Microwave drying at the highest power and halogen lamp–microwave combination
drying reduced the drying time to an extend of 98% in comparison to conventionalhot-air drying and a high-quality dried product
was obtained
Microwave drying of apples at constant temperatures follows typical drying curves. The middle stage of
the drying process shows a faster drying rate and accelerated moisture evaporation. Meanwhile, more flavors
are lost, surface color is degraded, and charring often occurs in this stage. To improve the drying
effects, drying curves were controlled and changed in this study. The drying curve was linearized by automatically
varying drying temperatures in the middle stages. The controlled drying process therefore led
to an optimized temperature profile. To simplify the drying methods, apples were further dried with the
obtained temperature profile, while drying curves were online monitored but not controlled. It was
proved that slowing down the drying rate in the middle stage could improve the product quality in terms
of color, flavor, and overall appearance, while the drying time and energy consumption were still acceptable
(within 200 min and 22 kJ/g, respectively).