The pasteurization is carried out for a prolonged time at around 170°F and is supposed to kill only heat susceptible organisms and their spores, while sterilization works at a temperature of 250°F and is supposed to kill all organisms.
Fractional sterilization or tyndallization is a method used to destroy bacteria and endospores in preparation of grain spawn(rye, wheat, millet, birdseed.
With oven sterilization similar problems arise as with the microwave: The heat conduction of dry air compared to steam (as in the pressure cooker and a covered pot of boiling water) is much smaller - therefor the sterilization time has to be longer -> this dries out the substrate.