Why onions make us cry

Research from Case Western Reserve University School of Medicine has provided further insight into why onions make us cry.

A group of researchers from CWRU and Cornell University studied its structure and figured out how it works to cause tears in their new study published in the ACS Chemical Biology journal.

Those involved in the project include Marcin Golczak, PhD, assistant professor of pharmacology at the School of Medicine; Josie Silvaroli, the leading student author; Philip D. Kiser, PhD;  Matthew J. Pleshinger, a student from the College of Wooster who worked on the study and Surajit Banerjee, PhD, of Cornell University crystallized lachrymatory factor synthase (LFS).

The burning you feel in your eyes as you’re slicing into an onion? Call it nature’s tear gas. It’s the last line of defense employed by the onion before becoming food. A chemical known as lachrymatory factor (LF) is realised as the cells break down, and it’s only found in a few other plants such as guinea hen weed and Sicilian honey garlic.

“Our research is not generally this popular,” laughed Golczak. “The whole story is about the shape of an enzyme, and by that we know more or less how it works. The lachrymatory factor found in onions is characterized by an enzyme (LFS), which structure was not known. So we went out to find to out how this lachrymatory factor is made.”

The research project took over a year to complete. Previous research from the 1970s showed that LF irritated the eyes, and research from 2002 confirmed the existence of LFS. Because of the protective nature of the compound, LF can be activated easily. This present challenges for a researcher: How do you study a compound that disappears so quickly with little provocation? By binding it to a crotyl alcohol, researchers were able to stabilize the compound.

By crystallizing LFS and evaluating the structure, Golczak’s team discovered the enzyme’s active binding site. Combining the observations of LFS and analyzing information about similar proteins, they developed a detailed chemical reaction mechanism that explains the creation of lachrymatory factor.

“We collected data, which allows us to map each atom in the protein. So now, we know the architecture of the protein of the enzyme. And by this we can find out how this enzyme works,” said Golczak.

When cut, the onion’s last attempt to avoid becoming someone’s meal is to release its LF. Unfortunately for onions, that has not deterred many people.

“This is a very volatile compound,” said Silvaroli. “So it will hit your eyes, and then it will hit the nerves in your eyes which will then cause the pain receptors to go off and click something in your brain [that says] this is burning. I need to produce tears to get it out.’”

The chemical causes general irritation in the eyes, releasing tears upon hitting those nerves. For the weary eyed, putting onions in the freezer may reduce the amount of LF released when slicing into an onion. Onions genetically engineered by Japanese researchers and then flooded with ions lack the LF factor, so one can slice without having to fight back tears.

Although this is a viable option, Dr. Golczak scoffed at the concept.

“As any enzymatic reaction, you can do many things,” he said. “You can lower the temperature, you can dilute the substrate, you can cut underwater there will be nothing, but you are also diluting all the flavors which comes with the onion… Production of the LF is associated with production of the molecules that are important for flavor, so I don’t think there is a good solution. If you eat an onion, you’ve got to cry. Otherwise it is not an onion.”