Onion

Why they make you cry

Cutting an onion ruptures cells and releases two compounds that meet for the first time — an enzyme (alliinase) and a sulfur-containing amino acid. Their reaction produces a volatile sulfenic acid, which is then converted to propanethial S-oxide — a gas that diffuses to your eyes, where it dissolves in tear-film moisture and forms sulfuric acid. Your tear glands flood the eye to wash it out.

The compound was identified as recently as 2002, when researchers in Japan also found that onions evolved a new enzyme (lachrymatory factor synthase) specifically to produce this irritant — likely as defense against herbivores.

Reducing the cry

• Chill the onion 30 minutes before cutting (slows enzyme activity).
• Use a very sharp knife (less crushing means less enzyme activation).
• Cut near a flame or vent fan (oxidizes the volatile gas).
• Cut under cool running water (water dissolves the gas).
• Wear well-sealed goggles (the only really effective method).

Caramelizing — slow and slow

The sweetness in caramelized onions comes from two reactions: pyrolysis of natural sugars (caramelization proper) and the Maillard reaction between sugars and amino acids. Both require time and moderate, even heat — typically 45 minutes to over an hour with frequent stirring. Recipes that promise “10-minute caramelized onions” are producing browned, half-cooked onions, not actual caramelization.

Layered chemistry

Each onion layer is a modified leaf. The outer dry papery layers are the same tissue, just dehydrated. The chemical pungency increases toward the center; the milder, sweeter parts are in the lower outer layers — useful to know when you want to control the bite of a raw onion garnish.