Slicing into an onion and finding your eyes welling up with tears. This involuntary reaction is not due to sadness but rather a fascinating chemical defense mechanism within the onion itself. Understanding the science behind this phenomenon reveals the intricate biological processes at play when preparing this vegetable.
The Onion’s Defense Mechanism
Onions have developed a defense mechanism to protect themselves from pests. This protective strategy involves storing specific compounds and enzymes separately within their cells. These components remain inert until the onion’s cellular structure is disrupted.
When cut or damaged, these previously isolated compounds mix. This mixing triggers a rapid chemical cascade designed to deter consumption. The resulting volatile chemicals are what cause the familiar irritation associated with cutting onions.
From Onion Cells to Tears
The chemical process begins when onion cells are ruptured during slicing. This damage releases the enzyme alliinase, which acts upon sulfur-containing amino acid derivatives like isoalliin. Alliinase converts these compounds into sulfenic acids.
These sulfenic acids are unstable and quickly transform. Another enzyme, lachrymatory factor synthase (LFS), rearranges these sulfenic acids into a volatile compound called syn-propanethial-S-oxide. This specific chemical is the primary irritant responsible for causing human eyes to tear up.
How Your Eyes React
As syn-propanethial-S-oxide is released into the air, it diffuses to your eyes. Upon contact with the moist surface of the eye, this volatile chemical dissolves and forms a dilute solution of sulfuric acid. This acid then irritates the sensory nerves in the cornea.
The irritation of these nerves triggers a reflex response in the brain, signaling the lachrymal glands to produce tears. These tears serve a protective function, acting as a natural wash to dilute and flush away the irritating sulfuric acid from the eye’s surface. This response safeguards the eye.
Simple Ways to Reduce the Tears
To minimize tears, chill the onion before cutting. Lowering the onion’s temperature slows down the enzymatic reactions that produce syn-propanethial-S-oxide.
Cutting under running water or submerged in a bowl also helps, as water dissolves and washes away irritants.
Using a very sharp knife can also reduce tearing because it causes less cell damage, thereby releasing fewer irritating chemicals into the air.
Cutting onions near an open window or under a range hood can help dissipate the volatile compounds away from your face.
Wearing protective eyewear, such as glasses or goggles, creates a physical barrier that prevents the irritant from directly contacting your eyes.