Anyone who has left a loaf of sandwich bread in the cupboard for too long has no doubt seen a moldy loaf. To say that that bread will "spoil" or "rot" if kept for too long is something of a misnomer. The presence of visible mold growth indicates that the bread itself is not rotting but rather a species of fungus has taken up residence in the bread and is beginning to break it down. There are several species of mold that are known to commonly infect breads.
Mold and Bread
Along with meat and poultry, sugary foods and fruits and vegetables, bread is among the foods that are most commonly attacked by food-borne molds. Some common molds found in and around foods, according to the U.S. Department of Agriculture, include: Alternaria, Aspergillus, Botrytis, Cladosporium, Fusarium, Geotrichum, Monilia, Manoscus, Mortierella, Mucor, Neurospora, Oidium, Oosproa, Penicillium, Rhizopus and Thamnidium. Some of these molds are capable of producing toxic chemicals known as mycotoxins, and others can produce cancer-causing chemicals called aflatoxins.
One of the most common bread molds is Rhizopus stolonifer. This is a filamentous species mold that tends to grow in more moist environments. Rhizopus stolonifer is used medicinally as an ingredient in birth control medications for women due to its ability to produce steroids such as progesterone. This species prefers breads over other foods because it needs both sugars and starch to survive.
Rhizopus stolonifer belongs to the zygomycetes group of fungal species, and, though Rhizopus stolonifer may be the most common example, any zygomycete species can be found on bread and similar foods. This group of fungi gets its name because they all produce a certain kind of spore called a zygospore. Zygospores collect and form a fruiting body called a sporangium. A sporangium is the black, fuzzy patch of mold you see on a piece of bread that has been infected.
Any potential problems with bread mold growth can be avoided with some protective steps. Never eat any bread with any amount of mold growth on it. Resist the temptation to cut off the visible mold growth from the bread and eat the rest of it. If a mold colony is visible on the surface of the bread, it indicates that its mycellium "roots" have taken hold inside the bread. Molds that produce dangerous toxins contain the toxins in these root threads, so it is always better to err on the side of caution by throwing away any bread that has mold growing on it.