Why do onions become brown and fragrant as they cook?

A cooking onion changes from sharp and pale to softer, fragrant and brown. Several physical and chemical changes overlap. Water evaporates, tissue changes, and new molecules form. The familiar phrase 'caramelized onions' is useful in a kitchen, but it does not identify every reaction responsible for the result.

One pathway is the Maillard reaction. A reducing sugar reacts with an amino group, such as one supplied by an amino acid or protein. Further reactions generate a mixture of compounds, including some that contribute aroma and brown color. Studies have detected early Maillard products in cooked onions.

Caramelization is a different family of reactions: sugars break down and recombine without requiring amino groups. It can also contribute to browning. Onion pigments and aromas cannot be assigned to one pathway merely by looking at the finished color.

Time, temperature, acidity, composition and water availability all influence these reactions. Evaporation carries energy away and changes the local conditions. Wet interiors and drying surfaces therefore behave differently. Browning is a changing reaction network, not a single switch that turns on at one universal temperature.