You can put the aspen on a baking sheet and place it in the oven set at 300 F. 300 F is well below the flash point of most materials and as long as you keep an eye on it you can let it bake for an hour or so. I've done this with a few substrates to remove any moisture from it, but I'd bet that it would kill most things as well. After I bake the substrate I stick it in the freezer to cool it off.