Why are raw red onions spicy?
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Same reason raw white onions are spicy…they’ve got a sulphur compound in them that reacts with water to form a chemical that we taste as spicy (and pisses our eyes off). Heating breaks the compound down so you don’t get this with cooked onions.
All onions contain sulfur, an element. As a defense mechanism (remember onions don’t *want* to be eaten) when an onion is broken (cut, smashed, etc) the onion mixes an enzyme (a digestive chemical) with the sulfur that cuts the sulfur loose and turns it into a gas. This isn’t an active mixing (it’s a plant after all) it’s more like the onion keeps the enzyme and the sulfur in two different rooms. When you cut or smash the onion the wall between the rooms is broken, they mix, and bingo, sulfur gas.
That sulfur gas travels around in the air round the onion until it touches water, for example, on our eyes or in our mouths/nose. Once it touches water it reacts chemically and becomes Sulfuric Acid, as you can image having acid on our eyes/in our mouths isn’t very pleasant and our bodies sense the damage the acid is causing and send a “pain” signal to your brain. What you call “spicy” is that “help, you’re touching acid, ouch” signal in your brain.
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