We’ve all been there – you’re cooking a recipe that calls for onions and in the process of chopping, dicing or slicing them you get that stinging sensation in your eyes. Before you know it, you’re crying and your vision is blurred (definitely not a good thing when handling knives!) but you must press on. I mean, it’s not like you can rely on anyone to come to your rescue. Heck, my kids (and husband) don’t even like being in the kitchen (or in the vicinity of the kitchen) when I’m chopping especially potent onions! While all this is happening, have you ever wondered exactly what is going on and if it’s necessary to endure this ‘pain’ for the lovely, savory flavor associated with this vegetable? Let’s explore the science behind what is actually happening and see if there are ways of avoiding the tears!
Onions are chock-full of organosulfur compounds, which are responsible for so many of the health benefits associated with this veggie – however there is a downside. One of these organosulfur compounds, syn-Propanethial-S-oxide (C3H60S), is responsible for that “irritating” feeling. (Irritating as in annoying and painful!). This compound is a lachrymator, meaning it’s a substance that ‘irritates’ the eyes and causes tears to flow. In actuality, syn-Propanethial-S-oxide is produced by a chain of chemical reactions. When onions are cut in any way—through chopping, dicing, or slicing— damage is caused to cell walls, releasing enzymes called allinases. These enzymes break down amino acid sulfoxides and generate sulfenic acids. One specific sulfenic acid is then acted on by a second enzyme, the lacrimatory factor synthase (LFS), producing the offending compound – syn-Propanethial-S-oxide (a combination of sulfuric acid, sulfur dioxide and hydrogen sulfide). When this gas reaches your eyeballs, it activates your lachrymal glands, which produce tears to dilute and flush the irritant out.
The degree to which this process occurs varies among Allium species, based on the level of sulfenic acids they produce. Allium family members include garlic, leeks, chives, scallions, shallots, and all kinds of onions including sweet onions, red onions, and green onions. They all differ in pungency, which is why some types of onions make you cry more than others. To this end, the New Zealand Institute for Crop and Food Research created “no-tear” onions through genetic modification (eliminating LFS) and Bayer introduced LFS-silenced “Sunions” in 2018, a product of 30 years of cross-breeding. However, the decrease in sulfur ingestion by plants lacking LFS would likely result in inferior flavor and health benefits.
So, what can we do to stop the flow of tears and still enjoy that indispensable delicious onion flavor? Luckily, there are some culinary tricks you can try to minimize the tears!
How To Minimize Onion Eye Irritation
Here are a few ways to avoid irritation associated with chopping onions:
- Cut onions under running water or submerged in water (syn-Propanethial-S-oxide is water soluble, so it will be washed away before it can get in the air).
- Leave the root end intact as this part of the onion has higher concentrations of sulfur compounds.
- Chill your onions by refrigerating for 30 minutes before cutting to slow down the enzymes responsible for producing syn-Propanethial-S-oxide.
- Use a food processor to chop large amounts.
- Wear onion goggles (I mean we’re all science nerds anyway – might as well even throw on a lab coat while you’re at it!)
If you’re desperate you could even try using a fan to blow the gas away from your eyes or chop your onions outdoors, although neither option is very practical!
In the end, while it might take a little effort (and a few tears), the flavor and health benefits of onions make them well worth it.
