What Causes Onions to Make Us Cry?

Onions date back to the Bronze Age, where they were revered by the Ancient Egyptians (and consumed by the Israelites during their captivity in Egypt). They were used by Roman gladiators to soothe sore muscles, served as payment for rent in the Middle Ages, and eventually made their way to the Americas, where we now fry, caramelize, pickle, grill, and enjoy them in various ways.

Cutting an onion often results in tears for many of us. It's a small sacrifice we make for the delightful flavor. Here's how the process unfolds from reaching for the knife to tearing up like a baby:

1. When you slice into an onion, its broken cells release a mix of compounds, including allinase enzymes and amino acid sulfoxides. The allinase enzyme breaks down the sulfoxides into sulfenic acids.

2. The sulfenic acids, being rather unstable, quickly transform into thiosulfinates. These compounds emit a strong odor that was once believed to be the cause of our tears. The acids also undergo conversion by the LF-synthase enzyme into a gas known as syn-propanethial-S-oxide, which is commonly referred to as the lachrymatory factor (or the crying agent).

3. Syn-propanethial-S-oxide then travels through the air and reaches our eyes. It first encounters the cornea, which contains autonomic motor fibers that connect to the lachrymal glands. Upon detecting the gas, these fibers send signals to the lachrymal glands, triggering them to begin producing tears to flush out the irritant.

4. Our eyes automatically begin to blink and produce tears, which help to wash away the irritant. Naturally, our instinct is to rub our eyes, but this only worsens the situation since our hands also carry traces of syn-propanethial-S-oxide.

It takes only about 30 seconds from the first cut to start tearing up; that's how long it takes for syn-propanethial-S-oxide to reach its peak concentration.

WHY DON'T GREEN ONIONS MAKE US CRY?

The onion’s relatives, such as green onions, shallots, leeks, and garlic, also produce sulfenic acids when cut. However, they generally contain fewer (or none) of the LF-synthase enzymes, meaning they don't produce syn-propanethial-S-oxide.

HOW CAN I STOP MYSELF FROM CRYING?

Since I tend to use a lot of onions when cooking at home, I've been experimenting with various methods recommended online to lessen or avoid the effects of the lachrymatory factor. Here’s what I tested:

Method #1: Chill or slightly freeze the onions before cutting, in hopes that this will alter the chemical reactions and reduce the gas released.Result: The onion from the fridge had me crying just as fast as the room-temperature ones. The one that was in the freezer for 30 minutes spared me for a bit, but by the time I finished chopping, my eyes started to sting a little.

Method #2: Cut quickly! Finish chopping before the gas can reach your eyes.Result: If I just hack away at the onion, I can get it in the frying pan without so much as a tear. However, the onion looks a mess. If I take my time and do a proper dice, I end up tearing up. If you don't mind a mangled onion, this is the way to go.

Method #3: Place a slice of bread in your mouth and cut the onion with most of the bread sticking out to "trap" the fumes.Result: The bread I have is stale. I abandon the experiment and add bread to my shopping list.

Method #4: Chew gum while chopping. It encourages mouth breathing, keeping the fumes away from your eyes.Result: This works fairly well as long as you keep your head in the right position. If you lean toward the cutting board or look directly down at the onion, your eyes are back in the line of fire.

Method #5: Cut the onions under running water. This stops the gas from reaching your eyes.Result: Cutting an onion in the sink is a challenge. I believe Confucius said that. My leaky Brita filter is spraying me in the face, and I’m worried about cutting myself, but at least I'm not crying.

Method #6: Wear goggles.Result: In an attempt to look somewhat dignified, I first try my eyeglasses and sunglasses, but neither helps. The old chemistry lab safety glasses make me look ridiculous, but do offer a bit more relief. I suspect swim goggles would be ideal, but I don’t have any.

Method #7: Change your onion. “Tear-free” onions have been developed in the UK through special breeding and in New Zealand using “gene silencing” techniques.Result: My nearest store, Whole Foods, doesn’t carry genetically modified produce or onions from England. Tonight, it’s leeks for dinner!