When slicing an onion, many people experience an involuntary flow of tears and a stinging sensation in their eyes. This common kitchen phenomenon is a direct result of a complex biochemical reaction occurring within the onion. Understanding the science behind these tears reveals a fascinating natural defense mechanism.
The Onion’s Chemical Defense
Onions contain sulfur compounds that contribute to their distinctive flavor and aroma. When an onion is cut, its cellular structure is ruptured, releasing these sulfur-containing molecules into the air. Syn-propanethial S-oxide is the primary irritant responsible for eye irritation and tear production. This volatile chemical is often referred to as the lachrymatory factor (LF). Its production serves as a defense mechanism for the onion against pests and microbes, it deters potential predators by releasing this irritating gas upon damage.
The Enzymatic Reaction
The process that leads to the formation of the tear-inducing compound begins when the onion’s cells are damaged. Inside the onion, two components, sulfur compounds and an enzyme called alliinase, are normally kept separate. When the cell walls break, alliinase is released and acts on these sulfur compounds. This interaction converts the sulfur compounds into sulfenic acids.
Sulfenic acids are unstable and quickly undergo further transformation. A second enzyme, lachrymatory factor synthase (LFS), then converts these sulfenic acids into the volatile syn-propanethial S-oxide. This gas diffuses into the air and, upon reaching the eyes, reacts with the water on the eye’s surface to form a dilute solution of sulfuric acid. This acid irritates the nerve endings, prompting the lachrymal glands to produce tears as a protective reflex to flush out the irritant.
Mitigating the Tears
Several practical methods can help reduce or prevent onion-induced tears, each based on disrupting the chemical reaction. Chilling the onion in the refrigerator or freezer for a short period before cutting can slow down the enzymatic activity. This reduction in enzyme activity means less of the irritating gas is produced. Alternatively, cutting the onion under running water or near an active fan helps to disperse the volatile compounds before they reach your eyes.
Using a very sharp knife also minimizes the release of the irritant. A sharp blade creates a cleaner cut, damaging fewer cells and thus releasing fewer enzymes and sulfur compounds compared to a dull knife which crushes more cells. Some individuals find relief by wearing protective eyewear, such as goggles, which create a physical barrier against the airborne irritant. These simple approaches can make the onion-cutting experience much more comfortable.