What chemicals do onions release when cut open?

When onions are cut open, they release a volatile chemical compound called propanethial S-oxide (also known as syn-propanethial S-oxide). This compound is responsible for the characteristic strong, pungent odor and tear-inducing properties of onions.

Here's how it works:

1. Enzymes and Precursors: Onions contain enzymes called alliinases and a sulfur-containing compound called alliin. These two components are separated within the onion cells.

2. Cutting the Onion: When the onion is cut, the cell walls are ruptured, allowing the alliinases and alliin to mix.

3. Chemical Reaction: The alliinases catalyze a chemical reaction with alliin, converting it to propanethial S-oxide. This volatile compound is released into the air.

4. Tear-inducing Properties: Propanethial S-oxide is a powerful irritant that stimulates the lachrymal glands in the eyes, causing them to produce tears.

Other chemicals released in smaller amounts include:

* Sulfenic acids: These contribute to the pungent odor and flavor.

* Pyruvic acid: This is a flavor component.

* Flavonoids: These are antioxidants that provide health benefits.

Fun Fact: Different varieties of onions produce varying amounts of propanethial S-oxide, which is why some onions are considered "stronger" or "milder" than others.