1. Breakdown of Cell Walls:
* Heat: The heat breaks down the cell walls of the onion, releasing the compounds that contribute to its flavor.
* Enzymes: Enzymes within the onion cells are activated by heat, which further breaks down these compounds.
2. Sulfur Compounds:
* Alliin: This amino acid is abundant in onions. When the cell walls are broken, alliin reacts with an enzyme called alliinase, producing allicin.
* Allicin: This volatile compound is responsible for the pungent, sharp, and slightly bitter flavor of raw onions.
* Heating: As the onion cooks, allicin breaks down into other sulfur compounds, including diallyl disulfide, diallyl trisulfide, and other sulfur-containing compounds. These compounds contribute to the sweet and savory notes we experience in cooked onions.
3. Sugar Formation:
* Fructose: Onions contain fructose, a type of sugar. When heated, fructose undergoes a process called Maillard reaction.
* Maillard Reaction: This reaction involves the interaction of amino acids and sugars, producing a complex array of flavor molecules. It's responsible for the browning and caramelization that occurs during cooking, and it contributes to the sweetness and depth of flavor in cooked onions.
4. Volatile Compounds:
* Other Volatiles: The heat also releases other volatile compounds from the onion, such as pyrazines, which are responsible for the nutty, roasted, and earthy notes.
Overall: The combination of these chemical reactions creates the complex and layered flavor profile of cooked onions. The specific compounds produced, and thus the final flavor, are influenced by the cooking method, time, and temperature.
Note: This is a simplified explanation. The chemical reactions in onions are incredibly complex and involve numerous other compounds and reactions.
