Why Does Toast Turn Brown and Smell More Aromatic?

When a slice of bread goes into a toaster, it first loses some water. Its surface then turns golden brown and develops nutty, roasted aromas that fresh bread does not have. The colour and most of those new aromas do not come simply from “drying out”. They arise from a group of chemical processes known as Maillard reactions.

Reducing sugars in the bread react with nitrogen-containing groups in proteins and amino acids. The early products then break down, rearrange and combine, producing many volatile aroma compounds as well as darker, larger molecular mixtures. Heat speeds the reactions and reduces surface moisture. The inside remains wetter and cooler, so the outside browns much more readily than the crumb.

Maillard reactions are not the same as caramelisation. Caramelisation mainly involves changes in heated sugars and does not require amino compounds. Nor is this the enzymatic browning that occurs when a cut apple meets oxygen. Taking the same toast from pale gold to black is not simply “more flavour”: excessive heating destroys desirable aromas, adds bitterness and may form more acrylamide. The central point is simple: toasting rearranges molecular relationships; it does more than remove water.

https://www.acs.org/pressroom/reactions/library/maillard-the-most-delicious-chemical-reaction-in-the-world.html
https://cen.acs.org/articles/90/i40/Maillard-Reaction-Turns-100.html