A Chemical Reaction
Onions contain amino acid sulfoxides that produce sulfenic acids inside the cells of the onion. When you start cutting an onion, it disrupts the cells and the normal cell enzymes mix with the sulfenic acids and it produces propanethial S-oxide. Propanethial S-oxide is a sulfur chemical that is a gas that floats through the air and into your eyes. When this gas interacts with the water in your tear film, sulfuric acid is produced. This acid is not compatible with your tears and your eyes begin to burn. Your brain says, “There is something in my eye!”
(Cooked onions won’t produce this same effect because the process of cooking the onion inactivates the enzymes needed to make propanethial-S-oxide.)
Basic Tear Production
There are two types of tear production that occur inside the eye. Basal tears, the ones that provide basic lubricant to the eye, and reflex tears, the type of tears we typically think of when we are crying. Reflex tears are produced in response to emotion and some external irritant. An external irritant, such as dust or smoke, triggers nerve endings in the cornea to communicate with the brain which turns on the lacrimal gland. The lacrimal gland, which is under the upper eyelid on the side of your temple, creates reflex tears. Your tear glands begin secreting tears to potentially dilute or wash out the offending agent.
How to Avoid Tearing Up While Cutting an Onion
There are several life hacks known to minimize tears when cutting onions. Here are some of the most effective:
Store onions in the refridgerator and consider popping them in the freezer 10-15 minutes before cutting. This freezes the enzymes and stops them from mixing with the sulfenic acids which diminishes the production of propanethial S-oxide. As a result, sulfuric acid is reduced when you chop the onions. Soak the onions in water before cutting. The gas will be absorbed into the water. Simply allow the onions to sit in a small bowl of water for about half an hour before beginning to chop them. Do as little damage to the onion as possible. Use a very sharp knife to chop the onion. Crushing an onion, instead of cutting it sharply, tends to release more gas. A sharp knife will allow for quick, clean slices, and less damage to the flesh of the onion. Cut off the stem, and leave the root end in tact. The root has the highest amount of sulphuric properties and causes more tears. Use sweeter onions, like red, green or vidalia onions, as opposed to the more pungent white or yellow onions. Use an electric food processor instead of a knife. Chop onions where there is some air flow. Turn on the range hood, chop near an open window, or turn on a fan. Point the fan toward your work surface so that the fumes are drawn away from you. Wear safety goggles while chopping the onions. You may get some funny looks but it will definitely keep the acid from getting into your eyes. And always remember not to touch your eyes when you are chopping onions. Make sure to wash your hands thoroughly after you are finished.
A Tear-Free Onion?
Crop & Food Research of New Zealand and House Foods Corporation of Japan have developed genetically engineered onions that do not produce the sulfur compounds that make us tear while chopping them. The onions contain onion flavor without the weepy side effects—but it’s not clear when, or if, these will ever be available to consumers.
There is also a new brand of tearless onion developed by meticulous cross-breeding over the past 30 years (and not genetically modified), called Sunions. These onions can be purchased at the food market now, albeit a little more expensive.
