why does onions make you cry when mixed?

The Deep Dive

Cutting into an onion ruptures its cells, releasing the enzyme alliinase which interacts with sulfur-rich amino acids like trans-S-(1-propenyl) cysteine sulfoxide. This enzymatic reaction produces 1-propenesulfenic acid, which is quickly converted by lachrymatory-factor synthase into syn-propanethial-S-oxide, a volatile gas. The gas diffuses into the air and, upon reaching the eyes, dissolves in the tear film to form sulfuric acid and other irritants. These compounds stimulate sensory nerves in the cornea, sending pain signals to the brain, which responds by activating lacrimal glands to produce tears. The intensity depends on factors like onion variety—yellow onions often have higher sulfur content—and soil conditions. Some people have genetic variations reducing sensitivity. This defense mechanism evolved to deter pests, and it's also linked to onions' flavor and health benefits, as similar compounds contribute to their antioxidant properties when cooked or consumed.

Why It Matters

Understanding this chemistry helps home cooks reduce tears through methods like chilling onions, using sharp knives, or improving ventilation. For the food industry, it drives innovation in breeding tearless varieties, such as Sunions, enhancing user experience without sacrificing taste or nutrition. Onions offer health benefits like anti-inflammatory effects from sulfur compounds, so minimizing tears encourages more people to use them in cooking. This knowledge also serves as an accessible lesson in enzyme kinetics and plant defense mechanisms, bridging everyday life with scientific principles.

Common Misconceptions

A common myth is that holding a spoon in your mouth or chewing gum prevents tears, but these actions don't stop the gas from reaching your eyes; only physical barriers or ventilation are effective. Another misconception is that only red onions cause tears, but all onions contain the necessary compounds, with yellow and white varieties often being more potent due to higher sulfur content. Some also blame the smell, but it's specifically the volatile syn-propanethial-S-oxide gas, not general odor, that triggers the irritation. These myths persist from anecdotal advice, but science confirms the enzymatic release of the gas as the sole cause.

Fun Facts

• The tear-inducing compound syn-propanethial-S-oxide was first identified in the 1970s by Japanese researchers studying onion biochemistry.
• Onions are part of the Allium family, which includes garlic and leeks; all produce similar sulfur-based irritants when damaged, but onions are particularly potent.