Sugar (including cane sugar, dextrose, fructose and sugars in flour, etc.) decomposed under high temperature (150℃ to 200℃). Decomposed substance aggregate together into black substances (named coloring up)
Caramel reaction and Maillard reaction are two basic reaction in food. If we can deal with these two reactions well, the appearance of biscuit will be easily be handled.
Caramel reaction happened in whole backing course. But the most important step is in late period of backing. If we want little sugar in prescription and maintaining good color, we has to increase some coloring matter.