TL;DR: Onions release a tear-inducing chemical irritant called syn-Propanethial-S-oxide when their cells are cut, which stimulates the eyes to produce tears as a protective response.
Introduction
We’ve all been there: You start slicing an onion to prep your dinner, and before long, your eyes sting, your vision blurs, and tears stream down your cheeks. It’s as if the onion’s sole mission in life is to make you cry. But why do onions provoke such a dramatic reaction? Is it some form of vegetable revenge, a random quirk of chemistry, or something else entirely?
The truth lies in the onion’s evolutionary biology and chemistry. Onions (and their relatives like garlic and shallots) evolved to produce certain chemical compounds as a defense mechanism against herbivores and pests. When we slice into them, we release a cascade of chemical reactions resulting in a tear-inducing substance. These chemicals aren’t specifically designed to make humans cry—they’re more about deterring would-be onion grazers in the wild—but we just happen to be collateral damage.
Historical Background
Humans have cultivated onions for thousands of years. Ancient civilizations, including Egyptians and Romans, valued onions not just as a food source but sometimes as a symbol of eternal life or a currency for trade. Across cultures, onions became culinary staples, treasured for their pungent flavor and ability to enhance dishes.
However, chefs and home cooks throughout history have also struggled with the onion’s tears. In old texts, you might find folk remedies advising to cut onions under water, chew bread, or hold a piece of silver in your mouth. While these traditional methods vary in effectiveness, they reflect a long-standing effort to tame the onion’s tearful reputation.
In the modern era, scientists have focused their attention on identifying the specific compounds and enzymes responsible for causing tears. By the 20th century, advances in chemistry allowed researchers to isolate and understand the sulfur-containing compounds that trigger our tear ducts. Today, thanks to studies published in reputable scientific journals and extensive biochemical research, we know precisely which molecules are involved and how they work.
Scientific Explanation
The Chemistry Behind the Tears
Onions belong to the genus Allium, which also includes garlic, leeks, and chives. Allium plants are known for their strong odors and flavors, largely due to the sulfur-containing compounds they produce. These compounds serve as a natural defense mechanism against herbivores, fungi, and microbes.
When you cut into an onion, you’re slicing through its cells, releasing enzymes and volatile molecules that were previously kept separate. Think of the onion as having multiple chemical components stored in tiny compartments. Once these components mix, a chain reaction occurs.
Key Players:
- Sulfoxides (Specifically, S-alk(en)yl-L-cysteine sulfoxides):
These sulfur-containing compounds are abundant in onion cells. They are the “raw materials” from which the irritants form. - Alliinase:
This enzyme acts like a catalyst. When cell walls are ruptured, alliinase comes into contact with the sulfoxides and starts transforming them into other sulfurous compounds, including thiosulfinates and other intermediate molecules. - Lachrymatory Factor Synthase (LFS):
This is the key enzyme discovered relatively recently. It converts one of the intermediate compounds into syn-Propanethial-S-oxide (commonly known as the lachrymatory factor). This lachrymatory factor is what makes your eyes tear up.
The Lachrymatory Factor:
Once formed, syn-Propanethial-S-oxide is volatile, meaning it easily turns into a gas at room temperature. As you chop the onion, this gas wafts toward your face. When it reaches your eyes, it dissolves in the tears that naturally coat your eyeballs, forming a mild sulfuric acid. Your eyes’ sensory nerves sense this irritation and send a signal to your brain: something is stinging. In response, your brain triggers the lacrimal glands (tear glands) to produce more tears, attempting to wash away the irritant.
Why the Tears?
The tearing is essentially a defense mechanism of your own body. Your eyes are trying to flush out a perceived irritant. For the onion, the irritant’s original purpose wasn’t to make you cry; it evolved as a deterrent against herbivores and pests that might munch on its bulb. The fact that it also makes you cry is just a side effect of how our biology responds to sulfuric compounds.
Evolutionary Logic
From an evolutionary perspective, onions developed these sulfurous compounds to protect themselves. Many plants produce chemical defenses—spicy chilies, bitter leaves, and toxic seeds all serve to discourage predators. Onions’ sulfur compounds likely made them less palatable to certain predators. The tear-inducing factor is a byproduct of this chemical defense. After all, if you were a small herbivore that found biting into a plant caused irritation or pain, you might avoid it in the future.
While humans learned to love the onion’s flavor (and we can cook them to remove much of the pungency), we still haven’t evolved resistance to the tear-inducing chemistry.
Different Onions, Different Potencies
Not all onions are created equal. Some varieties have higher concentrations of sulfoxides and related compounds, making them more tear-inducing. Fresh onions straight from the field might be less harsh than older onions that have had more time to produce or concentrate these compounds. Additionally, “sweet” onion varieties like Vidalia onions have lower sulfur content, resulting in a milder eye-stinging effect.
Cooking and Preparation
When onions are cooked, the heat breaks down these sulfurous compounds, mellowing the flavor and ending their tear-inducing ability. That’s why sautéed or caramelized onions don’t make you cry—by that point, the irritating chemicals have been neutralized or vaporized away.
Freezing or chilling an onion before cutting can slow down the chemical reaction. The colder temperature reduces the volatility of the compounds and slows the enzyme’s activity, resulting in fewer tears.
Common Misconceptions
Myth: “It’s the Smell That Makes You Cry”
Reality:
While the smell is associated with the irritation, it’s not just a matter of fragrance. The tear-inducing chemical (syn-Propanethial-S-oxide) doesn’t just smell strong; it irritates your eyes directly. Odor alone wouldn’t cause tears; it’s the chemical’s effect on your eye’s surface and tear ducts that leads to crying.
Myth: “Onions Contain Acid That Directly Burns Your Eyes”
Reality:
The chemicals released don’t start as acid. They react with the moisture in your eyes to form a weak sulfuric acid solution. This process is what irritates the sensitive eye tissues. The onion isn’t squirting acid; it’s just releasing compounds that turn into irritating molecules in your tears.
Myth: “Putting a Spoon in Your Mouth or Chewing Gum Stops the Tears”
Reality:
While some people swear by odd kitchen hacks—like holding a spoon between your teeth—they mostly serve as distractions. They don’t fundamentally change the chemical reaction. However, certain tricks like cutting the onion under running water or chilling it first can reduce the irritant’s effect.
Applications and Broader Implications
Agricultural Breeding and Engineering
Plant scientists and breeders have explored ways to create “tearless onions.” By tweaking the enzyme or reducing the sulfur content, it’s possible to breed onion varieties that cause fewer tears. In 2008, Japanese researchers developed a “tearless onion” by reducing the activity of lachrymatory-factor synthase. While not yet widespread, such varieties hint at a future where chopping onions is less of a weepy affair.
Culinary Considerations
For chefs and home cooks, understanding why onions make you cry can help you minimize discomfort. Chilling onions, using a sharp knife (which reduces cell damage and thus fewer irritants), or using a fan to blow vapors away can make a big difference in the kitchen.
Scientific Curiosity and Broader Plant Studies
Onions are just one example of how plants use chemical defenses. Studying them gives us insights into plant biology, chemical ecology, and co-evolution with herbivores. Similar chemical warfare goes on throughout the plant kingdom, with capsaicin in chili peppers or the bitter compounds in Brussels sprouts playing parallel roles.
Health and Nutritional Value
Onions are nutritionally beneficial. They’re rich in antioxidants, vitamins, and minerals. Interestingly, some of the same sulfur compounds that make you cry also contribute to onions’ potential health benefits, such as anti-inflammatory properties. Understanding onion chemistry helps us appreciate these healthful aspects, even as we curse their tears.
Conclusion
So, why do onions make people cry? It boils down to chemistry. Slicing an onion sets off a chain reaction that releases sulfurous compounds, which a special enzyme transforms into a tear-inducing gas. This gas irritates your eyes, causing you to produce tears in self-defense.
Though it may be annoying, this crying isn’t malicious. It’s merely how our bodies and onions’ evolved defenses interact. As we learn more about plant chemistry, we not only discover ways to reduce kitchen discomfort but also gain a deeper appreciation for the intricate biochemical dance between plants and the creatures that eat them.
Next time you find yourself tearing up over a chopping board, remember: it’s not personal—just nature’s chemistry at play.
Have your own tips for cutting onions tear-free? Or do you have questions about related plant chemistry? Share your thoughts in the comments and let’s explore these fascinating culinary-scientific intersections together.
Key Points
- Onions contain sulfurous compounds and enzymes that form syn-Propanethial-S-oxide when cut.
- This compound, when it reaches your eyes, transforms into a mild sulfuric acid-like solution, irritating your eyes and causing tears.
- Different onion varieties and preparation techniques can influence how tear-inducing they are.
- Research into “tearless onions” may one day make cooking less weepy.
- Understanding onion chemistry helps us appreciate plant defenses and their role in our diet.
References
- Nature – Search “onion lachrymatory factor” for peer-reviewed articles
- ScienceDirect – Research on onion chemistry and breeding
- Wikipedia: Onion – Background and general info
- Eady, C. C., et al. (2008). Silencing of a gene (lachrymatory-factor synthase) and its effect on tear production in onion. Journal of Agricultural and Food Chemistry.
- Block, E. (2010). Garlic and Other Alliums: The Lore and the Science. Amazon Link
