Stefan Jan Kowalski

The aim of this article was to study the influence of drying parameters like air temperature and velocity on the effectiveness of convective (CV) and ultrasound-assisted convective drying. The apple samples were used as the testing material by drying at air temperatures of 313 and 323 K and three different air velocities: 2, 3, and 4 m/s. The drying kinetics, the quality of dried products, and the energy consumption in each drying process were analyzed. The model of drying elaborated by authors was performed to estimate numerically the effect of ultrasound enhancement of convective drying. It was found that application of ultrasound significantly accelerates the CV drying, mainly due to “vibration effect,” and the values of drying parameters like air temperature and velocity affect the drying effectiveness in a different way.

The aim of this work is to promote ultrasound as an energy source that is suitable for the enhancement of drying processes, and in particular of biological materials. The study aims at a more profound recognition of the interaction between ultrasonic (US) waves and the tissues of fruits and vegetables, which may contribute to an intensification of moisture removal during their drying. Absorption of acoustic energy causes heating and structural changes of the drying material due to a series of rapid material compressions and decompressions. The research hypothesis is based on the expectation that the ultrasound waves may enhance moisture removal from the fruits and vegetables during drying due to a special “heating effect,” “vibration effect,” and “synergistic effect.” Convective ultrasonic-assisted drying tests were conducted experimentally on a new hybrid dryer with ultrasonic equipment, and the effects of ultrasonic enhancement by drying are presented based on a drying model and assessed numerically.

This paper presents the results of experimental studies on convective drying of beetroot (Beta vulgaris L.) in non-stationary conditions. The effect of stepwise changing air temperature and pretreatment methods such as osmotic dehydration and blanching on the material quality were investigated. The main aim of the studies was to find a drying mode by which the product quality degradation is minimized. Therefore, except for drying kinetics, the total color change, water activity, retention of betanin, and ability to rehydration were examined. It was proved that drying of beetroots in intermittent conditions contributes to shorter drying time, higher retention of betanin, better color preservation, and less water activity.

This article examines the influence of apple osmotic dehydration (OD) on the kinetics of convective drying and the quality of the products obtained. Fresh apples (Malus domestica), variety Ligol, were used as the experimental material. Samples were first dehydrated in three osmotic agents (glucose, fructose, and sucrose), each at a concentration of 40%, for 30, 60, and 120 min, and then dried convectively. Efficiency of osmotic dewatering was assessed on the basis of water loss (WL) and solids gain (SG). The influence of OD on convective drying kinetics was analyzed by comparing the drying curves and the basic kinetic parameters (time and drying rate). Quality of products was assessed on the basis of colorimetric measurements and sensory evaluation. It was found that OD significantly improved the quality of dried products through better retention of the original color, shape, texture, aroma, and taste, but it did not remarkably affect the rate of convective drying.

The main aim of this paper is to mathematically describe and experimentally determine the self-fracturing of fluid-saturated porous materials during drying processes. The thermo-hydro-mechanical model of drying that is suitable for a mathematical description of these phenomena as well as the acoustic emission (AE) technique for their experimental detection are presented. Numerical solutions based on the thermo-hydro-mechanical drying model are used to analyze stress development and to formulate the crack criterion in dried material. The studies concern the convective drying of kaolin-clay material. Bar-shaped samples with a rectangular cross-section were used for the mechanical tests and cylindrically shaped samples of kaolin-clay were used for the drying tests. Good agreement of the theoretical predictions to the experimental data is shown.

This article presents a quality analysis of convectively dried fruits and vegetables with preliminary osmotic dehydration. Tests were carried out on banana fruit and red beetroot samples. Hypertonic solutions of fructose for the banana and those of sucrose for the red beetroot were used, each one at three different concentrations. After osmotic dewatering treatment conducted at different time intervals and after osmotic dehydration the samples were dried convectively until an equilibrium with the surroundings was attained. Osmotic dehydration and convective drying curves were determined. The values of Solids Gain (SG), Water Loss (WL) and Weight Reduction (WR) were measured and changes in the samples’ colour and shape after convective drying with and without osmotic pretreatment were assessed.

The mechanistic theory of drying is developed for the purpose of stress and strain analysis in wet clay-like materials subjected to convective drying. The aim of these studies is to show the difference in stress and strain patterns for cases when the material is considered to be elastic, viscoelastic, or elastic-plastic. Such an analysis is justified by the fact that many materials under drying, e.g., clay-like or woody materials, reveal different mechanical properties and safer, mostly inelastic deformations. In addition, inelastic models are able to describe some additional phenomena as, for example, stress reverse that often appears in dried materials. It is displayed through comparison of the experimental measurements with the numerical results obtained on the basis of inelastic constitutive equations applied to drying materials. These results show that drying models that include inelastic deformations are more realistic and present more natural stress and strain patterns than the purely elastic model. The analysis is presented on an example of kaolin-clay plate subjected to convective drying. The numerical results concerning inelastic deformations are visualized on plots and photographs taken from the samples tested experimentally.