Writers 
The Little Chemistry Lesson We Don’t Expect
Imagine.
You cut an onion. A few seconds later. Your eyes start to sting. Then - the tears.
It all happens so fast that it seems like the onion is fighting back.
And in a way, it is.
An onion has a fascinating chemical defense mechanism which causes it to "fight back" when cut.
🔪 WHAT HAPPENS WHEN YOU CUT AN ONION?
A whole onion contains a variety of chemical substances in their cells.
As long as the onion is intact, these substances do not interact with each other.
However, when you start cutting, chopping or dicing, you disrupt those cells and allow those previously isolated substances to interact.
Enzymes contained within the cells are instrumental in catalyzing the chemical reactions that produce a number of sulfur-containing substances.
The end result of these reactions is a volatile, irritating substance known as "lachrymatory factor".
Being volatile, it easily becomes airborne and wafts its way into your tear ducts.
👁️ WHY DO YOUR EYES START WATERING?
Your eyes have a fascinating defense mechanism which is triggered when this irritating substance comes into contact with the eye surface.
It activates sensory neurons which signal the brain to produce tears.
Tears will then neutralize and wash away the irritating substance.
So, not only is the mechanism fascinating - it is useful and highly effective.
Your eyes are essentially screaming "Irritating substance detected! Initiating decontamination procedure: tears."
🧪 WHY DOES THE CHEMISTRY MATTER?
It's all an example of a clever chemical defense mechanism.
An onion plant does not have the luxury of running away from predators, and so it has developed a clever defense mechanism.
When its cells are broken, it releases chemicals that can deter insects, animals, humans alike.
It's the same mechanism that causes our eyes to water, but it's also a survival mechanism that keeps the onion from becoming someone's meal.
🌱 WHY WOULD AN ONION NEED SUCH A DEFENSE?
Plants do not have the luxury of running away when attacked by animals, insects or fungi.
As such, they have to rely on other, indirect means of defense.
They can make themselves bitter or unappetizing.
They can generate insect-repelling substances.
Or they can generate structures that deter herbivores (such as thorns or prickly surfaces).
The tear gas defense mechanism of an onion falls under the first two categories.
Onions are known to have a variety of sulfur-based biochemicals within their cells.
🧅 WHY DON'T ALL ONIONS MAKE YOU CRY EQUALLY?
It's all a matter of preference.
Some varieties have more of these substances than others.
The temperature and storage conditions can also affect the reaction.
Lastly, the way you cut the onion matters.
A sharp knife will slice through the onion, making fewer cuts than a dull knife.
A dull knife, however, will crush and tear the onion, resulting in more of the eye-irritating chemicals being released.
It's not foolproof, but it can certainly help.
❄️ CAN CHILLING AN ONION HELP?
Some swear that chilling an onion before cutting it reduces the amount of eye-watering it causes.
Chilling it inhibits some of the reactions that result in the lachrymatory factor being generated.
It also reduces the movement of airborne substances, meaning the volatile ones have less opportunity to move around.
However, it is not a universal remedy.
It depends on a number of different factors, including the type of onion and the temperature of the room.
Other methods, such as providing adequate ventilation and using a sharp knife, may be even more effective.
🧠 A SMALL KITCHEN EXPERIMENT
Next time you prepare an onion, pay attention to what happens.
Specifically, try and observe when and why your eyes start watering.
CUT → CELLS BREAK → CHEMICAL REACTIONS → IRRITATING COMPOUND → EYE RESPONSE → TEARS
What seems like a small annoyance is, in reality, a fascinating chain of biochemical and physiological events that happen within seconds.
🌍 THE BIGGER LESSON
Onions are a great example of how science is not confined to sterile laboratories.
It surrounds us in our everyday lives, often in the most unsuspecting places.
The same kind of chemical reactions, biochemical pathways and physiological responses take place all the time, all around us.
We simply have to be willing to observe.
That's the beauty of science: wherever there are questions, there are answers waiting to be discovered.
🌿 AN ETHICAL REFLECTION
The ability to observe and investigate the world around us can be an important part of developing a strong ethical and moral compass.
By thinking critically about commonly accepted ideas, we can often discover new and better ways of looking at things.
Instead of accepting the world the way it is, we can analyze it and improve it.
It's a great lesson that can be learned from something as simple as an onion and a few tears.
❓ QUICK QUESTIONS
Why do onions make us cry?
Cutting an onion triggers a chain of chemical reactions that release an irritating volatile substance that makes us tear up.
Are the tears emotional?
No. In this context, they are purely a physical response to the irritation of the eyes.
Why does cutting damage matter?
Cell damage is necessary to release enzymes and other substances that participate in the chemical reactions that take place.
Can a sharp knife help?
It may reduce the amount of crushing of cells that takes place, thus reducing the amount of irritating chemicals released. This, in turn, reduces the likelihood of tears forming.
📌 KEY TAKEAWAYS
• Cutting an onion damages its cells
• The damaged cells release enzymes and other substances
• Those substances then participate in chemical reactions
• One of the products of the reactions is a volatile, eye-irritating substance
• The eyes react to the substance by watering up
• Different onions and cutting techniques produce different results
• A single kitchen experiment can produce fascinating insights into chemistry and biology
CONCLUSION
Next time you find yourself crying over onions, remember: it's not your emotions that are to blame. It's the chemistry taking place in the kitchen. The eyes are reacting to an airborne, eye-irritating substance released after the onion cells are damaged.
It really is a fascinating defense mechanism that plants use - and a handy way of demonstrating the wonders of science to young inquisitive minds.
WRITERS ARENA QUESTION
What other common food or kitchen experience makes you wonder why it takes place?
SOURCE
American Chemical Society - Educational resources on onions and tear production.
https://www.acs.org/
