According to the MadSci Network at Washington University, the type of bread that grows mold the fastest depends on the “water activity” present in the bread. The water activity is what helps scientists to determine the shelf life of a product.
Pure water has an activity of 1.0. Bacteria generally does not grow when the water activity recorded is less than around .91. However, mold can grow on drier types of foods. In order for mold to develop, the water activity of a food needs to be below .81. Breads, in general, have a water activity of .95. The pH level of bread is another factor that determines how quickly it grows mold.