The aim of these studies was to investigate the influence of airborne ultrasound-assisted convective drying and microwave-assisted convective drying, as well as their combination, on process kinetics, total color change, water activity, content of carotenoids, polyphenols and antioxidant activity of carrots (Daucus carota L.). The global model of drying kinetics based on coupled ordinary differential equations was used to describe the moisture and material temperature profiles during drying. Application of ultrasound and microwave in convective drying reduced drying time in the range of 9-81%, but the shortest drying time was observed for simultaneous action of convection, ultrasound and microwave. The results of qualitative analysis showed a product improvement due to ultrasound as compared to convective drying and microwave-convective drying. The proposed mathematical model of drying kinetics successfully simulated real drying processes. The proposed mathematical model of drying kinetics successfully simulated real hybrid drying processes.

The drying kinetics, microstructural alteration, and rehydration properties of raspberry samples were studied experimentally in this work. Five different drying programs with the application of microwaves (MW) and ultrasound (US) were used, including convective drying (CV) as a reference test. The drying experiments were performed using a hybrid chamber dryer equipped with airborne ultrasound and microwave generators. The modified Page model was successfully used to describe the drying kinetics of raspberry fruits. Next, microstructural properties (porosity and total pore volume) of the dried samples were determined from the postprocessing of the images which were acquired using a lab-scale X-ray microtomograph. The results show a significantly shorter drying time by 54–64% for CVUS, 69% for CVMW, and 79% for CVMWUS; and a lower energy consumption resulting in energy saving of 14–23% for CVUS, 54% for CVMW, and 59% for CVMWUS as compared to CV. It is also shown that the average drying rate of raspberry samples increases by maximum fourfold with the application of both ultrasound and microwave radiations in CV. In addition, a higher porosity, total pore volume, and a better rehydration property were found for the raspberry samples dried with US than with MW assistance.

The aim of this study was to assess the impact of ultrasound on two subsequent processes - initial osmotic dehydration and convective drying of kiwifruit (Actinidia deliciosa). The effect of ultrasound (at a frequency of 25 kHz) was assessed both in terms of process kinetics and product quality. During the study, three different osmotic agents were used - erythritol, sorbitol, and sucrose - in an aqueous solution at a concentration of 50% (w/w). The essential kinetic parameters were analyzed - water loss and increase of dry matter during osmotic dehydration, and evolution of moisture content and temperature of samples during convective drying (drying curves). Product quality was assessed on the basis of color, water activity, and content of relevant bioactive components - polyphenols and carotenoids. It was found that the application of ultrasound during osmotic dehydration resulted in higher water loss and solid gain. This surely results from the phenomena occurring during the propagation of the elastic waves in the liquid medium (mainly related to cavitation) and enhancement of the mass transfer. The use of ultrasound during convective drying also had a positive effect on the kinetics of this process. In most cases, during the ultrasonically assisted drying operations, a significant increase in drying rate was observed, leading to a reduction in drying time. This may be due to the intensification of both heat and mass transfer as a result of the mechanical and thermal effects of ultrasound. The assessment of product quality showed that the use of sugar alcohols was a good alternative to sucrose, and ultrasound-assisted convective drying increased the retention of valuable carotenoids and polyphenols. Moreover, in all dried kiwifruit slices, water activity was below 0.6, which can guarantee the microbiological stability of the tested samples.

A drying technique using a combination of high power airborne ultrasound, microwaves and hot air was applied to investigate the effect of intermittent drying on the process kinetics and several quality indicators of red beetroot. An innovative hybrid dryer was used in drying experiments. Six sets of drying programs were carried out. Ultrasound and microwaves were applied in convective drying continuously (hybrid processes) or periodically (hybrid intermittent processes). The drying processes were assessed in terms of drying time, drying rate, and energy consumption. Moreover, the total color change, retention of natural dye (betanin), water activity, texture, and microstructure of dry product were examined. The drying kinetics was well-fitted with the use of the Midilli-Kucuk model. It was found that hybrid intermittent drying reduces the total drying time and energy consumption, enhances both the drying rate and product quality. Furthermore, it was demonstrated that the hybrid intermittent drying can serve as an alternative to traditional hot air drying that could produce a more porous, nice color, and crispy vegetable product.

• The whole cranberry fruit was impregnated. • Ultrasound intensified the mass transfer during vacuum impregnation. • The degree of impregnation depends on the stage in which ultrasound was used. • No negative impact of vacuum impregnation on quality was observed.