Mold is a type of fungus, and like other fungi, it does not require light for its growth. In fact, it thrives in dark and warm places.
The spores of mold are what give it its color: white, yellow, green, gray, or black. These spores travel throughout the air and grow on other objects, such as bread.
Preservatives in the bread will affect how fast the bread molds. Many commercial breads have chemical preservatives to prolong their shelf life, and these will slow down the growth of mold.
Environmental conditions will also impact how fast the bread molds. If you put a slice of fresh wheat bread in the refrigerator and another in the kitchen cabinet, the one in the fridge will grow mold faster because it is in a warmer and darker environment.
The acidity of the bread will also determine how fast it molds. High acidity types of bread will mold much slower than low-acidity varieties.
Moisture level of the bread will also contribute to how quickly it molds. Molds love damp environments and need moisture to thrive, so the moistier the bread, the faster it will mold.
Salt can help to slow down the growth of mold on bread. The higher the amount of salt in a slice of bread, the less likely it is to mold.
The best way to prevent mold from growing on bread is to store it properly and keep it dry. If you don't have time to store your bread in the refrigerator, you can also freeze it. Freezing stops the growth of mold without changing the texture of the bread too much.