Dynamics of drying bentonite in superheated steam and air as a model food system

The objective of this research was to compare temperature patterns, drying rates, and diffusivities of bentonite as a food model with a high concentration of solids dried in hot air and superheated steam at 160 degrees C. In our preliminary investigations on the drying of liquid food droplets in hot air and superheated steam, spherical samples of 13.5 mm in diameter were prepared from chemically stable bentonite paste. In superheated steam drying, bentonite samples reached the saturation temperature for steam (100 degrees C) after the first minute of drying. During this period the samples gained approximately 0.1 kg/kg dry basis (db) of moisture, due to condensation of water on the surface of the sample. In air drying at 160 degrees C, the temperature of a similar sized sample gradually increased to a wet bulb temperature of 80 degrees C over a 10 min period. The drying rate in the superheated steam was higher by 8 to 10% than in the 160 degrees C air in the initial stage of drying. However, this situation reversed when samples reached the falling rate-of-drying stage. For this period and below moisture content of 0.10 kg/kg db, the overall diffusion coefficient was 50 to 80% higher for samples dried in the 160 degrees C air than in superheated steam of the same temperature.