why do onions make your eyes water when wet?

The Deep Dive

When you slice into an onion, you break open its cells. This releases enzymes like alliinase, which were previously kept separate from sulfur-containing compounds called amino acid sulfoxides. The mixing of these components triggers a chemical reaction. Specifically, alliinase converts the amino acid sulfoxides into sulfenic acids. These sulfenic acids are unstable and quickly rearrange to form syn-propanethial S-oxide, a volatile gas. This gas is the culprit behind the tears. When it reaches your eyes, it dissolves in the watery film covering your cornea. There, it reacts with the water to form a weak solution of sulfuric acid. Your eyes perceive this as an irritant and signal your tear ducts to produce more tears to dilute and flush out the offending chemical, which is why you end up crying.

Why It Matters

Understanding this chemical process helps us appreciate the complex defense mechanisms plants have evolved. For cooks, it's a practical piece of knowledge that can lead to techniques for minimizing eye irritation, such as chilling onions before cutting or using ventilation. It also highlights how seemingly simple actions, like chopping an onion, involve fascinating biochemical reactions that protect the plant and, in turn, trigger a physiological response in us.

Common Misconceptions

A common misconception is that onions contain ammonia, which is the irritating agent. In reality, onions do not produce ammonia when cut. Another myth is that only certain types of onions cause tears, but all onions contain the necessary enzymes and sulfur compounds to produce syn-propanethial S-oxide. The intensity of the tears can vary based on the onion variety and how it was grown, but the fundamental chemical reaction is the same across all onions.

Fun Facts

• Some onions are bred to be 'non-')])
• The tear-inducing chemical produced by onions is called syn-propanethial S-oxide.