An Insightful Exploration of the Tear-Producing Chemistry of Onions

You slice an onion.

A few seconds later, your eyes begin to water.

What seems like an instantaneous response is actually a chain of biochemical events.

It is easy to say the onion "makes you cry."

Actually, onions have a built-in chemical defense mechanism that makes their predators (including humans) quite uncomfortable.

🔪 WHAT HAPPENS WHEN YOU CUT AN ONION?

An unbroken onion contains several chemical substances within its cells.

These substances are largely segregated while the cells are intact.

However, cutting, crushing or chopping the onion causes release of enzymes that facilitate the production of several sulfur-containing substances.

One of these is a volatile (easily airborne) irritant called the "lachrymatory factor."

Because it is volatile, the lachrymatory factor can easily reach your eyes, causing your tears to flow.

👁️ WHY DO YOUR EYES START WATERING?

Your eyes have a built-in response to irritants in the form of a chemical signaling system that tells your tear ducts to create more tears.

The tears then carry the irritants away from your eyes.

The process of making tears and washing the irritants away is basically a protective response by your eyes.

Your eyes aren't "sad" or "angry" when you cut an onion - they're simply reacting to the presence of these irritating chemicals.

🧪 WHY DOES THE CHEMISTRY MATTER?

This is a wonderful example of basic chemistry in the context of the natural world.

The onion plant does not have to "fight" potential predators (animals that might eat it) because it cannot move from place to place.

Instead, when its cells are broken (by insects, animals or humans), it can release toxic, bitter or unpleasant substances.

In fact, this defense mechanism is what makes us want to cry in the first place!

The chemical reaction within the broken cells of the onion is actually useful to us (it keeps us from eating potentially harmful plants) and to the plant itself (it keeps us from eating its cells in the first place).

🌱 WHY WOULD AN ONION NEED SUCH A DEFENSE?

Plants can't simply run away when something wants to eat them.

They have to rely on other forms of protection.

Many plants make chemical substances that are distasteful or harmful to animals and insects that want to eat them.

Some give off smells or tastes that repel insects.

Many plants develop physical adaptations like spines or thorns as deterrents.

Onions are actually part of a family of plants that have several unique sulfur-based biochemical substances that contribute to their aroma and flavor.

The tear-making potential of the onion is just another characteristic of this plant's unique chemical makeup.

🧅 WHY DON'T ALL ONIONS MAKE YOU CRY EQUALLY?

Some people find that the degree to which they cry when cutting onions varies from onion to onion.

Each onion has its own biochemistry.

The type and quantity of tear-inducing substance present in each onion can also depend on storage conditions and overall freshness.

The way the onion is cut can also affect the likelihood of it making you cry.

If you use a particularly sharp knife, the cells of the onion will be broken more efficiently than with a dull knife.

The former probably minimizes damage to the cells, thereby minimizing the release of enzymes and other cell contents that play a role in tear production.

❄️ CAN CHILLING AN ONION HELP?

You might have heard that storing an onion in a refrigerator slows the rate at which it begins to spoil.

Cooling it can also slow the release of irritating (and airborne) substances like the lachrymatory factor.

However, it's not foolproof.

Another common approach is to store onions in ventilated containers and to use a sharp knife to do your cutting.

🧠 A SMALL KITCHEN EXPERIMENT

The next time you cut onions, take notice of how your eyes feel when you first begin to cut.

You might notice the following:

CUT → CELLS BREAK → CHEMICAL REACTIONS → IRRITATING COMPOUND → EYE RESPONSE → TEARS

Actually, the process of cutting an onion is a fascinating study in the interaction of biology and chemistry.

It is helpful to remember that science doesn't only happen in test tubes and beakers.

Much of it takes place all around us in the world - sometimes, even in our kitchens!

🌍 THE BIGGER LESSON

In general this experience serves as a wonderful reminder that science is everywhere, and that we can learn about our world no matter where we are.

Chemistry is happening around us all the time.

Biology is also taking place in all living things, including us.

By understanding the way our eyes work and why they might begin to water unexpectedly, it's possible to not only be prepared for what may happen - it's possible to be able to explain why it happens.

After all, science really isn't that hard.

It's not some complex, inaccessible endeavor.

Science is simply being curious about the world around us and asking, "Why?"

🌿 AN ETHICAL REFLECTION

Understanding the way our world works can help us to gain important perspective.

In a larger sense, an investigation of something as seemingly simple as why we cry when cutting an onion can help us to become more thoughtful people.

Perhaps we can learn to be less judgmental of others, because we can see the world through their eyes (literally).

We can also foster a sense of humility by recognizing that there is still much about our world that we do not understand, despite the many things we do know.

Knowledge doesn't begin in a laboratory or a textbook.

It begins when we choose to ask questions about the things that surround us.

It begins with curiosity.

❓ QUICK QUESTIONS

Why do onions make us cry?

Cutting an onion triggers a series of biochemical reactions that result in the formation of an airborne irritant that causes our eyes to water.

Are the tears emotional?

No. They are primarily a protective response to the irritant produced by a cut onion.

Why does cutting damage matter?

Cutting the onion facilitates the mixing of various substances that had previously been segregated, resulting in a series of (unpleasant) chemical reactions.

Can a sharp knife help?

Yes. Using a sharp knife may cause less mechanical damage to the cells compared to a dull knife, resulting in less irritation.

📌 KEY TAKEAWAYS

• Cutting an onion damages its cells.

• Broken cells release substances that interact chemically.

• The reactions that occur generate volatile sulfur-containing substances.

• One of these can irritate eyes.

• The eyes respond by making tears.

• Different onion varieties and preparation techniques yield different results.

• A simple kitchen experience often involves complex chemical and biological processes.

CONCLUSION

When you next find yourself watering eyes while slicing onions, understand that your eyes are simply reacting to an irritation.

You can thank them (or blame them) for trying to wash away the volatile substances released when you damage the cells of the onion.

This seemingly simple observation serves as just one example of the fascinating ways in which science plays a role in our lives.

🧅 FEEDCOVER QUESTION

What other typical food or household experience makes you wonder, "Why does that happen?"

SOURCE

American Chemical Society — educational resources on the chemistry of onions and tear production.