Fruits like Apples, Pears and Bananas turn dark when exposed to air. All my dehydrator books recommend to soak them a few minutes in a lemon juice & water solution or a fruit preserver like Fruit Fresh. You can find Fruit Fresh in the canning aisle of your supermarket. I use about 2 tablespoons of Fruit Fresh to about 1 1/2 quarts of water. I think it's a little stronger than what the package says. Soak Fruit for about 5 minutes, drain (do *not* rinse) and dry. I have experienced some fruit to still turn a little darker even with using the Fruit Fresh but I think it was attributed to my fruit (pears) being *really* ripe.

Hope this helps and happy dehydrating!!
Toni :)