How to tell apart mold and flour on sourdough bread? I bought some sourdough bread from a bakery three days ago, and put it (still in its brown bag) in a ziploc bag. Now, I know these loafs are covered in flour, but the bottom looks slightly different than flour. 





Are there any tricks to tell if it's mold or not?
Scrape at it with your fingernail. Flour will come off in a... floury manner, revealing the cranny it was stuck in. Mold won't be limited to the crannies. Also, it'll appear on the cut surface before it appears on the crust. Additionally, it'll be different from the color of the flour (in the same area of the loaf).

FWIW, that's definitely flour on most of the bottom, though I do see a couple of lighter spots that could be mold.