Does mold grow on peanut butter?

Sarah P. asks, "Dr. Volt, why doesn't mold grow on peanut butter?"

Oh dear, Sarah, I hope that you are not eating any strangely-colored peanut butter thinking that this is the case! In fact, peanuts are susceptible to certain molds which can then produce a particularly dangerous chemical called aflatoxin, which is a carcinogen believe to cause liver cancer. However, most major brands of peanut butter contain sufficient preservatives to prevent the growth of molds in the finished product for weeks or even months without requiring refrigeration (assuming the jar is kept tightly sealed). But eventually the peanut butter will go bad and can become quite dangerous to eat. Natural peanut butters do require refrigeration, as they lack the same preservatives (the oils also tend to separate and can go rancid).