The aim of this article is to present a modern method of convective drying intensification caused by the external action of ultrasound. The purpose of this study is to discover the mechanism of ultrasonic interaction between the solid skeleton and the moisture in pores. This knowledge may help to explain the enhancement of drying mechanism affected by ultrasound, particularly with respect to biological products like fruits and vegetables. The experimental kinetics tests were conducted in a hybrid dryer equipped with a new ultrasonic generator. The drying kinetics curves determined on the basis of drying model developed by the author were validated with those by the ones obtained from experimental tests. The intensification of heat and mass transfer processes due to ultrasound induced heating effect and vibration effect are analysed. The obtained results allow to state that ultrasound makes drying processes more effective and enhance the drying efficiency of biological products without significant elevation of their temperature.

This paper concerns convective drying of carrot preliminary dehydrated in aqueous solutions of three types of osmotic agents (sucrose, fructose, glucose). Three solution concentrations (20, 40 and 60%) were examined to work out efficient conditions of osmotic dewatering. The parameters such as water loss (WL), solid gain (SG) and osmotic drying rate (ODR) indicating the real efficiency of osmotic dehydrations (OD) were determined. The samples dehydrated with osmotic solutions underwent further convective drying to analyze influence of dehydration process on drying kinetics and final products quality. The quality of products was assessed on the basis of visual appearance of the samples and colorimetric measurements. It was found that osmotic pretreatment improves significantly the final product quality as the samples were less deformed and their colour was better preserved compared to samples, which had not been preliminarily dehydrated. Preliminary dehydration, however, did not influence significantly the overall drying time of the samples.

The aim of the present theme issue was to study the influence of ultrasound enhancement on the kinetics of osmotic dehydration and the effect of convective drying from the point of view of drying time and quality of dried products. Apple fruit was used as the experimental material. The kinetics of osmotic dehydration with (UAOD) and without (OD) ultrasound enhancement were examined for 40% fructose and sorbitol solutions. The effective dehydration time of osmotic process was determined. Preliminary dehydrated samples with OD and UAOD were next dried convectively with (CVUS) and without (CV) ultrasound assistance. The influence of OD and UAOD on the kinetics of CV and CVUS drying was analysed. The parameters of water activity and colour change were measured for the assessment of product quality after drying process.

This article presents experimental studies on drying kinetics and quality effects of red beetroot (Beta vulgaris L.) after convective drying with a preliminary osmotic pretreatment. The effects of the osmotic agent (NaCl) concentration and the osmotic bath time on the product colour and nutrient content preservation, the water activity, and rehydration ability after drying were analysed. Osmotic dehydration curves and Solid Gain (SG), Water Loss (WL), Weight Reduction (WR) were determined. It was proved that drying of beetroot with osmotic pretreatment contributes to shorter drying time, smaller water activity, higher retention of betanin, better colour preservation, and a greater degree of water resorption.

Evaluation of moisture absorption in foodstuffs such as black chickpea is an important stage for skinning and cropping practices. Water uptake process of black chickpea was discussed through normal soaking in four temperature levels of 20, 35, 50 and 65 °C for 18 hours, and then the hydration kinetics was predicted by Peleg’s model and finite difference strategy. Model results showed that with increasing soaking temperature from 20 to 65 °C, Peleg’s rate and Peleg’s capacity constant reduced from 13.368×10-2 to 5.664×10-2 and 9.231×10-3 to 9.138×10-3, respectively. Based on key results, a rise in the medium temperature caused an increase in the diffusion coefficient from 5.24×10-10 m2/s to 4.36×10-9 m2/s, as well. Modelling of moisture absorption of black chickpea was also performed employing finite difference strategy. Comparing the experimental results with those obtained from the analytical solution of the theoretical models revealed a good agreement between predicted and experimental data. Peleg’s model and finite difference technique revealed their predictive function the best at the temperature of 65 °C.

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.