In the process of baking bread, the surface will turns brown and exudes a pleasant aroma. This is because during the baking process, a series of chemical reactions happened. The most important reaction is Maillard reaction. This reaction is very complex, and i’m not completely clear how it works. Simplest way to explain is, the Maillard reaction happened at high temperatures when sugar occurs with amino acids. This reaction will release some fragrance substance, while the substances combined together and produce some dark substances. This also happened when barbecuing.
If the baking time is too long, the bread will be scorched black. Caramelization reaction will happened in the bread. Under continuous high temperature, the starch molecules will be cleaved into smaller sugar molecules. These sugars will go into molecular structure, forming some sort of volatile substances, some of these will also have pleasant aroma. Some molecules will re-aggregated to form caramel. As a synthetic pigment, caramel is widely used in modern food.
If the baking temperature is too high, the bread will be "carbonized." More than 80% of the main components of flour is starch. Starch and protein contains a large amount of organic carbon. While heating in the absence of water, even when the temperature is not high enough to burn, these organisms will be cleaved. Elements like hydrogen, oxygen will gradually lost, leaving only carbon and minerals on the bread. This remaining carbon, of course is black in color and this black color bread is not suitable for consumption.
