Moisture and temperature dependence of the dielectric properties of pharmaceutical powders

The dielectric properties of pharmaceutical powder-(paracetamol, aspirin, lactose, maize starch, adipic acid) solvent (water) mixtures were measured at 2,450 MHz at a range of moisture contents (0-1.0 kg kg^-1, dry basis) and temperatures (20-70 C). The dielectric constant (ε′), loss factor (ε″) and penetration depth (d _p) were found to be dependent on frequency, moisture content, temperature and powder type. For powder-water mixtures, a linear increase in the dielectric properties with moisture content was observed, whilst the temperature dependence was of quadratic form. The penetration depth was also significantly affected by temperature and moisture content. Although, ε″ also increased with increasing temperature, variation with moisture content was temperature dependent. This information on dielectric properties is essential for mathematical description of the pharmaceutical product temperature history during microwave heating and for the design of microwave drying equipment.