Onions are a nutritional powerhouse—low in calories and rich in vitamins, minerals, and antioxidants. They enhance countless dishes, from salads and omelets to guacamole. Yet, any home cook knows the downside: chopping onions triggers tears. But why does this happen?
Onions grow underground, where pests lurk. To defend themselves, they harness sulfur from the soil, forming amino acid sulfoxides—sulfur compounds that readily turn into gas. When you slice an onion, these sulfoxides mix with enzymes, producing sulfenic acid. This then reacts to create syn-propanethial-S-oxide, a volatile gas.
This gas rises from the cut onion, repelling insects (and irritating human eyes). The process involves precise chemical reactions. Not all onions are equal: white, yellow, and red varieties produce more tears due to higher enzyme levels, while sweet, green, and spring onions generate less.
How Onions Affect Your Eyes
Your eyes are highly sensitive and quick to protect themselves from irritants. The gas triggers tear production to flush it out. Sensitivity varies—some people tear up minimally, while others react from across the kitchen.
Tips to Minimize Tears
Practical strategies include wearing swim goggles to block the gas (kitchen-friendly over lab gear). Submerging onions in water while cutting helps, as does using eye drops beforehand. Tear-free onions aren't on shelves yet, so keep tissues nearby for your recipes.
