Onions are a fundamental ingredient in many cuisines worldwide, adding flavor and aroma to a wide variety of dishes. However, anyone who has ever peeled or chopped an onion knows that it comes with an unpleasant side effect—tears. While it’s not uncommon to feel emotional when preparing a favorite dish, the tears we shed when peeling onions have nothing to do with feelings and everything to do with science.
In this article, we’ll explore why onions make us cry, the chemistry behind this phenomenon, and various methods to reduce or prevent tears while working with onions. By understanding the process, you’ll be better equipped to tackle this kitchen chore without having to wipe away tears constantly.
Why Do Onions Make Us Cry?
The simple act of slicing into an onion sets off a chain reaction that leads to the release of sulfur compounds—irritants that cause the eyes to water. But the process is a little more complex than that. To understand why we cry when peeling or chopping onions, we need to dive into the anatomy of an onion and the chemical reactions that occur when the onion’s cells are damaged.
The Anatomy of an Onion
An onion is made up of layers of cells, and within these cells are compounds that remain harmless as long as the onion is intact. These compounds exist in two main parts of the onion:
- The Cell Wall: The onion’s cells contain sulfur compounds known as sulfoxides, which are organic compounds that contain sulfur atoms.
- Enzymes: Inside the onion’s cell walls are enzymes known as allinases.
These two components are kept separate while the onion is whole. It is only when the onion is cut, crushed, or damaged that these compounds come into contact with each other and trigger the process that leads to tears.
The Chemistry Behind the Tears
When you slice through an onion, you rupture its cell walls, causing the sulfur-containing compounds (sulfoxides) and the allinase enzymes to mix. This initiates a chemical reaction that converts the sulfoxides into sulfenic acid.
Sulfenic acid is highly unstable and quickly rearranges into a more reactive form known as syn-propanethial-S-oxide. This volatile compound is a gas that easily vaporizes and disperses into the air. Once airborne, syn-propanethial-S-oxide reaches your eyes, where it dissolves in the moisture on the surface of your eyes.
Your eyes have a corneal nerve, which is sensitive to irritants, including sulfur compounds. When the syn-propanethial-S-oxide comes into contact with your cornea, it triggers the nerve endings, sending a signal to your brain that there’s an irritant in your eyes. Your brain’s response is to flush out the irritant by producing tears to wash away the sulfuric compound.
In summary:
- Cutting an onion releases sulfoxides and allinases.
- These compounds react to form sulfenic acid, which quickly transforms into syn-propanethial-S-oxide.
- This gas irritates the corneal nerves in your eyes, leading to tears.
Why Doesn’t This Happen With All Vegetables?
You might wonder why onions make us cry, but other vegetables do not. The key difference lies in the high concentration of sulfur compounds in onions. Onions, along with other members of the allium family like garlic, leeks, and shallots, contain particularly high levels of these sulfur-containing compounds. However, onions have a unique ability to release volatile gas (syn-propanethial-S-oxide) in large amounts when cut, which is what makes them the tear-inducing vegetable they are.
Other alliums may also cause mild eye irritation, but onions are by far the most notorious for bringing on tears due to their high sulfur content and the rapid conversion of sulfenic acid into the irritating gas.
How to Prevent Tears When Cutting Onions
Now that we understand the chemistry behind onion-induced tears, the next question is: How can we prevent them? While it’s difficult to completely eliminate the possibility of tears, several techniques can help reduce eye irritation and make the process of cutting onions more comfortable. Here are some of the most effective methods:
1. Chill the Onion Before Cutting
Cooling the onion before cutting it can slow down the chemical reactions that release the irritating gas. When the onion is cold, the sulfur compounds and enzymes become less volatile, meaning fewer irritants are released into the air.
How to do it:
- Place the onion in the refrigerator for 30 minutes before cutting it.
- Alternatively, you can put the onion in the freezer for 10-15 minutes to achieve a similar effect.
This technique reduces the intensity of the onion’s sulfuric compounds, making the cutting process less likely to induce tears.
2. Use a Sharp Knife
A sharp knife is essential when cutting onions. A dull knife crushes the onion cells, causing more cell damage and releasing more irritants into the air. In contrast, a sharp knife slices cleanly through the onion, minimizing the release of sulfur compounds and reducing eye irritation.
How to do it: Always use a well-sharpened knife when cutting onions to limit cell damage and reduce the release of syn-propanethial-S-oxide.
3. Cut the Onion Under Running Water
Water can help dissolve and carry away the irritating gas before it has a chance to reach your eyes. By cutting the onion under running water, you prevent the syn-propanethial-S-oxide from becoming airborne and irritating your eyes.
How to do it:
- Run cold water over the onion while cutting it.
- Alternatively, you can cut the onion in a bowl of water or near the sink with water running.
The water acts as a barrier, keeping the sulfuric compounds from reaching your eyes.
4. Cut Onions Near a Fan or Ventilation Source
Cutting onions near a fan or in a well-ventilated area can help disperse the gas before it reaches your eyes. This prevents the irritants from building up around your face and causing you to tear up.
How to do it:
- Use a fan to blow the gases away from your face while cutting the onion.
- Cut onions near an open window or in a kitchen with good ventilation to ensure the gases disperse quickly.
5. Use Onion Goggles
Onion goggles or any kind of protective eyewear can help shield your eyes from the irritating gas. These goggles create a barrier between your eyes and the sulfuric compounds, preventing the gas from reaching your corneal nerves.
How to do it:
- Purchase onion goggles, which are specifically designed to protect against onion fumes.
- Alternatively, you can use swimming goggles or safety glasses to achieve the same effect.
This method is particularly effective if you’re cutting a large quantity of onions and need maximum protection.
6. Soak the Onion in Vinegar
Vinegar can help neutralize the sulfur compounds in onions, reducing the release of syn-propanethial-S-oxide. By soaking the onion in vinegar before cutting, you can reduce the chances of tearing up.
How to do it:
- Peel and slice the onion in half.
- Soak the onion halves in a bowl of vinegar for 10-15 minutes before chopping.
While this method can help reduce eye irritation, keep in mind that the vinegar may slightly alter the flavor of the onion.
7. Use a Food Processor
A food processor can help you quickly chop onions without the need to manually handle the onion for an extended period. By reducing the time spent cutting the onion, you limit your exposure to the gas.
How to do it: Use a food processor with a sharp blade to finely chop or dice the onion in seconds.
This method works well for recipes that require finely chopped onions, and it minimizes direct contact with the onion’s sulfuric compounds.
8. Freeze the Onion’s Enzymes
In addition to chilling the onion, freezing it before cutting can help reduce the activity of the enzymes that release the irritating gas. While freezing the onion won’t stop the chemical reaction completely, it can slow it down significantly.
How to do it:
- Place the onion in the freezer for about 10-15 minutes before cutting.
- Proceed with cutting the onion while it’s still cold.
This technique is particularly effective when combined with other methods, such as using a sharp knife or cutting the onion under water.
The Role of Genetics: Are Some People More Sensitive to Onions?
Interestingly, some people may be more sensitive to the tear-inducing effects of onions than others. This could be due to genetic factors that make certain individuals more prone to eye irritation. People with more sensitive corneal nerves or those who produce less natural eye moisture may experience more intense tearing when exposed to syn-propanethial-S-oxide.
On the other hand, some people may hardly tear up at all when cutting onions, which could be due to less reactive corneal nerves or more effective tear production that quickly washes away the irritants.
Are There Tear-Free Onions?
In recent years, scientists have developed tear-free onions through selective breeding and genetic modification. These onions contain lower levels of the enzymes that produce syn-propanethial-S-oxide, reducing the release of the irritant gas. Tear-free onions still retain the flavor and nutritional benefits of regular onions but without the unpleasant side effect of tearing.
However, tear-free onions are not yet widely available in all markets. If you’re someone who frequently experiences irritation when cutting onions, keep an eye out for tear-free onion varieties in the future.
Why Do We Sometimes Tear Up More With Certain Onions?
Not all onions are created equal when it comes to making you cry. The level of sulfur compounds in an onion varies depending on the type of onion and how it was grown. Some onions, especially those grown in sulfur-rich soil, have higher concentrations of sulfoxides and allinases, leading to more intense tears when they are cut.
White onions and yellow onions tend to contain higher levels of these compounds, making them more likely to cause tears than sweet onions or red onions, which have lower sulfur content.
Additionally, onions that are fresher or have just been harvested may contain more volatile sulfur compounds, leading to more intense eye irritation compared to older onions that have had time to dry out.
Conclusion
The reason we cry when peeling or cutting onions boils down to a fascinating chemical reaction that releases irritating sulfur compounds into the air. The gas produced, syn-propanethial-S-oxide, irritates the eyes and triggers a tear response as the body attempts to wash away the irritant. While this process is inevitable when handling onions, there are several effective methods to minimize or prevent tears, including chilling the onion, using a sharp knife, cutting under water, and wearing protective goggles.
By applying these techniques, you can make your time in the kitchen more pleasant and tear-free, allowing you to enjoy the flavorful benefits of onions without the discomfort. So, the next time you’re prepping onions for your favorite dish, you’ll be better equipped to tackle the task without crying—at least not from the onions!
