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 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.

The aim of this work was to study the influence of microwave power variation during microwave–vacuum drying on both drying time and dried material quality. Red beetroot and carrot were chosen as the material. The quality of the dried material was described by its color change and β-carotene loss (for carrot) due to drying. The first series of experiments consisted of drying under constant microwave power. Only the lowest microwave power gave an undamaged product. During the second series of experiments, the microwave power was changed in time intervals. However, the use of interval conditions either had a small influence on the process time or did not provide satisfactory quality of the dried samples. During the third series of experiments, initial drying was performed with higher microwave power and then the process was continued with the lowest applied power. This method shortened the process time and gave the best quality of material.

One of the characteristic features of clay-like materials is the change of their mechanical properties during drying as the moisture content is changed. At high moisture contents they are viscoelastic and then become elastic-plastic and finally brittle-elastic or even stiff. Elastic strains are reversible but viscous and plastic strains are not reversible. The permanent strains are usually undesirable in dried products because they cause warping. In brittle materials, on the other hand, microcracks and cracks are generated. The number of cracks depends on the material properties, sample shape, and drying conditions. The aim of this work is to examine experimentally the permanent strains during drying. The samples used in experiments were made of different materials (two types of clay and kaolin) and were dried convectively. Constant but nonsymmetric drying conditions were applied. The strains of the samples were measured online using a video camera.

During drying or desiccation of clay-type materials, some stresses appear. Usually they are compressional inside of the material and tensional close to the surface. If the tensional stresses exceed the material strength, the clay cracks. This article is devoted to the modeling and numerical simulation of this phenomenon. The proposed model consists of two parts. The mass transfer is described by a simple diffusion equation together with convective boundary conditions. In the mechanical part it is assumed that the clay is composed of small particles linked together by cohesive forces. These forces are described with the use of mesh models. Two models are proposed: elastic (mesh consists of springs) and viscoelastic one (mesh consists of Maxwell elements). Four types of clays were tested experimentally to obtain the model parameters. The tested materials were selected with respect to different mineralogical compositions that determine the water-bonding ability. Simulations of the convective drying of bricks made of these clays were performed. It was shown that the degree of cracking depends on the quartz content of the clay. The obtained results were compared with experimental ones and good agreement between simulations and experiment was obtained. Additionally, the inner forces caused by drying are analyzed and discussed in this work.