Cutting onions often results in streaming tears, a reaction many home cooks accept as an unavoidable nuisance. This temporary irritation is not emotional distress, but a defensive chemical reaction triggered by the plant. Understanding the mechanics behind this process allows for targeted, effective strategies to neutralize the irritant. This article explains the chemical cause of the tears and provides practical methods utilizing environmental control, physical barriers, temperature, and moisture to keep your eyes dry while preparing a meal.
The Chemical Reason for Eye Irritation
The tearing reaction begins when the onion’s cellular structure is damaged by cutting. Onion cells store sulfur compounds and two key enzymes separately within the cell structure. When the knife slices through the cell walls, the contents mix, initiating a rapid chemical cascade intended as a defense mechanism against predators.
The enzyme alliinase first reacts with stored amino acid sulfoxides to produce unstable sulfenic acids. A second enzyme, called lachrymatory factor synthase (LFS), immediately converts these sulfenic acids into a volatile gas known as syn-propanethial S-oxide. This gas quickly diffuses into the air and travels toward the eyes.
Upon contact with the moist surface of the eye, syn-propanethial S-oxide reacts with water in the tear film to form a small amount of sulfuric acid. This acid stimulates sensory nerves in the cornea, prompting the lachrymal glands to produce tears as a reflex to dilute and wash away the irritant.
Strategies Using Environmental Control and Physical Barriers
One direct way to prevent irritation is to establish a physical barrier between the volatile gas and the eyes. Sealed protective eyewear, such as swimming goggles or safety glasses, effectively blocks the airborne irritant from reaching the sensitive corneal surface. This method prevents the initial chemical reaction from occurring on the eye’s surface.
Airflow Control
Controlling the airflow is a powerful technique to redirect the gas away from the user’s face. Placing a small fan next to the cutting board, positioned to blow air across the board and away from your eyes, effectively disperses the syn-propanethial S-oxide. Cutting the onion directly under an active kitchen exhaust fan or range hood also draws the volatile compounds upward, preventing them from reaching eye level.
Cutting Technique
Using a very sharp knife minimizes cellular damage and reduces the amount of enzyme and sulfur compounds released into the air. A dull blade crushes the onion cells, causing a more violent expulsion of fluid and irritant-laden aerosols. Leave the root end of the onion intact until the very last cut, as the base typically contains the highest concentration of the sulfur compounds responsible for the reaction.
Strategies Based on Temperature and Moisture
Temperature Reduction
Reducing the onion’s temperature is a common technique used to slow the enzymatic reaction that produces the irritant gas. Chilling the onion in the refrigerator for 10 to 15 minutes before cutting decreases the activity of the lachrymatory factor synthase enzyme. A slower reaction rate means the syn-propanethial S-oxide is produced less rapidly, allowing it to disperse before it concentrates and reaches the eyes.
Moisture Neutralization
Moisture can neutralize the irritant gas before it becomes airborne. Cutting the onion close to or under a gentle stream of running cold water allows the water-soluble gas to dissolve immediately upon release. This prevents the volatile compound from vaporizing and traveling to the eyes, though it can make the cutting surface slick and potentially unsafe.
For a similar effect without submerging the onion, you can place a damp paper towel next to the cutting board. The moisture in the towel acts as an alternative, closer water source for the gas to react with instead of the moisture in the eye. The mechanism of slowing the enzyme or dissolving the gas remains the scientific basis for these popular methods.