The Kitchen Scientist

Why Do Onions Make Us Cry? And How Do We Make Them Stop?

October 30, 2020

In The Kitchen Scientist, The Flavor Equation author Nik Sharma breaks down the science of good food, from rinsing rice to salting coffee. Today, he's unpacking why cutting onions make us weep—and what to do about it.


When it was time to decide on a career, I debated on attending culinary school. My mother, however, wasn’t too keen.

Having worked in hospitality in India, she didn’t think I had what it took to make it in a cutthroat industry, and her justification involved onions: “I don’t see you in a cold room chopping onions all day long.” To her, chopping onions was a true test of determination.

I ended up pursuing a career in molecular biology. With the thought of cooking professionally at the back of my mind, I continued to cook for my family and friends. And years later, I took a leap of faith and left research to pursue a culinary career, exchanging the lab for the kitchen—and onions.

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“I wear contact lenses. My eyes don't water when I slice onions. ”
— Elizabeth N.
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Onions and shallots are notorious at making us cry when we chop or slice them. This mechanism evolved as a way for the plant to protect itself from damage. Onions belong to the allium family, which also includes garlic and chives. When an onion is cut, an enzyme called allinase is released from the broken cells, converting the amino acid alliin (an amino acid not present in proteins) to a substance called allicin. Allicin is extremely volatile and, as soon as it’s produced, allicin moves through the air, reaches the membrane on our eyes, and irritates it. In response, our eyes secrete tears to wash away the allicin and we begin to cry and smart.

There are a couple of ways to avoid crying: either stop the enzyme, reduce its activity, or cover your eyes with goggles or another airtight protective eye gear. Since allicin production is an enzyme-dependent reaction, knowing the optimal conditions at which this enzyme works is very helpful. All enzymes need certain environmental conditions met, so they can do their job to the best of their abilities. In the case of the enzyme alliinase, the optimal pH is near neutral (neutral pH is 7.0, allicin’s optimum pH to function is 7.5), and the optimum temperature is 104 degrees Fahrenheit. Working under these conditions, alliinase will work efficiently to produce allicin. Thus, making conditions inhospitable for the enzyme can avoid or lessen the crying effect.

Adding an acid like citric acid or vinegar, or an alkaline ingredient like baking soda, would alter the pH sufficiently to reduce or destroy the enzyme’s ability to function. We do this with onions pickled in vinegar—have you noticed how they taste sweeter after just 30 minutes of pickling, and lose their pungency and eye-irritating character? This is because these acids prevent the enzymes from performing their job (unable to bind alliin well, can’t convert it to allicin) and the enzymes, being proteins themselves, lose their shape and get denatured.

This loss in allinase function also heightens the sugars in the pickled onions, making them taste sweeter (onions are rich in sugars, especially fructose, and contain polymers of fructose called fructans). Do not add baking soda to onions as an attempt to play with the pH: The onions will eventually fall apart due to an unrelated phenomenon—baking soda makes pectin, a carbohydrate that provides structural integrity to onion cells, and the onions will turn mush, more so when heat is applied. I ran an experiment on this for my book, The Flavor Equation.

Since temperature is also important to allinase’s ability to function, chilling onions in the freezer for 30 to 45 minutes—or at least two hours in the refrigerator—helps reduce the tearing effect. This is the hack I use at home. It does require a little bit of planning ahead for the onion to chill (coincidentally, this is what my mother was referring to when she mentioned my inability to chop onions in cold rooms). You need to chop the onion quickly because, as it warms up, the activity of the enzyme increases. Also as it warms up, any sliced or chopped onion will start to produce allicin, so keep that in mind—either submerge the cut onion in a bowl of chilled water or keep in a sealed airtight container in the fridge; this will lower the temperature and also prevent contact with air.

The third option is to seal off any airflow or reduce it to a minimum. Eye goggles achieve this and are very popular with cooks, as are onion-chopping tools. If you’re wondering about the latter, many look like boxes where onions are sliced or pressed down over blades that chop or dice them. Here the compartmentalization and short prep time reduce exposure to allicin. My grandmother used one of these devices when she needed to prep large quantities of onions for her dinner parties, though I don’t own one myself.

The quantity of onions (or shallots) determines how I go about chopping at home. If it’s just one or two, I can do it quickly, with little crying and no extra steps. For larger onions, I will prechill them. And for a large quantity of onions, as is the case when I make crispy onions and onion jam, I wear a pair of goggles.

What’s your favorite tried-and-tested onion chopping hack at home?

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Nik Sharma is a molecular biologist turned cookbook author and food photographer who writes a monthly column for Serious Eats and the San Francisco Chronicle and is a contributor to the New York Times. His first cookbook, Season: Big Flavors, Beautiful Food, was a finalist for a James Beard Foundation award and an International Association of Culinary Professionals award. Nik resides in Los Angeles, California and writes the award-winning blog, A Brown Table. Nik's new book, The Flavor Equation will be released in October 2020.

20 Comments

JV November 12, 2020
Allicin is extremely healthy - it fights cancer and many diseases, and also boosts immunity. I opt for the swimming goggles and keep the allicin! I recommend checking out Fiber Fueled written by a doctor for more details on allicin
 
Chip T. November 11, 2020
There is an "onion hack" video currently circulating that recommends chewing gum while slicing onions to prevent tears. Any truth to that?
 
Nara L. November 10, 2020
My son gave me his paintball goggles/mask. It worked like a charm! These days I wear my sunglasses. Works every time.
 
Frank P. November 9, 2020
Soak a paper towel in water, ring it out a decent amount, and hang it above your cutting board on a cabinet or place it next to the board. That, or run your sink water while cutting. Learned it when I used to slice unions on the slicer at a pizza place. Works every time!
 
Frank P. November 9, 2020
Soak a paper towel in water, ring it out a decent amount, and hang it above your cutting board on a cabinet or polace it next to the board. That, or run your sink water while cutting. Learned it when I used to slice unions on the slicer at a pizza place. Works every time!
 
MargeryM November 9, 2020
I got great "home remedy" advice a few years ago. Sniff vanilla just before and during cutting onions. It greatly reduces crying!
 
MaryNorberg November 8, 2020
In the case of onions try holding your breath. Move face away from cutting are for next breaths. Not totally foolproof but does wonders. Perhaps something to do w sense of smell requiring oxygen? And smell perhaps contributes to the burn? Or inhalation pulls acrid air deeper into sinuses closer to tear ducts?
 
Bikegirl227 November 4, 2020
I refrigerate my onions and shallots but I also wear a pair of "onion goggles". They are goggles that have foam surrounding each part of the eye so no allicin can waft under the eyepiece. They sell them in cookware stores and they work great!
 
Annette November 1, 2020
I own a cooking school that teaches home cooks. This question comes up all the time. Yes colder onions but storing onions for the long term is not good for an onion. So yes one has to plan to place in fridge. Best to also a very sharp knife. Dull knives cause cooks to press hard and saw and literally squeeze/mist out more onion juice. I also find the taller one is the further away from the cutting board the better...but I"m only 5'1" so I am much closer to the action. Also very important...the 'greener' the onion, meaning picked soon will have more issues.
 
Judy G. November 8, 2020
Yes, I agree
 
Elizabeth N. November 1, 2020
I wear contact lenses. My eyes don't water when I slice onions.
 
JEANINE A. November 1, 2020
I cut them on a board next to the stove with a burner turned on. It works extremely well to avoid tears.
 
Judy G. November 8, 2020
interesting....I'll try it. this wouldn't work in our Foods lab at school, but interesting!
 
Judy G. November 1, 2020
I teach high school cooking classes and found this article so helpful. When we do knife skills I have 20 students cutting onions at a time! Some are bothered and most aren't. I often put a fan on to "move the fumes" away but I guess that isn't wise. I have often wondered if the sharpness of a chef's knife makes a difference?
 
Annette November 1, 2020
I teach adult home cooks and YES the sharpness of the knife makes a huge difference. Instead of pressing hard and sawing to cut the onion which makes the juices atomize a sharp knife is essential...actually for all work in the kitchen.
 
Arpita November 1, 2020
I wear swim goggles when I cut onions. I look ridiculous, but they work well to protect my eyes from the allicin fumes.

When I was a kid, my stepmom used to tell me that the more tears you cry when slicing an onion, the more "jhaj" (flavor, pungency) it has. I am unsure if that is true or just an old Indian (Bengali) cooking myth, or a way she helped me feel better about my tears. It makes me wonder: does cutting the onion cold, and/or soaking the cut pieces in water, make it lose its flavor?
 
Anusha J. November 2, 2020
On the "...the more tears you cry when slicing an onion, the more "jhaj" (flavor, pungency) it has...", as a fellow South-Asian, Indian I can attest to the fact that I have also heard of such tales from literally everyone in my family :) We say the onion is "teekha" (spicy) which I think is in reference to the pungency of the onion rather than actual spice, as in heat.
 
Theresa November 1, 2020
Enjoyed the article! I will start prechilling onions. Another lesson from high school chemistry lab besides googles is to cut the onions under the exhaust hood (turned to high exhaust) of your stovetop. Not perfect but helps too.
 
Debby K. November 1, 2020
I have found that wearing contact lenses reduce significantly the likelihood that cutting onions will make me cry.
 
Judy G. November 8, 2020
I'll remember this and ask my students that cry if they wear contacts!! It could definitely be a part of protecting our eyes from the "fumes" ...