I was thinking of heat for the first effort but I have been looking at vacuum approaches as well. Basically, it's a box that can accept screened racks which hold the food. Air is brought in at the bottom, heated and distributed via a fan. Exhaust is at the top. Humidity would be sensed at the inlet and outlet. air temp would be controlled based on a differential humidity profile. The lower the differential humidity, the lower the heating. As the food dries, the differential would drop. A given food would have a specific profile. This way, it would prevent over drying.
The only question I have about capacitive is how much of a difference between 30% humidity and 70% (for example) there would be. I guess I'll need to do some tests.