Microbes are hungry. To survive, they need sugar. But flour is not sugar. It is starch—long, complex chains of glucose molecules locked together in granules. For a yeast cell, a starch granule is like a boulder. It is too big to eat. Before the yeast can feast, someone has to break the rock. This is the job of Amylase.

Refined Flour (often labelled as White Bread Flour) has had the bran and germ mechanically removed, leaving only the starchy endosperm. While this creates a flour that produces high volume and airy bread (due to strong, uninterrupted gluten), it is a biological "desert."