If the unit is burning or severely shriveling the food first try to reduce the pressure and temperature for the recipe. For example, some melons may need to be reduced down to 800mtorr and 40 °F (4 °C) in order to freeze dry without shriveling. 

If the unit is severely burning food every cycle regardless of changing the set temperature, clean the unit out and turn the unit off then back on. With the unit on the home screen wait 45 minutes and then feel the shelves. If the shelves are hot while they are not turned on and the unit is still on the home screen then the machine will need a new power board. This issue is caused when one of the triacs on the power board has failed. Specifically the triacs fail closed so they will continue to provide power to the heat rack even if not commanded to do so.