqueous film (tears). This mechanism prevents your eyes from drying and keeps them clean and lubricated.

The reason why we do not notice this is that these tears are usually quickly drained away by a pair of lacrimal ducts (those pink thingies in the inner sides of your eyes). However, when there is excessive production of tears the drainage system cannot keep up and we cry. These drains also direct the tears through nasolacrimal canal to your nose, hence the reason why we have runny nose while crying.

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Apart from basal tear secretion we cry due to different reasons. It might be mechanical stimulation – we cry when something happens to fall into our eyes. Then emotions – humans cry due to anger, fear, sadness. And last, but not least crying due to presence of chemical stimuli (will cover that in a moment). What have all of these reasons for crying in common? They all happen to be due to increased activity of responsible parasympathetic nervous pathways.

The answer – chemicals

You probably guessed it right. While chopping onions specific chemicals are released that stimulate responsible parasympathetic nervous pathways and as a result we start crying. But let’s take a closer look at this process.

When you slice onion, damaged cells release enzymes called alliinases which catalyse series of chemical reactions that lead to formation of sulfenic acid. This acid is then converted to a volatile gas termed lachrymatory factor (syn-propanethial-S-oxide) by enzyme lachrymatory factor synthase. Lachrymatory factor then reacts with water on the surface of cornea and result of this reaction is the production of several noxious compounds such as sulphuric acid or hydrogen sulphide- these activate the defense mechanisms of eyes and thus cause snivelling. There is also another compound, that is being secreted, called allicin, which has similar effects as lachrymatory factor. All these chemical substances cause your eyes to sting and burn until they are diluted or washed away.

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How to prevent this?

Well, there are numerous ways how to prevent this emotional display from happening. For example you could refrigerate the onions prior cutting, hence reducing the enzyme reaction rate. Or wear glasses. Also practice makes master- the more regularly you chop onions the more used to them you will get. Another way is the usage of special GMO onion that do not release these chemicals, but unfortunately you probably won’t get them in your local supermarket.

Best advice at the end – Make someone else chop the onions for you! :D

Concluding remarks

So, fellow Steemians, I do believe you learned something new today. Heck, I did too. It is really interesting how many things happen everyday and we do not even stop to think why. Now you know why you cry while chopping onions. Great right. Well, if you have any ideas or suggestions or really questions, then leave them in the comments.

Thank you and have a great day!

References:

Article about onion tears by Professor Ian Gibbins – great article, check it out!
Plant biochemistry: An onion enzyme that makes the eyes water – Article from Nature about enzymes in onions
Onion – Wikipedia article