Why Onions Make You Cry
Onions contain a chemical called syn-propanethial-S-oxide that is released when the onion’s cells are damaged during cutting. This chemical mixes with the air and forms sulfuric acid, which then comes into contact with your eyes and causes irritation and tearing up.
When you cut into an onion, you break open its cells and release enzymes that mix with the sulfur compounds to produce the tear-inducing chemicals. So the more onion cells you break, the more of this chemical is produced and the more pronounced the tear-inducing effect.
Tips to Prevent Tearing Up
Here are some tips to prevent tearing up when cutting onions:
Use a Very Sharp Knife
A sharp knife slices cleanly through the onion’s cells with minimal crushing, releasing fewer of the tear-inducing chemicals. So be sure your knife is as sharp as possible before cutting.
Chill the Onion First
Chilling an onion in the freezer for 10-15 minutes before cutting slows down the chemical reaction that produces the irritating gas. Just make sure you remove the chilled onion from the freezer promptly or it could actually freeze and become harder to cut.
Cut Under Running Water
If you cut the onion under a gentle stream of cold running water, the irritating gases will be diluted and swept away before they have much chance to reach your eyes.
Use a Paper Towel
Holding a paper towel near your eyes as you cut onions can absorb some of the sulfuric compounds before they waft upwards. Just make sure the paper towel doesn’t obstruct your vision while using the knife.
Step-by-Step Guide to Cut an Onion with a Paper Towel
Follow these simple steps to cut an onion without crying using the paper towel method:
Step 1: Choose and Prep the Onion
Pick a firm, dry onion that feels heavy for its size. Avoid soft or moist onions as they will have more moisture and tear-inducing compounds.
Peel off the papery outer layer of skin. Leave the root end intact for now, as this holds the onion layers together.
Step 2: Ready a Sharp Knife and Cutting Board
Use a large chef’s knife that’s been freshly sharpened. Have a clean cutting board ready to chop on.
Step 3: Hold Paper Towel Near Your Eyes
Take a sheet of paper towel and hold it about 6 inches from your eyes, just under your eyebrows.
The goal is to put a barrier between your eyes and the onion gases, while still being able to see what you’re cutting.
Step 4: Cut the Onion in Half Lengthwise
Leaving the root end intact, cut the onion in half from top to bottom. This exposes more surface area to cut while keeping the layers together.
Step 5: Make Horizontal Cuts
Hold each onion half cut-side down on the cutting board. Make several horizontal cuts through the onion, leaving the root end uncut.
Slice from the top down to the root. Space cuts about 1/4-1/2 inch apart.
Step 6: Make Vertical Cuts
Now make vertical cuts across the onion, again keeping the root end intact.
These should also be about 1/4-1/2 inch apart. The onion will fall apart into diced pieces.
Step 7: Separate the Pieces
Once fully diced, cut through the root end to fully separate the onion pieces. Discard the root.
Step 8: Rinse Under Water
For maximum tear reduction, rinse the diced onion briefly under cold running water to wash away any lingering irritating compounds.
Pat dry with a paper towel before using.
Other Effective Methods
Here are a few other tricks that can help reduce onion tears:
Use a Fan
Position a fan to blow air across your cutting board, away from your face. This blows the gases away before they can waft upwards.
Wear Swimming Goggles
Protect your eyes fully by wearing well-fitted swimming goggles. You may look silly, but this blocks vapors completely.
Use a Food Processor
Chop onions in a food processor instead of by hand. The faster chopping releases less of the irritating chemicals.
Light a Candle
Some people claim that lighting a candle near your cutting board helps neutralize the tear-inducing fumes through the smoke.
Wear Onion Goggles
Specialized goggles called onion goggles form a seal around your eyes, preventing vapors from getting in. These allow you to see clearly while protecting your eyes.
Why Use a Paper Towel?
Of all the techniques to stop onion tears, using a paper towel is one of the simplest:
- Paper towels are readily available in every kitchen
- They’re disposable, so you avoid onion residue on reusable goggles or glasses
- A paper towel is thin enough to see through easily while cutting
- The fibrous material traps sulfur compounds before they reach your eyes
- You can hold the paper towel with one hand while cutting with the other
Just be sure to stand far enough away from your cutting board so the towel doesn’t create a safety hazard with the knife. Pay attention and keep your fingers out of harm’s way behind the towel.
Understanding Onion Chemistry
Let’s take a closer look at the compounds responsible for onion tearing:
Syn-Propanethial-S-Oxide
This sulfur-containing organic compound is the main irritant found in onions. When onion tissue is damaged, an enzyme called alliinase acts on amino acids to produce propanethial S-oxide.
Lachrymatory Factor
Propane-thial-S-oxide rapidly rearranges itself into an unstable compound known as the lachrymatory factor. This gas interacts with the water in your eyes to form sulfuric acid.
Sulfenic Acids
The lachrymatory factor breaks down into sulfenic acids, which irritate nerve endings in the eyes and cause stinging and watering.
Onion Chemistry in Action
Here’s a play-by-play of the onion’s tear-inducing powers:
- Knife blade cuts into onion, breaking open cells.
- Amino acids come into contact with enzyme alliinase.
- Alliinase converts amino acids into propanethial S-oxide.
- Propanethial S-oxide rearranges into the lachrymatory factor.
- Lachrymatory factor is released into air and reaches eyes.
- It interacts with eye water to form mild sulfuric acid.
- Sulfenic acids activate nerve receptors, causing stinging and irritation.
- Brain responds by releasing more tears to wash the irritants away.
This fascinating process happens every time you slice into an onion!
Avoiding onion tears
Here are 5 tips for avoiding onion tears:
Use a sharp knife
A sharp knife damages fewer onion cells, releasing less of the irritating chemical.
Chill the onion
A chilled onion slows down the chemical reaction that produces the tear-inducing gas.
Cut under running water
The water washes away the gases before they reach your eyes.
Use a fan
Blowing air across the cutting board keeps the gases from wafting upward.
Wear goggles
Goggles form a seal around your eyes to block the gases completely.
Science of Onion Tears
When you cut an onion, a chemical reaction takes place that produces tear-inducing vapors:
Onion Chemistry
- Onions contain amino acids and an enzyme called alliinase.
- Cutting the onion ruptures cells and exposes amino acids to alliinase.
- The enzyme breaks down the amino acids into sulfenic acids, including propanethial S-oxide.
- Propanethial S-oxide rapidly rearranges into an unstable compound called the lachrymatory factor.
Reaching Your Eyes
- The lachrymatory factor is released into the air when you cut onions.
- It reaches your eyes and interacts with the water in tear film.
- Mild sulfuric acid forms on the eye’s surface, stimulating nerves.
- Nerves signals the brain to dilute acid by releasing more tears.
This automatic response is your body’s defense against the irritating gas.
Preventing Onion Tears
You can take steps to prevent onion tears while chopping:
Use Proper Cutting Technique
- Use a very sharp knife to minimize cell damage.
- Make clean, diagonal slices instead of hacking or crushing.
- Keep the onion root intact for stability.
Create Barriers
- Wear shielded goggles to block vapors.
- Use a fan to blow vapors away from your face.
- Hold a paper towel near your eyes to trap gases.
Change Chemical Process
- Chill onion in the freezer before cutting to slow chemical reaction.
- Cut under running water to dilute and rinse away vapors.
With the right techniques, you can chop onions tear-free and enjoy their flavor!
Conclusion
Cutting an onion causes a complex chemical reaction that produces irritating sulfuric compounds and gases. These ultimately cause stinging, watery eyes when they come into contact with the sensitive nerves in your eyes.
Using a paper towel as a barrier between the cut onion and your eyes can absorb some of these tear-inducing sulfur compounds before they waft upwards towards your face. This simple kitchen staple, along with other tricks like chilling the onion and using proper cutting technique, can help minimize the tear factor allowing you to chop onions without crying. Understanding the science involved gives you an advantage in preventing this pesky kitchen problem.