When preparing onions, many people experience stinging eyes and involuntary tears. This common kitchen phenomenon is a direct result of a precise biochemical reaction that occurs when onion cells are damaged. Understanding these mechanisms reveals how the onion defends itself and why our bodies react.
The Chemical Culprit
The primary irritant responsible for onion-induced tears is a volatile sulfur compound known as syn-propanethial S-oxide. It is a highly reactive substance that readily forms sulfuric acid when combined with water, contributing to the irritation experienced. When an onion is cut, syn-propanethial S-oxide is released as a volatile gas. As it disperses, it reaches the sensitive membranes of the eyes, initiating the tear-jerking process.
The Enzyme’s Role
The formation of syn-propanethial S-oxide begins when an onion’s cells are ruptured during slicing or chopping. This damage releases specific enzymes and precursor compounds that are normally kept separate within the intact onion. One of the initial steps involves the enzyme alliinase, which acts on amino acid sulfoxides found in onion cells. Alliinase converts these sulfoxides into unstable sulfenic acids.
Following this, a distinct enzyme called lachrymatory factor synthase (LFS) plays a crucial part. LFS converts 1-propenyl sulfenic acid into syn-propanethial S-oxide. This enzymatic conversion leads to the rapid production of the volatile lachrymatory factor. The presence of LFS is unique to onions and some other Allium species, differentiating their chemical defense mechanisms from those of garlic, for example.
How Tears Are Triggered
Once syn-propanethial S-oxide reaches the eye, it dissolves in the tear film, creating a dilute solution of sulfuric acid on the eye’s surface. This acid then irritates nerve endings in the cornea and conjunctiva, which are part of the trigeminal nerve system. These stimulated nerve endings send signals to the brain, interpreting the irritation as a painful sensation. In response, the brain triggers the lacrimal glands to produce tears, which serve as a protective mechanism, diluting the irritating chemical and flushing it away. The entire process is a rapid reflexive action designed to protect the eyes from potential harm.
Strategies to Reduce Tears
Minimizing eye irritation when cutting onions can be achieved through several practical approaches:
- Chilling onions in the refrigerator for 10-15 minutes before cutting can slow down the enzymatic reactions that produce the irritant. The reduced temperature decreases the activity of alliinase and LFS, thus limiting the amount of syn-propanethial S-oxide released.
- Cutting onions under cold running water or near a strong vent helps to dissipate the volatile chemical before it reaches the eyes. The water can absorb some of the gas, and airflow directs it away.
- Using a very sharp knife is beneficial because it causes less cell damage. A dull knife crushes onion cells, releasing more enzymes and thus more irritant, whereas a sharp knife makes cleaner cuts.
- Cutting the onion from the root end to the top can help, as the root contains a higher concentration of the tear-inducing compounds.
- Wearing protective eyewear, such as goggles, creates a physical barrier that prevents the gas from contacting the eyes directly.