Moisture is the No. 1 element behind determining how quickly mold spots appear on any kind of bread. The internal dampness of a type of bread is a major determining factor in neutral environments, so the more light and moist a slice, the sooner it shows growth.

Darker breads tend to hold more moisture, thus, rye, bran, oat and Boston loaves develop mold much faster than dense and dry variants. White and potato bread fall in the middle range for decay with spores appearing less quickly than more moist bread, but more rapidly than on dryer loaves.

The rate of decay and molding is also influenced by the amount of preservatives present. Commercially distributed bread contains many chemicals that make it stable for weeks of transportation and sitting on shelves, so even more moist breads of these types take longer than fresh varieties before showing mold spots. Sourdough bread of either manufactured or homemade types never grows mold because of its high acidity.

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