We’ve all been there before — you’re preparing a dish and it hits you: the dreaded task of chopping onions stands before you. You reluctantly take out your knife and cutting board and brace for burning tears that are soon to come. But what exactly is is about chopping, cooking or biting into onions that can cause such a harsh reaction?
A recent articles explains that onions and other alliums have a chemical irritant called syn-propanethial-S-oxide, which is best described as a chemical defense system that deters critters who are digging in the ground looking for a snack.
“The way the onion bulb is designed, is ingenious,” chemist Eric Block said in the article. Within each onion cell, there’s a little glob — a sealed vacuole — filled with enzymes, he explains. When you bite or cut into an onion, these enzyme-filled blobs break open, releasing their contents, which then mix with other chemicals inside the onion cells. “And then,” Block says, “a whole cascade of chemical processes happen within an instant.”
A bunch of syn-propanethial-S-oxide molecules form as a result of all those chemical reactions — and because these guys are really tiny, and light, they float up into the air and then they find their way to the eye of the person that’s cutting the onion,” Block added.
That being said, Block does have some suggestions to help us deal with the onion’s hostile chemistry.
The best strategy to avoid tearing up, Block says, is to chop onions in a well-ventilated area, near a fan. “If you just cut the onion in a stream of air blowing away from your face,” he says, “then you’ll pull the molecules away and they won’t get to your eyes.”
Refrigerating the onions for a few minutes before chopping can help as well, he notes, since the volatile compounds that cause eye irritation “are less volatile when they’re cold.” Molecules of any sort tend to move faster when the’re hot and slower when they’re cool — so by chilling the onion you’re ensuring that the irritating chemicals don’t rush up to your face all at once.
“There are some ridiculous suggestions that have been made — like wear goggles,” Block says. “That doesn’t really work.” Really tight goggles may stop the chemical from getting directly to your eyeballs, but they won’t prevent it from seeping up through your mouth and nose. “Plus,” Block points out, “goggles just look silly.”
Of course, it’s virtually impossible to avoid watery eyes entirely when you’re cooking with onions, Block says. Ultimately, these recalcitrant chemicals in onions are what give it its unique flavor. “Besides,” he added, “sometimes pain is pleasure.”
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For a visual of what the complete process looks like, check out this video by the American Chemical Society.