why do onions make your eyes water when cooled?

The Deep Dive

The act of cutting an onion breaches its cell walls, initiating a fascinating chemical cascade designed as a defense mechanism. Onions absorb sulfur from the soil, storing it in compounds called S-alk(en)yl cysteine sulfoxides. When cells are ruptured, an enzyme named alliinase is released, which rapidly converts these sulfoxides into sulfenic acids. For most alliums, these sulfenic acids spontaneously rearrange to form various thiosulfinates, which are responsible for the distinct flavors and aromas of garlic and other alliums. However, in onions, a unique enzyme called lachrymatory-factor synthase (LFS) intervenes. Instead of allowing the sulfenic acids to spontaneously rearrange, LFS acts upon them to specifically produce syn-propanethial S-oxide. This compound is a highly volatile, sulfur-containing gas that quickly disperses into the air. When this gas reaches the moist surface of your eyes, it dissolves in the tear film and reacts to form a dilute solution of sulfuric acid. This acid irritates the nerve endings in your cornea, prompting the lacrimal glands to produce tears to wash away the irritant. Cooling an onion before cutting it significantly reduces this reaction because lower temperatures decrease the activity of the enzymes alliinase and LFS, and also reduce the volatility of the syn-propanethial S-oxide, meaning less gas is produced and less reaches your eyes.

Why It Matters

Understanding why onions make us cry has practical implications for food preparation and offers insight into plant biochemistry. Chefs often refrigerate onions before chopping them to minimize eye irritation, a direct application of this scientific knowledge. This chemical defense mechanism also explains the onion's pungent flavor, which has been utilized in cuisines worldwide for centuries. Beyond the kitchen, studying these compounds helps researchers understand plant stress responses and could inform agricultural practices aimed at developing less irritating onion varieties or even new pest deterrents. It's a prime example of how basic biological defense mechanisms can intersect with human experience and culinary traditions.

Common Misconceptions

A common misconception is that the "juice" directly causes tears, or that the onion simply releases a strong acid. In reality, it's a volatile gas, syn-propanethial S-oxide, produced through a series of enzymatic reactions, that irritates the eyes. Another frequent misunderstanding is that cooling an onion makes the eye-watering worse. On the contrary, cooling actually reduces the effect. Lower temperatures slow down the enzymes responsible for producing the irritant gas and also decrease the gas's volatility, meaning less of it gets into the air and into your eyes, thus lessening the irritation.

Fun Facts

• The enzyme lachrymatory-factor synthase (LFS) was only identified in 2002, solving a long-standing mystery about onion tears.
• Onions are one of the oldest cultivated vegetables, with evidence of their use dating back over 7,000 years.