|MadSci Network: General Biology|
The mold has always been there. However, the conditions were optimum for it to grow. All things around you (unless they are specifically sterilized) comtain molds. Mold and spores (the reproductive component) of yeasts and molds, are microscopic and very tough. They can survive dry, hot, and even cold conditions. The bread you purchased had mold spore on it but there was sufficient time to eat it before the spores produced visible mold. Either the bag is quite old or the conditions were perfect for mold growth and they just triggered it (i.e. dark, warm, moist- as on the top of your fridge!?) Though the bag was hermetically sealed, this only prevents things from entering/exiting the back. It doesn't destroy what's already on the bread. I wouldn't take a chance on eating it, though. If you can visibly see the mold, I'd throw it out or feed it to the birds.
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