The objective of this study was to determine the effect of output power of a microwave generator on the kinetics of convection-microwave drying process and on the colour of foamed apple pulp, as well as to suggest a model describing the drying process kinetics. The apple pulp was foamed with 2 % albumin and 0.5 % methylcellulose during a 5 minute period. A 4 mm thick layer of the foamed pulp produced was dried in a combined convection-microwave system at 60 °C, at an air speed of 1.8 m/s. The output power of the microwave generator was: 0, 80, 120, 180, and 250 W, respectively. After the drying process was completed, the moisture content and the surface colour of samples were determined in CIEL*a*b* system using a Minolta chromameter. The analysis of kinetics of the convective-drying process showed that there was no period of a constant drying rate. For the most part, only a period of falling drying rate was found. The microwave output power significantly impacted the decrease in the drying period of foamed apple pulp. At a power of 250 W applied, the drying period was 5 times shorter compared to the drying using solely convection. When drying at 180 W, it was possible to obtain a product of a lighter colour, and, at the same time, the drying rate at a water content of 1.0 kg/kg d.m. was 15 % lower than the respective values obtained at 250 W.
foam drying, microwave power, drying kinetics, colour