Dielectric properties of bentonite water pastes used for stable loads in microwave thermal processing systems

The dielectric properties of bentonite water pastes relevant to microwave thermal processing were measured over 300-3000 MHz and 22-120 °C. Effects of bentonite content (7.5-25%, wb), salt (NaCl) content (0.3-1.2%, wb), vegetable oil content (5-15%, wb) and sucrose content (30%, wb) on dielectric properties were investigated. Regression equations were developed to reveal the influences of temperature and different ingredients on the dielectric properties of bentonite pastes at 915 and 2450 MHz. Results illustrated that dielectric properties of bentonite pastes could be adjusted with different ingredients to match those of a wide range of food materials with similar response to increasing temperature. Vegetable oil and salt were good additives to reduce dielectric constant and increase loss factor, respectively. Adding sucrose reduced both dielectric constant and loss factor. Derived from the regression equations, the influence factor of each ingredient was calculated to reveal its influence on the changing rate of dielectric properties with increasing temperature. The bentonite pastes can be formulated with stable dielectric properties and be used as dummy loads for evaluating performance of industrial microwave assisted thermal processing systems.