well it doesn't matter they mold at the same rate unless one is covered. If his is for a paper write that bacteria can't live in dehydrated food or freeze-dryed food because there is no moisture and...
White bread doesn't necessarily mold faster than whole wheat bread. (White bread is usually made from wheat.) It depends upon the formulations, the quality of the ingredients, the processing...
The white bread has been ground more finely, and has been bleached. More organisms survive when processing is less.
Another possibility.
Because white bread has more preservatives in it than wheat...