This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Dehydrating food works on the principal that both microbes and enzymes
in your food require free water to work. (To a lesser extent, this is
how freezing works. The water is frozen instead of evaporated off.).
Generally, you get rid of the water in food by gentle, even heat (sun,
oven, dehydrator) and air movement (wind, open oven door, fan)
otherwise water just stays in the food or condenses on it. You especially
need to be cautious, though, about several types of mold that produce
mycotoxins (e.g. aflatoxin) while growing on the surface of your dried
Dried foods which take more than 1 to 2 hours to rehydration or
reconstitution should be rehydrated either in the refrigerator or in
simmering water to prevent the growth of microorganisms. Once
vegetables are rehydrated, they will support the growth of Clostridium
botulinum so they must be handled safely. Any dried foods with signs
of spoilage or mold growth should be discarded. (section taken from
Susan Brewer) [See also the section of aflatoxin under Spoilage.
Check out Part 6 of this FAQ (different file), for additional web
sites, and ISDN numbers, authors, etc of books recommended below.