why do potatoes make you cry

The Deep Dive

The culprit is a chain reaction initiated by physical damage to the potato's cells. Inside the tuber, the enzyme alliinase is stored separately from sulfur-containing compounds called sulfides. When a knife ruptures the cells, these components mix. Alliinase rapidly converts the sulfides into a new, unstable molecule: sulfenic acid. This acid then spontaneously rearranges into syn-propanethial-S-oxide, a volatile sulfur compound that readily vaporizes at room temperature. This gas is the true tear-jerker. When it drifts into your eyes, it dissolves in the thin film of moisture on the cornea, forming a mild sulfuric acid. This acid directly stimulates the nerve endings of the trigeminal nerve, the same nerve responsible for sensing irritants in the nose and eyes. The brain's response is a reflexive production of tears to dilute and wash away the perceived threat. This process is an evolutionary defense for the potato, deterring animals from feeding on the damaged plant.

Why It Matters

Understanding this reaction is key for both home cooks and the food industry. It explains why some potato varieties are more irritating than others and informs techniques to minimize the effect, like cutting under water or chilling the potato first. More broadly, it’s a classic example of enzymatic browning and chemical defense in plants, principles that apply to onions, garlic, and many fruits. This knowledge helps in developing less tear-inducing crop varieties and in food processing to preserve flavor and texture without unwanted reactions.

Common Misconceptions

A common myth is that the gas is pure sulfur or that it's the potato's 'juice' that stings your eyes. In reality, it's a specific, synthesized compound (syn-propanethial-S-oxide) created by the enzymatic reaction, not elemental sulfur or simple sap. Another misconception is that cooking potatoes beforehand will prevent tearing. While heat denatures the enzyme, if you cut a cooked potato and then re-expose it to the raw enzyme from another source, the reaction can still occur, though typically on a much smaller scale.

Fun Facts

• The same enzyme responsible for potato tears, alliinase, is also what creates the pungent compounds when you crush garlic.
• The 'tear gas' from potatoes was first identified and named 'lachrymatory factor' in the early 20th century, with the exact structure confirmed only in the 1960s.