Magdalini Krokida

Abstract: Objectives: The degradation of maxillofacial prosthetic elastomers that occurs during physical weathering is usually responsible for the replacement of prostheses. In this study the mechanical behavior of 4 non-pigmented facial prosthetic elastomers, exposed to outdoor weathering for 1 year, was investigated. The hypothesis investigated was that irradiation time did not affect the properties measured. Methods: The samples were exposed to solar radiation for 1 year in Thessaloniki (Greece). Three different types of polydimethyl siloxane (PDMS) and chlorinated polyethylene (CPE) samples were tested in this study. Mechanical tests (compressive-tensile) were performed using a universal type testing machine. Hardness tests were evaluated using a durometer tester. Simple mathematical models were developed to correlate the measured properties with irradiation time. The stress-strain data of compression and tensile tests were modeled using parameters such as maximum stress (Ï�_max), maximum strain (ε_max), elasticity parameter (E), and non-linearity parameter (p), while the mathematical model used for hardness data involves initial hardness of materials (H₀). Results: Two of the silicone prosthetics (Elastomer 42, TechSil 25) seem to become harder and more brittle contrary to the other silicone (M 511) and chlorinated polyethylene (CPE) samples that become softer and more ductile. Duration of exposure increases these phenomena. Conclusion: The effect of irradiation time on the mechanical behavior was introduced through its effect on the models' parameters. The hypothesis was rejected since changes were observed in the model parameters. © 2009 Academy of Dental Materials.

Abstract: The effect of UVA-UVB irradiation on the mechanical properties of three different industrial types of polydimethylsiloxane and chlorinated polyethylene samples, used in maxillofacial prostheses, was investigated in this study. Mechanical properties and thermal analysis are commonly used to determine the structural changes and mechanical strength. An aging chamber was used in order to simulate the solar radiation and assess natural aging. Compression and tensile tests were conducted on a Zwick testing machine. Durometer Shore A hardness measurements were carried out in a CV digital Shore A durometer according to ASTM D 2240. Glass transition temperature was evaluated with a differential scanning calorimeter. Simple mathematical models were developed to correlate the measured properties with irradiation time. The effect of UVA-UVB irradiation on compressive behavior affected model parameters. Significant deterioration seems to occur due to irradiation in samples. © 2009 IOP Publishing Ltd.

Abstract: The seasonal moisture transfer to and from the walls of a building is the most important factor concerning the deteriorating effect of moisture in buildings envelopes. The presented simulator takes into account the (a) moisture transfer mechanisms to and from the building (capillary rise, drying, etc.), (b) wall configuration (materials and size), (c) construction materials properties, (d) seasonal region meteorological data (air temperature, humidity and velocity) and calculates the (a) seasonal wall moisture content along with the corresponding equilibrium moisture height, (b) capillary rising water flow rate, (c) wall drying flow rate, etc. The simulator has been developed in an Excel platform in a user-friendly environment and consists of four units: (a) process model, (b) problem solution algorithms, (c) database and (d) graphics interface. The proposed simulator is a powerful tool in decision-making concerning the building deteriorating evolution and the selection of appropriate protecting strategy, e.g., the plaster selection (material, size, replacing time), contributing to the sustainability of masonries. © 2005 Elsevier Ltd. All rights reserved.

Abstract: Additional types of silicone biopolymers are widely used in maxillofacial prosthetics. Therefore, the knowledge of the solar radiation's effect on their structural stability is highly important. Four different industrially synthesized biomaterials were examined, called Episil Europe 1, Europe 2, Europe 3 and Africa 3, which were exposed to solar radiation (UVA, UVB) for eight different time periods (from 8 to 168 h). Structural damages due to irradiation exposure were investigated by mechanical tests (compression) and differential scanning calorimetry (DSC) methods. Simple mathematical models were developed, containing parameters with physical meaning such as maximum stress (�_max), maximum strain (ε), elasticity parameter (E), and viscoelastic parameter (p), for the compression test, and melting temperature (T _m) and Enthalpy in melting point (Heat) for DSC. With increasing irradiation time their maximum stress and strain decreased significantly, and the materials lost their elasticity and molecular stability. A decrement in their melting points and heats was observed as irradiation time was increasing. Finally, experimental results demonstrated that solar radiation has a severe effect on the structural stability of the examined biomaterials. © 2007 Springer Science+Business Media, LLC.

Abstract: The presence of water in masonry is one of the main factors in deterioration. Capillary rise is the most usual mechanism of water penetration into building materials. In this study the kinetics of the capillary rise phenomenon was studied for various building materials: four stones, two bricks, and six plasters. A first-order kinetic model was proposed, in which the equilibrium moisture height derived from Darcy law. The capillary height time constant found to be strongly affected by the material characteristics. Moreover, the capillary height time constant can be predicted if the average pore radius of the materials is known. © 2004 Elsevier Inc. All rights reserved.

Abstract: Moisture is one of the most deteriorating factors of buildings. The deteriorating effect of moisture occurs mainly during the drying phase, and not in the wetting phase. Appropriate parameters of the drying kinetics are required for the building materials. Environmental factors, such as air temperature, air humidity, and air velocity affect drying. An experimental air dryer of controlled drying air conditions was used to investigate the drying performance of 4 stone materials, 2 bricks and 7 plasters. Drying kinetics was examined at 4 air temperatures, 6 air humidities, and 3 air velocities. A first-order kinetics model was obtained, in which the drying time constant was a function of the drying conditions, and the equilibrium material moisture content was described by the Oswin equation. The parameters of the proposed model were found to be affected strongly by the material and the drying air conditions. The results obtained are very useful in selecting the appropriate plaster to protect existing historic buildings.

Abstract: Moisture is one of the most deteriorating factors of buildings. The deteriorating effect of moisture occurs mainly during the drying phase, not in the wetting phase. Environmental factors, such as air temperature, air humidity, and air velocity affect drying. An experimental air dryer of controlled drying air conditions was used to investigate the drying performance of 4 stone materials, 2 bricks, and 6 plasters. Drying kinetics was examined at 3 air temperatures, 5 air humidities, and 3 air velocities. A first-order kinetics model was obtained in which the drying time constant was a function of the drying conditions, and the equilibrium material moisture content was described by the modified Oswin equation. The parameters of the proposed model were found to be affected strongly by the material characteristics. Copyright © 2005 Taylor & Francis, Inc.

Abstract: Moisture is one of the most deteriorating factors of buildings. The masonry moisture content depends on hygroscopic equilibrium between building materials and environment. Moisture sorption (adsorption and desorption) isotherms of 4 sandstones, 2 bricks, and 6 plasters were determined at 15, 25, and 35°C. A modified Oswin equation was used to predict experimental data for water activity in the range between 0.03 and 0.90. The hysteresis phenomenon between adsorption and desorption was observed for all the examined materials and classified at various isotherm types. A correlation of the hysteresis type with the microstructural characteristics of the materials was attempted. Copyright © 2005 Taylor & Francis, Inc.

Abstract: Dehydrated products readily take up moisture when immersed in a liquid medium, leading to significant changes in their thermophysical properties. The rehydration kinetics, the structural properties (apparent density, true density, specific volume and internal porosity), the viscoelastic behavior (compression tests), and the flavor losses were investigated during rehydration of various fruits and vegetables. The effect of temperature on the above properties was also investigated for some fruits and vegetables dehydrated by different drying methods (convective, vacuum, freeze, and osmotic drying). The results showed that the water temperature influences the rehydration kinetics and the equilibrium moisture content of the rehydrated. The structural properties of rehydrated foods appear to show a hysteresis compared to those measured during dehydration processes. The shrinkage that takes place during dehydration prevents rehydration and produces products with lower apparent density and higher porosity. Structural damages that occur during drying seem to affect also the viscoelastic behavior of the rehydrated foods and the hysteresis phenomenon is also observed at the textural properties, with a degree varying between the different drying methods. More specifically, freeze-dried materials present the highest hysteresis after rehydration, losing their elasticity and becoming more viscous. Osmotic pretreatment seems to help freeze-dried materials to keep their elastic nature probably due to solids gain. Air-and vacuum-dried materials showed the smallest hysteresis tense, keeping their viscoelastic characteristics during rehydration close to those of dried materials. Flavor losses seem to have lower rates during rehydration comparing to those observed during drying. The maximum retention of flavor has been observed in the rehydrated products after freeze-drying, and flavor levels are close to those observed for fresh boiled foods. Copyright © 2005 Taylor & Francis, Inc.

Abstract: The classical problem of heat recovery from convective dryer exhaust air is considered under modern easy-to-programming techniques towards effective decision making environment. Heat exchangers as well as heat pumps are considered versus variou dryer performance characteristics. © 2004 by Marcel Dekker, Inc.

Abstract: Olive-mill cake is one of the most widespread biomaterials for bioenergy exploitation in Greece. It is a sludge-type material, produced as byproduct from olive-mill extraction process. Its energy content is higher than 15 MJ/kg db and it can be used for direct burning, after drying. The drying process of olive cake is examined in the present paper. Drying kinetics data as well as the related thermophysical properties are obtained experimentally. The appropriate dryer model is proposed, validated and used to design an industrial rotary dryer. Economic analysis of the process is also discussed. A characteristic case study of an industrial rotary dryer for olive cake is included to illustrate the effectiveness of the proposed approach.

Abstract: Recently reported heat and mass transfer coefficient data in drying were retrieved from literature and were classified per dryer type and material. Most of the data were available in the form of empirical equations using dimensionless numbers. All available empirical equations were transformed in the form of Heat or Mass Transfer Factors versus Reynolds Number (j_H=aReⁿ or j_M=aReⁿ). In the case that more than one equations were reported for the same dryer type, a new similar equation was re-fitted to consolidate the existing literature equations. It is expected that the resulting equations are more representative and predict more accurately the heat and mass transfer coefficients. Equations for each dryer type are also proposed.

Abstract: Moisture loss and oil adsorption kinetics, structural properties (apparent density, true density, specific volume and internal porosity), color changes and viscoelastic behavior (compression tests, crispness) were investigated during deep fat frying of french fries. The effect of frying conditions (oil temperature, sample thickness and oil type), drying pretreatment and osmotic dehydration pretreatment on the above properties was also examined. The results showed that oil temperature and thickness of potato strips have a significant effect on oil uptake, moisture loss and color parameters of french fries, while the use of hydrogenated oil in the frying medium does not affect these properties. The porosity of french fries increases with oil temperature increases and sample thickness and it is higher for products fried with hydrogenated oil. Maximum stress and maximum strain increase during frying, while crispness of potato strips is higher for hydrogenated oil, and lower for refined oil. Air drying and osmotic pretreatment increase porosity of fried potatoes but decrease their oil and moisture content. A negative effect on color development with drying time was also observed. Pre-fry drying as well as osmotic pre-treatment increases the maximum stress and maximum strain of french fries during frying. Air drying pre-treatment increases the crispness of potato strips while osmotic pre-treatment does not affect it, with the exception of sugar solutions.

Abstract: A generic structural model to estimate the effective thermal conductivity of granular foods is proposed based on a distribution coefficient. It is assumed that granular food materials in bulk can be considered as a two-phase system containing granules and an interstitial air phase, and while the granules are also be considered as a two-phase system containing dry mater and water. Two different distribution factors are defined, one for granular materials in bulk and another for the granules. The proposed model was applied successfully to granular starch.

Abstract: Most of the research efforts in the field of fluidized dryer design, focus on the evaluation of the appropriate structural and operational process variables so that total annual plant cost involved is optimized. However, the increasing need for dehydrated products of the highest quality, imposes the development of new criteria that, together with cost components, determine the design rules for any drying process. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is represented by three parameters: redness, yellowness and lightness. These three parameters of a dried product should deviate from that of its original material as little as possible. In this case, fluidized bed dryer design is a complex multi-objective optimization problem, involving the color deviation and the unit cost of final product as an objective vector and as constraints described in process models. The mathematical model of the dryer was developed and the fundamental color deterioration laws and critical financial parameters were determined for the fluidized bed drying process. In this paper, non-preference multi-criteria optimization methods were used and the Pareto-optimal set of efficient solutions was evaluated. An example for drying of sliced potato was studied in detail to demonstrate the design procedure, the process performance as well as the effectiveness of the proposed approach. (C) 2000 Elsevier Science S.A. All rights reserved. Most of the research efforts in the field of fluidized dryer design, focus on the evaluation of the appropriate structural and operational process variables so that total annual plant cost involved is optimized. However, the increasing need for dehydrated products of the highest quality, imposes the development of new criteria that, together with cost components, determine the design rules for any drying process. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is represented by three parameters: redness, yellowness and lightness. These three parameters of a dried product should deviate from that of its original material as little as possible. In this case, fluidized bed dryer design is a complex multi-objective optimization problem, involving the color deviation and the unit cost of final product as an objective vector and as constraints described in process models. The mathematical model of the dryer was developed and the fundamental color deterioration laws and critical financial parameters were determined for the fluidized bed drying process. In this paper, non-preference multi-criteria optimization methods were used and the Pareto-optimal set of efficient solutions was evaluated. An example for drying of sliced potato was studied in detail to demonstrate the design procedure, the process performance as well as the effectiveness of the proposed approach.

Abstract: Drying related properties of apple are evaluated for various different drying methods (namely, convective, vacuum, microwave, osmotic and freeze drying). and their corresponding process conditions. The examined properties are drying kinetics, equilibrium material moisture content, density, porosity, color and viscoelastic characteristics. The effect of various process factors on these properties is described through particular mathematical models. The model parameters are estimated by fitting the corresponding model equations on a wide range of experimental data. Drying kinetics is greatly affected by the characteristic particle size and drying air temperature for convective drying, while for the case of microwave drying they are affected by the vacuum pressure and the emitted radiation power. Equilibrium material moisture content is affected by the temperature and the humidity of the surrounding air, while the osmotic pretreatment shifts the sorption isotherms to higher water activity levels. The quality properties examined, are significantly affected by the drying method. More specifically, osmotic dehydration decreases the porosity of the final product, while it prevents color deterioration and enhances the viscous nature of dehydrated apple. Freeze-dried apples develop the highest porosity, have the most elastic structure and the lowest rate of color deterioration. Drying related properties of apple are evaluated for various different drying methods (namely, convective, vacuum, microwave, osmotic and freeze drying), and their corresponding process conditions. The examined properties are drying kinetics, equilibrium material moisture content, density, porosity, color and viscoelastic characteristics. The effect of various process factors on these properties is described through particular mathematical models. The model parameters are estimated by fitting the corresponding model equations on a wide range of experimental data. Drying kinetics is greatly affected by the characteristic particle size and drying air temperature for convective drying, while for the case of microwave drying they are affected by the vacuum pressure and the emitted radiation power. Equilibrium material moisture content is affected by the temperature and the humidity of the surrounding air, while the osmotic pretreatment shifts the sorption isotherms to higher water activity levels. The quality properties examined, are significantly affected by the drying method. More specifically, osmotic dehydration decreases the porosity of the final product, while it prevents color deterioration and enchances the viscous nature of dehydrated apple. Freeze-dried apples develop the highest porosity, have the most elastic structure and the lowest rate of color deterioration.

Abstract: The effect of pretreatment on color of convective dried products (namely apple, banana, potato and carrot) was investigated. Five different types of pretreatment were taken into consideration; microwave, osmotic, sulfite, water blanching and steam blanching. Color characteristics were identified by measuring the color parameters (namely, Lightness (L), Redness (a) and Yellowness (b)) using a Hunter Lab chromatometer. The type of pretreatment was found to significantly affect the three color parameters. In addition, Redness (a) and Yellowness (b) were found to follow a first order kinetic model. Untreated dried materials showed an extensive browning, indicated by a significant drop of the L parameter and a corresponding increase of a, b parameters. Osmotically and microwave pretreated samples supressed browning compared to the untreated samples. In this case, lightness decreased slightly, while a, b increased slightly. Sulfite pretreatment prevented significantly color deterioration, while water and steam blanching also prevented enzymatic browning during convective drying. The effect of pretreatment on color of convective dried products (namely apple, banana, potato and carrot) was investigated. Five different types of pretreatment were taken into consideration; microwave, osmotic, sulfite, water blanching and steam blanching. Color characteristics were identified by measuring the color parameters (namely, Lightness (L), Redness (a) and Yellowness (b)) using a Hunter Lab chromatometer. The type of pretreatment was found to significantly affect the three color parameters. In addition, Redness (a) and Yellowness (b) were found to follow a first order kinetic model. Untreated dried materials showed an extensive browning, indicated by a significant drop of the L parameter and a corresponding increase of a, b parameters. Osmotically and microwave pretreated samples suppressed browning compared to the untreated samples. In this case, lightness decreased slightly, while a, b increased slightly. Sulfite pretreatment prevented significantly color deterioration, while water and steam blanching also prevented enzymatic browning during convective drying.

Abstract: The textural properties of apple, banana, carrot and potato were experimentally determined by uniaxial compressive tests of cylindrical specimens at a constant deformation rate of 5mm/min. Compression tests were performed, following air drying, at various moisture contents ranging from 0.2 to 6 kg/kg db. The tests were performed using a universal texture testing machine and simple mathematical equations were used to correlate the maximum experimental stress and the corresponding strain to the moisture content. It was shown that the maximum stress decreases as the moisture content decreases, until a critical moisture content of 1.8 kg water/kg dry solids. Further removal of water tends to increase the maximum stress. The maximum experimental strain was found to increase as water was removed. The stress-strain data of compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress (�(max)), the maximum strain (ε(max)), the elastic parameter (E) and the viscoelastic exponent (p). The effect of the moisture content on the compressive behavior of dried materials was introduced through its effect on the model parameters. The shift in compression behavior at 1.8 kg water/kg solids leads to the conclusion that there is an important change of structure at this moisture content. The textural properties of apple, banana, carrot and potato were experimentally determined by uniaxial compressive tests of cylindrical specimens at a constant deformation rate of 5mm/min. Compression tests were performed, following air drying, at various moisture contents ranging from 0.2 to 6 kg/kg db. The tests were performed using a universal texture testing machine and simple mathematical equations were used to correlate the maximum experimental stress and the corresponding strain to the moisture content. It was shown that the maximum stress decreases as the moisture content decreases, until a critical moisture content of 1.8 kg water/kg dry solids. Further removal of water tends to increase the maximum stress. The maximum experimental strain was found to increase as water removed. The stress-strain data of compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress (�_max), the maximum strain (ε_max), the elastic parameter (E) and the viscoelastic exponent (p). The effect of the moisture content on the compressive behavior of dried materials was introduced through its effect on the model parameters. The shift in compression behavior at 1.8 kg water/kg solids leads to the conclusion that there is an important change of structure at this moisture content.

Abstract: Design of fluidized bed dryers constitutes a mathematical programming problem involving the evaluation of appropriate structural and operational process variables so that total annual plant cost involved is optimized. The increasing need for dehydrated products of the highest quality, imposes the development of new criteria that, together with cost, determine the design rules for drying processes. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is determined as a three-parameter resultant, whose values for products undergone drying should deviate from the corresponding ones of natural products, as little as possible. In this case, product quality dryer design is a complex multi-objective optimization problem, involving the color deviation vector as an objective function and as constraints the ones deriving from the process mathematical model. The mathematical model of the dryer was developed and the fundamental color deterioration laws were determined for the drying process. Non-preference multi-criteria optimization methods were used and the Pareto-optimal set of efficient solutions was evaluated. An example covering the drying of sliced potato was included to demonstrate the performance of the design procedure, as well as the effectiveness of the proposed approach. Design of fluidized bed dryers constitutes a mathematical programming problem involving the evaluation of appropriate structural and operational process variables so that total annual plant cost involved is optimized. The increasing need for dehydrated products of the highest quality, imposes the development of new criteria that, together with cost, determine the design rules for drying processes. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is determined as a three-parameter resultant, whose values for products undergone drying should deviate from the corresponding ones of natural products, as little as possible. In this case, product quality dryer design is a complex multi-objective optimization problem, involving the color deviation vector as an objective function and as constraints the ones deriving from the process mathematical model. The mathematical model of the dryer was developed and the fundamental color deterioration laws were determined for the drying process. Non-preference multi-criteria optimization methods were used and the Pareto-optimal set of efficient solutions was evaluated. An example covering the drying of sliced potato was included to demonstrate the performance of the design procedure, as well as the effectiveness of the proposed approach.

Abstract: The rheological behavior of osmotically dehydrated apple and banana was examined under uniaxial compression and relaxation tests of cylindrical specimens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging from 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding strain were correlated to the moisture content using simple mathematical equations. It was shown that both parameters increase as water was removed and their values are significantly higher than the corresponding values for untreated air dried samples. The effect of moisture content on compressive behavior of osmotically dried materials was introduced through its effect on the four model parameters: the maximum stress (�(max)), the maximum strain (ε(max)), the elastic parameter (E) and the viscoelastic exponent (p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment increased the remaining force and it decreased the relaxation time of dehydrated samples. The osmotic dehydration and therefore the sugar gain tend to increase the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure. The rheological behavior of osmotically dehydrated apple and banana was examined under uniaxial compression and relaxation tests of cylindrical specimens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging from 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding strain were correlated to the moisture content using simple mathematical equations. It was shown that both parameters increase as water was removed and their values are significantly higher than the corresponding values for untreated air dried samples. The effect of moisture content on compressive behavior of osmotically dried materials was introduced through its effect on the four model parameters: the maximum stress (�_max), the maximum strain (ε_max), the elastic parameter (E) and the viscoelastic exponent (p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment increased the remaining force and it decreased the relaxation time of dehydrated samples. The osmotic dehydration and therefore the sugar gain tend to increase the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure.

Abstract: Color and sorption characteristics of osmotically treated and air dried apple and banana were studied during air drying at 70°C. The color parameters: Lightness (L), Redness (a) and Yellowness (b) were studied, using a Hunter Lab chromatometer. A first order kinetic model was fitted to the experimental data adequately for color parameters, while sorption data for treated and air dried products were fitted to the GAB model. Untreated fruits showed an extensive browning which was monitored by a significant drop of the lightness (L) and an increase of redness (a) and yellowness (b). Osmotically pretreated samples did not brown as much as the untreated samples and the lightness L decreased only slightly while a, b increased slightly. Osmotic pretreatment resulted in a shift in sorption isotherm for both fruits. Osmotic dehydration prevented color damages and decreased the sorption capacity of dehydrated products. Color and sorption characteristics of osmotically treated and air dried apple and banana were studied during air drying at 70°C. The color parameters: Lightness (L), Redness (a) and Yellowness (b) were studied, using a Hunter Lab chromatometer. A first order kinetic model was fitted to the experimental data adequately for color parameters, while sorption data for treated and air dried products were fitted to the GAB model. Untreated fruits showed an extensive browning which was monitored by a significant drop of the lightness (L) and an increase of redness (a) and yellowness (b). Osmotically pretreated samples did not brown as much as the untreated samples and the lightness L decreased only slightly while a, b increased slightly. Osmotic pretreatment resulted in a shift in sorption isotherm for both fruits. Osmotic dehydration prevented color damages and decreased the sorption capacity of dehydrated products.

Abstract: The effect of microwave and microwave-vacuum drying on some quality properties such as density, porosity, color and viscoelastic behavior of dehydrated apple, banana, carrot and potato was investigated. It is concluded that microwave drying and moreover microwave-vacuum drying tends to increase the product porosity and to prevent the color damages during drying. Microwave drying seems to decrease the maximum stress and maximum strain of dehydrated products, while it increases their elasticity and decreases their viscous nature. The effect of microwave and microwave-vacuum drying on some quality properties such as density, porosity, color and viscoelastic behavior of dehydrated apple, banana, carrot and potato was investigated. It is concluded that microwave drying and moreover microwave-vacuum drying tends to increase the product porosity and to prevent the color damages during drying. Microwave drying seems to decrease the maximum stress and maximum strain of dehydrated products, while it increases their elasticity and decreases their viscous nature.

Abstract: The operational performance of a paper production plant can be significantly improved by appropriate process interventions. This paper deals with energy aspects of the most important section of such a plant, the paper drying section. A significant intervention that would lower the operational cost of the drying process is proposed. It refers to the recompression of low pressure steam leaving the dryers by means of a thermocompressor that would utilize high pressure steam coming from the plant burners. The mathematical model of the thermocompressor is evaluated and its design procedure is formulated as a mathematical programming problem involving appropriate objective function and constraints. Copyright © 1999 by Marcel Dekker, Inc.

Abstract: Viscoelastic properties for carrot and potato were experimentally determined by stress relaxation tests using uniaxial compressive tests of cylindrical specimens at various deformation rates (5,10 and 20 cm/min). Stress relaxation tests were performed following conventional drying at 70°C and 15% humidity for various moisture contents ranging from 10 to 80%. The tests were performed using a Zwick testing machine and the stress relaxation data were modeled using a two-term Maxwell model. It was shown that the relaxation behavior of carrot and potato was not affected by deformation rate, but it was sensitive to moisture content. The remaining force and relaxation time of the elastic component were found to be depended on moisture content. Drying tends to decrease the remaining force and the elastic relaxation time of carrot and potato until a certain moisture content (1.7 and 1.9 kg/kg db). Further uptake of water tends to increase the remaining force and the elastic relaxation time for both materials. The viscous component of Maxwell model does not seem to be affected by moisture content. Copyright © 1998 by Marcel Dekker, Inc.

Abstract: The effect of drying conditions on color changes of apple, banana, carrot and potato during conventional and vacuum drying was investigated. The Hunder color scale parameters redness, yellowness and lightness were used to estimate color changes during vacuum and conventional drying at 50, 70 and 90°C. Air humidity during conventional drying was regulated at 15, 30 and 40%. Air temperature and humidity affected redness and yellowness, but not lightness. A first order kinetic model was fitted to experimental data adequately for both redness and yellowness. The rate of color deterioration was found to increase as temperature increased and air humidity decreased, for both drying methods and all the examined materials. Copyright © 1998 by Marcel Dekter, Inc.

Abstract: The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used in order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density �_w, the dry solid density �_s, the dry solid bulk density �_b0 and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.