Why does onion cause you to cry? –Kera, 5, Lawrenceville, GA

Dear Kera,

Try as we might, it’s hard to hold back tears while chopping up onions.

My friend Lindsey du Toit knows the feeling. She’s a scientist at Washington State University and works with lots of onions. Her research helps farmers grow good vegetables for us to eat.

“It’s not the onion itself that makes us cry,” she explained, “but a chemical reaction that starts when you cut into it.”

I wondered how exactly this chemical reaction worked. To find out, we set up a microscope in her lab and chopped up a Walla Walla sweet onion. I wiped a few tears from my cheek and slid a tiny piece of onion under the lens.

Under the microscope’s light, we could see rows of onion cells next to each other. Just like you and me, onions are made up of cells.

An onion sitting on the kitchen counter is pretty harmless because its cells are still together. But when we cut up an onion, we also cut up a bunch of the cells. This is where the chemical reaction begins.

Cutting the onion breaks open different parts in the cell and releases chemicals into the air. Some of these important chemicals contain sulfur.

“As the plants grow, they take up sulfur from the soil,” du Toit said. “It’s good for growing onions.”

This sulfur is important for the flavor, too. But some of the chemicals in onions that contain the sulfur also have the side effect of making us cry.

“It’s a sacrifice we pay for good-flavored onions,” du Toit adds.

The onion cells also contain parts called enzymes. It is the job of these enzymes to help chemical reactions happen. In the onion, the enzymes help convert the sulfur into a kind of acid.

This acid rearranges itself to form a new kind of chemical: syn-propanethial-S-oxide. It’s a bit of a tongue twister. It’s also a tearjerker.

When the chemical drifts up and meets the moisture in our eyeballs, it turns to sulfuric acid. Our eyes have many nerves and can sense that something unusual is happening—and that something is stinging.

Tear-producing glands in our eyes, called lachrymal glands, receive the message.

du Toit explained that an onion with more sulfur is often likely to produce more tears. For example, Walla Walla sweets are sweeter and don’t take up as much sulfur from the soil. They likely won’t provoke as many tears as some other onions might.

People have tried quite a few techniques to try to avoid crying when they chop onions. Some put onions in the fridge before cutting them to slow the chemical reaction. Others cut their onions under cold water to slow the chemical reactions with the sulfur compounds.

Chemical reactions often happen more slowly in cold conditions. So the idea is that cooling onions in the fridge before cutting them means that the sulfur chemicals are converted more slowly into the acid that reacts with your eyes —helping you chop more onions and slowing the waterworks.

Sincerely,
Dr. Universe

Ask Dr. Universe is a science-education project from Washington State University. Send your question to Dr. Universe at askDrUniverse.wsu.edu.