Dominik Mierzwa

Permanent Strains in Clay-Like Material During Drying

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.

US-Assisted Convective Drying of Biological Materials

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.

Influence of Osmotic Pretreatment on Kinetics of Convective Drying and Quality of Apples

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.

Convective Drying in Combination with Microwave and IR Drying for Biological Materials

The main aim of this article is to find an optimal method for drying of biological materials. Eight schedules of combined (hybrid) drying processes were carried out experimentally with the use of different techniques like convective, microwave, and infrared radiation. The drying time and the quality of dried products were the points of interest. Beetroot (Beta vulgaris) was used as the experimental material. The kinetic curves of drying for the beetroot samples were determined and the dried samples were photographed for visual assessment of the sample quality. Based on the result, attempts were made to suggest suitable drying methods for the application of biological materials in terms of drying time and product quality.

Hybrid Drying of Red Bell Pepper: Energy and Quality Issues

The research presented in this article is aimed at elaboration of possibly best drying technology for the red bell pepper (Capsicum annuum L. var. annuum) with the use of convective (CV), microwave (MW), and infrared (IR) heat sources applied separately and in different combinations. Six drying schedules were arranged and tested experimentally to find the best drying kinetics, at which the best quality of the products is attained by minimum energy consumption. Quality was judged through colorimetric measurements with the use of Konica Minolta CR400 colorimeter, and through sensory assessment. The energy consumption was measured with the use of electrical network analyzer MPR53 s (Entes). As expected, the purely convective drying was proved to be a long-lasting and a high-energy-consuming process as well as negatively affecting the quality of dried products. A combination of several drying technologies positively influenced both the time of drying and the product quality as well as the energy consumption.

Ultrasound-assisted convective drying of apples at different process conditions

ABSTRACT 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.

Hybrid and Non-stationary Drying—Process Effectiveness and Products Quality

During last decades a lot of new drying techniques have been developed. Some of them are focused on breaking the limits of convective drying usually by applying intermittent conditions or utilization of few drying techniques in one process (hybrid drying). The purpose of the chapter is to discuss these new opportunities. Hence the convective non-stationary drying and various hybrid drying techniques (convective–microwave, convective–microwave–infrared, convective–microwave–ultrasonic and microwave-vacuum drying) are discussed. Many examples are provided in this chapter to illustrate the impact of the applied drying conditions and techniques on time consumption, process energy consumption, and on the quality of the product obtained. In particular, the drying of kaolin clay, oak, pine and walnut wood, apple, carrots, kale, potatoes, raspberries and red pepper are presented. The results of the studies indicate that both variable drying conditions and hybrid techniques may result in improved drying kinetics, reduced process energy consumption, and increased product quality.

Hybrid drying of carrot preliminary processed with ultrasonically assisted osmotic dehydration

In this paper the kinetics of osmotic dehydration of carrot and the influence of this pretreatment on the post-drying processes and the quality of obtained products are analysed. Osmotic dehydration was carried out in the aqueous fructose solution in two different ways: with and without ultrasound assistance. In the first part of the research, the kinetics of osmotic dehydration was analysed on the basis of osmotic dewatering rate, water loss and solid gain. Next, the effective time of dehydration was determined and in the second part of research samples were initially dehydrated for 30 min and dried. Five different procedures of drying were established on the grounds of convective method enhanced with microwave and infrared radiation. The influence of osmotic dehydration on the drying kinetics and final product quality was analysed. It was found that it did not influence the drying kinetics significantly but positively affected the final product quality. Negligible influence on the drying kinetics was attributed to solid uptake, which may block the pores, hindering heat and mass transfer. It was also concluded that the application of microwave and/or infrared radiation during convective drying significantly influenced the kinetics of the final stage of drying. A proper combination of aforementioned techniques of hybrid drying allows reducing the drying time. Differences between the particular dehydration methods and drying schedules were discussed.