The Attack of the Onion

Most of our vegetables can’t fight back. So how do onions bring us to tears?

Onions  are one of those flavourful foods that compliment almost any dish. So  much so, we’ve even learned to power through the burning, watery eyes  produced when chopping the vegetable.

For  thousands of years, way back to the ancient Egyptians, humans have been  shedding tears in order to cook with the savoury onion. In fact, the  Egyptians had such a strong appreciation for onions that they were  actually treated as objects of worship. Onions have been found painted  on the walls inside pyramids, buried alongside the bodies of pharaohs,  and were often given as a religious offering.

To ancient Egyptians, the many layers of the onion were symbolic of eternal life [1].

The onion belongs to the genus Allium, which includes other delicious foods like garlic, leeks, chives, and scallions.

Of the Allium species, only the onion Allium cepa and Sicilian honey garlic Allium siculum are known to cause severe eye irritation [2]. I’m guessing that for  most of us, we’re much more likely to cook and encounter onions than  Sicilian honey garlic, making onions the main culprit when it comes to  evoking tears. This is what makes the onion so interesting. Most of our  food cannot, quite literally, fight back.

So, how exactly are onions such a vengeful vegetable?

We  know just staring at an onion doesn’t make our eyes water. Neither does  simply peeling an onion. Instead, it’s the act of cutting or chopping  an onion, that triggers a cascade of chemical reactions.

Normally,  onion cells are very organised. They have a fixed, standard structure,  with specialised compartments to hold specialised substances. When we  dig in with a knife, we effectively tear apart all the cell structures,  thereby scattering molecules everywhere. Everything spills out of the  compartments and floats around wherever it likes. Everything mixes with  everything else.

And that’s when allinase comes in contact with the amino-acid sulphoxides.

Originally,  allinase would be isolated in its own special compartments, while the  sulphoxides would be floating around outside. This means the two  substrates would never meet in an intact onion cell — and for good  reason. Allinase is an enzyme. It takes these amino acid sulphoxides,  and puts them together, converting them into another chemical.

Unfortunately for us, that chemical is the first step of many in triggering the production of tears [3].

If  you’re unfamiliar with enzymes, think of them as an online dating  service such Tinder, OkCupid, or whatever your favourite dating app is.

The  dating app can be used to introduce two people, let’s say Addy and  Bill. After Addy and Bill meet, the dating app continues to introduce  people to each other, without getting used up itself — and that’s how an  enzyme works too.

In  our case, these amino-acid sulphoxides are floating around in the cell,  but they need allinase, the enzyme, to help them get together.

It’s  possible Addy and Bill would have met without the dating app, but it  could have taken years for that to happen. Just like an enzyme, the  dating app speeds up reactions. It serves to bring things together,  whether that’s chemical compounds or people.

After allinase has done its job, the happy couples go on their way.

Except  they’re not couples: they’re sulphenic acid, the end result of their  respective chemical reactions. And that’s when another enzyme comes into  play.

‘Lachrymatory  factor synthase’ has a longer name, but apart from that it’s just  another enzyme. It imitates allinase, but at the next level: sulphenic  acid turns into a compound named syn-propanethial-S-oxide [4].

While  its name can twist your tongue, the compound’s speciality is irritating  your eye. It can easily vaporise into a gas, spread out through the  air, and enter your eye before you know it.

If you happen to be a syn-propanethial-S-oxide infiltrating the eye, the first thing you’ll encounter is a cornea.

The  cornea is the front part of the human eye (and lots of other eyes too).  It’s a transparent cover over the rest of the eyeball, protecting it  from the harsh world outside. But for a cover, it’s pretty sensitive:  the cornea also has several free nerve endings, which act as pain  receptors, having a direct hotline to the Trigeminal Nerve.

A  huge network linking the cavities of the mouth and nose, and going  finally up to the eyes, the trigeminal nerve responds not just to  irritation in the eyes, but also those in the mouth — like those caused  by eating spicy foods.

Capsaicin  is the molecule responsible for spiciness in products containing green  or red pepper ingredients. Capsaicin is an also irritant, which becomes  obvious when you realize capsaicin is one of the active ingredients in  defensive pepper spray. So rather than using taste buds to “taste”  capsaicin, it actually alerts the free nerve endings in our mouth that  are part of the trigeminal nerve.

The  fact that mouth, nose, and eyes are interconnected via this large nerve  network becomes evident when people eat spicy food. Not only is your  mouth burning, but your nose is dripping and your eyes are watering too.

Spiciness  isn’t the only sensation that free nerve endings in your mouth respond  to. Menthol is also a trigeminal stimulant. It’s most often found in  mint-flavored products like gums, mints, and toothpaste causing the  cooling effect in your mouth and throat. Astringency is another  trigeminal sensation that causes dryness or puckering of the mouth, most  often associated with wine or tea.

The Trigeminal Nerve is the pathway syn-propanethial-S-oxide exploits to strike.

Once  in the eye, it reacts with the water there, forming small amounts of  sulphuric acid. Alerted by such a strong acid present in the body, pain  receptors will be quick to react. They’ll report back to the nervous  system, which produces tears to dilute the acid and wash it away.

That sulphuric acid is, ultimately, what causes all the itching and burning.

Why  would the onion have such an odd pathway to hurting humans? Simply  said, it is the onion’s own version of self-defense. If this pathway can  inflict harm on humans, it’s also likely to cause pain in other  organisms. This makes the onion quite an unpleasant treat, tends to ward  off any predators, and ultimately protects the plant.

Just  because a plant is trapped in one place doesn’t mean it’s unarmed.  Plants can have very obvious mechanical barriers like thorns, prickles,  or spines to keep enemies away. Other defense mechanisms can be quite  sneaky like specialized cells called idioblasts. Idioblasts contain  sharp crystals and a toxin. As the predator munches on the plant, the  crystals tear open the mouth parts, and the toxin seeps in. Other cells  contain high amount of bitter tasting tannins to repulse a predator with  a disgusting taste.

Perhaps  most important to humans is when plants use enzyme inhibitors to ward  off predators (including us!) which limits normal digestion of  nutrients.

When  we eat plants, the plant essentially sees this as an attack. Under  stress, the plant could release some of its enzyme inhibitors, which  will bind to our digestive enzymes, halting digestion and absorption of  nutrients in our body. The idea behind this defence mechanism is that  since we get very little nutrition and fuel, we’re less likely to eat  this plant again.

Many  legumes we eat contain enzyme inhibitors. For example, soybeans have  trypsin inhibitors that are able to reduce our ability to breakdown  proteins during digestion. However, if the soybeans are heated to  deactivate any trypsin inhibitor, the beans are safe to consume.

There  is good news on the horizon. Plant breeders are working on a tear-free  onion called the Sunion. For over three decades, scientists have been  selecting onions that contain less and less of the tear-producing  compound. In the plant science world, this process is called traditional  cross-breeding. After years of selecting this trait, the Sunion onions  are nearly free of the volatile, sulphur-containing molecules.

The  downside is, Sunions are still hard to find in the grocery store. They  are only grown in Washington and Nevada, so if you live on the west  coast of the United States, you’ll have an easier time finding them than  from anywhere else.

While waiting it out for tear-free onions to become popular, there a couple methods you can try to minimize your water works.

Put  your onions in the freezer for about a half hour before cutting them.  At cooler temperatures, almost all reactions take place at slower rates,  including the production of syn-propanethial-S-oxide.

Try  cutting onions under running water preventing the vaporization of the  sulphur-compounds into the surrounding air, or at the very least  diluting them. If it’s a breezy day, chop onions outside on a porch,  allowing the wind to carry away the tear-inducing gas.

One  last trick is to place a piece of bread in your mouth, which should  absorb the gas before it makes it to your eyes. I love this last hint as  it includes getting your carbs in for the day!

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