Why Does Chopping Onions Make You Cry?

red onion
With their sharp taste, red onions can add lots of flavor but aren't for every recipe. (Photo: Bukhara Yurii/Shutterstock)

Q: I was making my mother’s famous French onion soup the other night and had to chop six onions. Before I was even halfway through the first one, I started to tear up. By the time I was on the second, it was a full-on sobfest. It got me wondering: Why does chopping onions make you cry?

A: Firstly — can I get that recipe?

Secondly — I hate cutting onions for that very reason.

It’s interesting, isn’t it? You can have whole onions sitting around your house for days, even weeks, and everything is just dandy. And then one day, you unsuspectingly cut into one, and ouch! Stinging and burning and crying like you wouldn’t believe. So what gives?

Allow me to explain: Lachrymatory-factor synthase, an enzyme discovered by Japanese researchers in 2002, is released when you cut into an onion. This enzyme converts the onion's sulfoxides into sulfenic acid. The sulfenic acid rearranges itself into an unstable compound known as syn-propanethial-S-oxide. Once this compound reaches our eyes, it irritates them, causing the crying reflex to kick in to flush out the irritants.

That's the chemistry basics. Now, on to biology ...

There are three types of tears — basal, reflex and emotional. Basal tears keep your eyes from drying out, like on a windy day. Reflex tears clear your eyes of irritants, such as when you get dirt in your eye. And finally, emotional tears can be a result of happiness, sadness or any other emotion that gets your cry on. (For me, it’s watching the last scene of "Garden State" ... or "Bambi.")

The kind of tears you produce when you're chopping onions is reflex tears, because your brain is sending the message to your eyes to flush out the unstable compound mentioned above.

So is there anything you can do to prevent it?

Well, a scientist at New Zealand’s Crop and Food Research Institute by the name of Colin Eady is creating a tear-less onion. That’s right, folks — no more crying. Using gene-silencing technology, Eady was able to shut down the lachrymatory-factor synthase gene and create an onion that didn't induce tears. The onion is a long way from grocery store shelves, but we could be on our way in the next few years.

Until then, here are some other things you can try:

First, try sticking your onions in the refrigerator for a few hours before you cut them. The National Onion Association suggests cutting the top end of the onion off and peeling downward, but cutting off the root end (the one with all the hair on it) last. This is because the cells that release the onion's bountiful sulfur are concentrated at the base.

It also might help to peel the onions under water. And food blogger Robin Shreeves swears by onion goggles.

Finally, if you peel and chop a lot of onions, it might pay to get a little exhaust fan from your local hardware store. Having the air blow on the onion scatters the sulfuric acid and makes the irritant less potent. Check out Chow.com’s nifty little how-to video.

That’s about all I've got on the onion front. I sincerely hope they hurry up and get those tear-free onions to us soon.