why does onions make you cry after cooking?

Q: why does onions make you cry after cooking?

Onions release volatile sulfur compounds when their cells are damaged, typically during chopping. Cooking can still cause tears if the onion was chopped first, as the irritant gas is already formed and may be released as steam. Heat eventually deactivates the enzyme responsible, but pre-formed gas escapes during heating.

The Deep Dive

The culprit is a sophisticated plant defense system. Onion cells store separate, inert precursors: sulfur-containing amino acid sulfoxides (like isoalliin) and the enzyme alliinase. When you chop an onion, cell walls rupture, allowing these components to mix. Alliinase rapidly catalyzes the breakdown of sulfoxides into highly reactive sulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, is then spontaneously converted by another enzyme (lachrymatory-factor synthase) into the volatile syn-propanethial-S-oxide. This gas diffuses through the air and, upon contact with the moist surface of your eyes, reacts with the water in your tears to form a mild sulfuric acid. This irritates the corneal nerves, triggering a reflexive increase in tear production to flush it away. Cooking applies heat, which denatures (unfolds and inactivates) the alliinase enzyme. However, if the onion was chopped before heating, the syn-propanethial-S-oxide gas may already be present and can be carried into the air by cooking vapors or steam, especially in methods like sautéing or frying where the chopped pieces are exposed. The intensity varies by onion variety; sweeter, storage onions often have higher concentrations of the relevant sulfur compounds.

Why It Matters

Understanding this mechanism has practical kitchen applications. It explains why chilling an onion before chopping slows the enzymatic reaction (reducing gas production) and why using a very sharp knife causes less cell damage, minimizing gas release. This knowledge drives agricultural research to breed 'tear-free' onions by selectively reducing the lachrymatory-factor synthase enzyme or its precursors, which could improve worker conditions in food processing and consumer experience. Furthermore, the same sulfur compounds are responsible for onions' characteristic flavor and aroma when cooked, making their controlled release central to culinary arts.

Common Misconceptions

A common myth is that cooking an onion completely eliminates its tear-causing potential. While heat denatures the enzyme, the irritant gas can still be released if the onion was chopped prior to cooking, as the gas is already formed. Another misconception is that only 'strong' or raw onions cause tears. In reality, any onion variety can produce the gas if chopped, though concentrations differ; even mild, sweet onions will cause some tearing, and cooking chopped onions can aerosolize the pre-formed irritant.

Fun Facts

The 'Vidalia' onion, famous for its sweetness, is legally required to be grown in specific low-sulfur soils of Georgia, USA, which reduces its pungency and tear-causing potential.
Ancient Egyptians revered onions for their spherical shape and concentric rings, symbolizing eternal life, and used them as currency and in burial rites, likely tolerating the tears.