What and how do they make raisins and prunes before the fruits sour?!
Answers: looks like this is or would be a fast process.
Did you try a food dehydrator or the oven on low?
It's the fruit's juice that spoils (sugar becomes alcohol which becomes vinegar). Drying the fruit takes away almost all the moisture, and the sugar cannot become sour without it. Take a look at your granulated sugar -- it will never spoil.