The use of dielectric heating in the drying of biological materials from the frozen state

Abstract

The rate of vacuum drying of biological materials from the frozen state may be extremely slow due to the lowering of the vapor pressure which results from evaporative cooling. Present means of supplying heat involve the danger of localized damage due to overheating at the surface. This can be avoided by the use of radio frequency (1000 Mc/s) energy from a radar generator. These wavelengths are not absorbed by dry proteins and thus the dielectric heating occurs only in that part of the material still containing water. Microscopic examination of tissues dried in this way revealed no evidence of localized overheating or other changes due to exposure to radio energy. In operation, a small antenna is placed next to the sample holder and the temperature, as indicated by a thermocouple in the material, is controlled by adjusting the power to the radar generator. The amount of power needed to produce a given amount of heating is a function of the rate at which water vapor can escape from the material as well as the dielectric properties of the material itself. When all water has been removed, no further heating can be produced.