During the baking process, the surface of the bread will turn brown and yellow, emitting a good smell. This is because a series of chemical reactions occurred during baking, the most important of which is Maillard reaction. This reaction is very complicated, and it has not been fully understood up to now. In short, Maillard reaction is the reaction between sugar and amino acids at high temperature. This reaction will release some aroma substances and polymerize to produce some dark colored substances. The unique aroma emitted by the barbecue process is also mainly from this reaction.
If you bake for too long, the bread will be scorched and turn black. At this time, the bread also had a caramelization reaction. Under continuous high temperature, the starch molecules in flour will split into small molecules of sugar. These sugars will undergo molecular structure transformation to form some volatile substances, some of which will also bring the smell of baking. Other molecules will re polymerize to form caramel pigment. As a synthetic pigment, caramel pigment is widely used in modern food. The black of coke is produced by caramel pigment.
If the baking temperature is too high, the bread will "carbonize". The main component of flour is starch, accounting for more than 80%. Starch and protein are organic substances containing a large amount of carbon. When heated in the absence of water, these organics will crack even if the temperature is not high enough to burn. Hydrogen, oxygen and other elements are gradually lost, leaving only carbon and mineral components. The carbon left behind is black. Of course, such dark bread is not edible.