Cutting onions releases sulfur-based compounds that irritate the eyes and trigger a tear response [1].

Understanding this chemical reaction allows cooks to use specific preparation methods to minimize discomfort. Because these compounds are volatile, they can affect anyone in the immediate vicinity of the cutting board.

According to Julien Ménielle, the irritation is caused by a volatile gas known as syn-propanethial S-oxide [1, 2]. This gas forms when the onion's cells are ruptured during slicing. The process serves as a defense mechanism for the vegetable [2]. Caroline Cormier said it is to protect the bulb when it is in distress [2].

This chemical reaction is not limited to standard onions. Four vegetables, including garlic, leeks, and shallots, contain these tear-inducing compounds [3].

To prevent crying, experts suggest several practical interventions. Chilling the onion before slicing can slow the chemical reaction [1]. Using a very sharp knife reduces the amount of cell damage, which in turn limits the release of the gas [1]. Another effective method is cutting the vegetable under running water to trap the compounds before they reach the air [1].

These methods work by interrupting the transition of sulfur compounds into the airborne gas that triggers the ocular response [2]. By controlling the temperature or the physical impact on the plant cells, the volume of syn-propanethial S-oxide is reduced.

“Cutting onions releases sulfur-based compounds that irritate the eyes”

The ocular response to slicing onions is a biological interaction between human sensitivity and a plant's chemical defense system. By applying basic physics and chemistry, such as temperature control and precision cutting, the release of the irritant gas can be managed, making the process more tolerable for the user.