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.
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?