COLLAPSE OF STRUCTURE DURING DRYING OF CELERY

Collapse of structure of foodstuffs during air drying affects quality. In many materials the soluble components, mainly sugars, are an important part of the tissue in which case collapse may be related to their glass transition temperature (T_g). It has been speculated that collapse occurs at a temperature (T_c) related to, but greater than, T_g. Plant tissues with high moisture contents, such as celery, have low T_gS. Therefore considerable collapse is expected at drying temperatures.

The aim of this study was to determine how air drying temperature affected the quantity characteristics of the tissue. Celery, air dried at temperatures between 5 and 80°C, was examined for volumetric shrinkage, rehydration characteristics and porosity changes. significant shrinkage occurred at all drying conditions. At low water content collapse was limited, probably due to a higher collapse temperature. porosity development was insignificant during drying until the sample was very dry. Lower air-drying temperatures gave a product with improved quality characteristics.     

STRUCTURE FORMATION OF COATED FILMS WITH DISPERSED PIGMENTS DURING DRYING

ABSTRACT

In the drying of coated films with dispersed pigments, such as floppy disks, the structure of the film is formed during the drying process and depends on the drying condition. It is important to understand the structure formation during drying for the design of the dryer and die better quality of the product. We measured die drying characteristics of the film and determined the structure of dried film experimentally. A qualitative model for the structure formation during drying of the coated film is suggested.     

SHRINKAGE OF STRAWBERRIES DURING MICROWAVE DRYING

ABSTRACT

Strawberries were pretreated with ethyl oleate and sodium hydroxide solution to make the skin transparent to moisture diffusion. The treated berries were dried with convective air at 45°C and microwaves at two power levels of 0.1 and 0.2 W/g. During the drying process change in shrinkage ratio with respect to moisture ratio was studied. Also, the change in equivalent diameter of strawberries in correlation with moisture ratio was investigated. During drying the variation in surface and centre temperatures with moisture content and time was studied. It was observed that there is a direct relation between the shrinkage and moisture ratio. The equivalent diameter of the berry has a logarithmic function with respect to the moisture ratio.     

SHRINKAGE OF POROUS MATERIALS DURING MICROWAVE DRYING

The dissipation of electromagnetic energy inside a material creates a thermal imbalance state producing some reactions different from those observed through slow classic drying processes. Important drying rates obtained by microwave application can be understood by taking into account induced pressure gradients which greatly accelerate the thermomigration mechanism and thereby modify the physical properties of the product. The shrinkage of porous materials during the drying stage is very sensitive to the internal vapour pressure.The quality of such products depends on the shrinkage behaviour, and it is therefore interesting to study this phenomenon in order to control the characteristics of the product. This study is an experimental approach leading to a theoretical model describing the shrinkage mechanism. This model is developed from results obtained by a computer controlled measurement system allowing to regulate the drying kinetics.     

PREDICTION OF DEGRADING REACTIONS DURING DRYING OF SOLID FOODSTUFFS.

Abstract

Nutrient loss during drying of a shrinking product was determined. A diffusion model that takes into account the shrinkage of the product in the mass balance equation was used, together with a simplified energy balance with no thermal gradients, and convective boundary conditions for both balances. Nutrient loss prediction was obtained, for first and second order kinetics. Finally, a new methodology was proposed, to determine the activation energy and the preexponential of the deterioration reaction, even when the differential equations that rules the drying process are unknown, and only the thermal history of the product is available. This approach is also valid for sterilization calculations, if temperature gradients can be neglected, i.e. when Biot number for heat transfer is small.     

SHRINKAGE OF DATES (Phoenix Dactilyfera L.) DURING DRYING

In this paper the shrinkage of Dates (Phoenix dactilyfera L.) during drying was observed. Samples were dried in a convective tray dryer under three different conditions (60°C-11.1%RH, 70°C-7.2%RH and 80°C-4.7%RH) and three different air velocities (0.5m/s, l.0m/s and 1.5m/s). Sample shrinkage was directly (using a caliper) and indirectly measured (snapshot area measured). The indirect manner shows a higher precision. The geometry applied in the shrinkage solution proved that the ellipsoidal geometry of the dates, may be successfully approached to a cylinder. Three mathematical models developed by Suzuki et al. (1976) were applied to correlate the dates surface to its moisture content. The core model presented the best fit to the experimental data.     

SHRINKAGE OF DATES (Phoenix Dactilyfera L.) DURING DRYING

ABSTRACT

In this paper the shrinkage of Dates (Phoenix dactilyfera L.) during drying was observed. Samples were dried in a convective tray dryer under three different conditions (60°C-11.1%RH, 70°C-7.2%RH and 80°C-4.7%RH) and three different air velocities (0.5m/s, l.0m/s and 1.5m/s). Sample shrinkage was directly (using a caliper) and indirectly measured (snapshot area measured). The indirect manner shows a higher precision. The geometry applied in the shrinkage solution proved that the ellipsoidal geometry of the dates, may be successfully approached to a cylinder. Three mathematical models developed by Suzuki et al. (1976) were applied to correlate the dates surface to its moisture content. The core model presented the best fit to the experimental data.     

CONTRACTION CHARAClFRlSTlCS OF MOLDED CERAMICS DURING DRYING

ABSTRACT

The characteristics of drvine and contraction (shrinkage) of molded ceramics were expenmentally studied to clarify the effects of drying conditions on contraction behaviors or deformation during drying processes. Mikawa-Shanshu clay molded into spheres or slabs was used as a sample which underwent convective drying by hot air.

Three periods of preheating, constanl rate and falling rate existed, and the contraction was observed mainly in the preheating and constanl rate periods for all samples. It is also observed that the behavior of ihe contraction of molded clay is inflienced significantly by drying conditions. The higher drying rate makes the contraction of clay more remarkable in the preheating and constant rate periods. This behavior could not be explained from the difference of the moisture content distribution among runs. However, it is suggested that the inside parts of molded ceramics may be subjected to the strong compressional stresses and the strain-stress in the sample would Influence the contraction behavior when he gradient of moisture content distribution becomes steeper with increase in the drying rate.     

EFFECT OF SHAPE ON POTATO AND CAULIFLOWER SHRINKAGE DURING DRYING

ABSTRACT

Shrinkage of potato cubes, parallelepipeds and cylinders was addressed during drying. A basic methodology by using image analysis was set-up in order to measure and monitor geometrical changes along the drying process. Both measurements by image analysis and directly with a calliper were carried out. Results from both methods showed a good agreement with no bias. Particle shape was found to influence shrinkage in a different way according to the axis. In fact the larger dimension shrinks less than the shorter one. This could be linked to the influence of the core drying in shrinkage. Experiments were also carried out for cauliflower stems. The difference in shrinkage between directions was greater for cauliflowers, due probably to the effect of alignment of fibers along the axis. Good agreement with existing data in the literature for specific volume was found.     

EFFECT OF SHAPE ON POTATO AND CAULIFLOWER SHRINKAGE DURING DRYING

Shrinkage of potato cubes, parallelepipeds and cylinders was addressed during drying. A basic methodology by using image analysis was set-up in order to measure and monitor geometrical changes along the drying process. Both measurements by image analysis and directly with a calliper were carried out. Results from both methods showed a good agreement with no bias. Particle shape was found to influence shrinkage in a different way according to the axis. In fact the larger dimension shrinks less than the shorter one. This could be linked to the influence of the core drying in shrinkage. Experiments were also carried out for cauliflower stems. The difference in shrinkage between directions was greater for cauliflowers, due probably to the effect of alignment of fibers along the axis. Good agreement with existing data in the literature for specific volume was found.     

STRUCTURE FORMATION OF COATED FILMS WITH DISPERSED PIGMENTS DURING DRYING

In the drying of coated films with dispersed pigments, such as floppy disks, the structure of the film is formed during the drying process and depends on the drying condition. It is important to understand the structure formation during drying for the design of the dryer and die better quality of the product. We measured die drying characteristics of the film and determined the structure of dried film experimentally. A qualitative model for the structure formation during drying of the coated film is suggested.     

MODELLING OF STRESS DEVELOPMENT DURING DRYING AND RELIEF DURING STEAMING IN PINUS RADIATA LUMBER

ABSTRACT

A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified.

Drying experiments were performed to dry Pinus radiata sap wood boards of 100×40×590 mm in a tunnel dryer. In the experiment, wood temperature, moisture content gradient and residual stress through board thickness were measured. The drying cycle included HT drying, cooling and final steam conditioning. The measured stress patterns were in agreement with the model predictions. However, more accurate calculations will be made once the detailed experimental data for radiata pine wood mechanical and rheological properties are available.     

EFFECT OF CREEP AND MECHANO-SORPTIVE EFFECT ON STRESS DEVELOPMENT DURING DRYING

ABSTRACT

Constitutive equations to quantify wood deformation under combined mechanical loading and moisture content change (1] were coupled with the moisture distribution developed during drying to predict stress and strain in 50 by 190-mm Douglas-fir heartwood lumber.

Two combinations of temperature and relative humidity were used to dry the wood. The overall board shrinkage and the immediately released and set strains were measured as a function of time. Those strains were compared with analytic results, which showed good agreement.

The roles that four strain components played in the development of stress-both at board surface and center were compared for different drying conditions. The significance of creep and mechano-sorptive strain in relieving the stress was demonstrated by varying the model parameters.     

DRYING KINETICS FOR SOME FRUITS: PREDICTING OF POROSITY AND COLOR DURING DEHYDRATION.

ABSTRACT

Drying kinetics of four fruits (prune, quince, fig and strawberry) were studied by using a simple mass transfer mathematical model involving a characteristic parameter (K) as a function of process variables. The model was tested with data produced in a laboratory air dryer, using non-linear regression analysis. The investigation involved three values of sample thickness (5, 10, 15mm) and three different air temperatures (50, 60, 70?°C).

The parameters of the model were found to be greatly affected by sample thickness and air temperature. The effect of moisture content on the porosity of three fruits (namely avocado, prune and strawberry) was also investigated. A simple mathematical model was used to correlate porosity with moisture content. It was found that porosity increased with decreasing moisture content.

Samples of three fruits (avocado, prune and strawberry) were investigated to estimate color changes during conventional drying at 70?°C. A first order kinetic model was fitted to experimental data with great success. It is found that the color of avocado and strawberry change while the color of prune remains the same.

Furthermore, the water sorption isotherms of three fruits (avocado, prune and strawberry) at 25° temperature were determined experimentally and the parameters at the GAB equation were evaluated by means of non-linear regression analysis. The use of the above equation produced a very good fit.     

CONVECTIVE DRYING OF PAPER CALCULATED WITH A NEW MODEL OF THE PAPER STRUCTURE

Paper drying models are based on the assumption of a rigid solid matrix. Because of the interaction of water molecules and paper fibre the dimensions of the sheet are changing, especially the thickness. These changes influence the moisture transport within the material. A model of the paper structure has been developed taking into account different kinds of pores in the paper.

By the assistance of this model the dependence between water content and thickness or transport parameters as permeability and thermal conductivity can be predicted. The only informations which are needed are sorption isotherms, apparent paper density and permeability of the single phase flow. Further informations as the pore size distribution are useful but not necessary.

The transport parameters are generated in the suggested way for wood pulp. It is shown that in the case of convective drying, the agreement between experimental results and model simulations is good.     

EXPERIMENTAL MEASUREMENT OF TEMPERATURE AND MOISTURE PROFILES DURING APPLE DRYING

An experimental system has been designed and constructed to investigate the drying rate, temperature and moisture profiles developed within food materials during forced convective air drying. The moisture profile was determined by employing the technique of gamma ray densitometry, and embedded thermocouples were used to sense temperature at different locations. The overall drying rate was measured using an electronic balance. A microcomputer and a computer front end were implemented for data acquisition and experimental control. Sample temperature and moisture profiles measured during drying are presented as a function of both drying time and the distance to the surface of the specimen. Both profiles described the drying phenomena inside the sample. The evaporation front was observed in a narrow region of the apple specimen, this front moved into the sample as drying progressed. The amount of apple shrinkage was determined to be linearly related to the distance which the evaporation front had recededfrom the initial surface.     

EXPERIMENTAL MEASUREMENT OF TEMPERATURE AND MOISTURE PROFILES DURING APPLE DRYING

An experimental system has been designed and constructed to investigate the drying rate, temperature and moisture profiles developed within food materials during forced convective air drying. The moisture profile was determined by employing the technique of gamma ray densitometry, and embedded thermocouples were used to sense temperature at different locations. The overall drying rate was measured using an electronic balance. A microcomputer and a computer front end were implemented for data acquisition and experimental control. Sample temperature and moisture profiles measured during drying are presented as a function of both drying time and the distance to the surface of the specimen. Both profiles described the drying phenomena inside the sample. The evaporation front was observed in a narrow region of the apple specimen, this front moved into the sample as drying progressed. The amount of apple shrinkage was determined to be linearly related to the distance which the evaporation front had recededfrom the initial surface.     

污水污泥干燥过程中表观形态变化及水分析出特性

The drying mechanism is important to the drying technology of sewage sludge. Using dynamic electronic balance and image acquisition technique, the paper studies the change of surface configuration and characteristics of moisture transference for three kinds of sewage sludge during drying. In the meantime, the paper also analyses the influence of sludge shape on moisture transference and the change of sludge apparent volume and porosity in the course of drying.     

EVOLUTION LIGNITE MESOPORE STRUCTURE DURING DRYING. EFFECT OF TEMPERATURE AND HEATING TIME

The knowledge of the intrinsic pore structure of coals is significant in elucidating the kinetics of mass transport and chemical reaction that leads to design of more efficient coal combustion and conversion equipment. The results of pore structure studies of Greek lignite are reported in this work. Isothermal drying of Greek lignite samples, under vacuum, caused mesopore structure evolution despite the severe (∼50%) particle size contraction due to heating. Mesopore volume and surface area were increased as the drying temperature was raised to 200°C while further drying up to 250°C caused a mesopore volume and surface area decrease. Lignite drying at 100°C for up to 3 h resulted in a monotonic increase of the mesopore structure properties while heating for a longer period i.e., 6 h, despite a slight increase of weight loss, caused pore volume and surface area reduction. Nitrogen sorption (77 K) hysteresis data obtained for partially dried samples have been processed to deduce BET surface area and pore size distributions (PSD) by using both the Roberts and a new method based on a Corrugated Pore Structure Model (CPSM-nitrogen) methods. The latter method was applied successfully in hysteresis loop simulations and predicted pore surface areas consistent with the respective BET values. Bimodal PSD have been detected with one peak at 3 nm and the second at 20 nm while surface area varied over the range 2.98-5.30 m²/g. Dry Greek lignite has shown a higher mesopore volume than that of several American and Canadian coals of varying rank. Mesopore volume distribution of dry Greek lignite, obtained from nitrogen sorption data, agree well with those deduced from mercury penetration data corrected for coal compressibility.     

DETERMINATION OF MOISTURE DIFFUSIVITY AND BEHAVIOR OF TOMATO CONCENTRATE DROPLETS DURING DRYING IN AIR

ABSTRACT

The drying mechanism and diffusion coefficient of water in spherical droplets (1.73 – 2.08 mm diameter) of tomato concentrates were successfully interpreted and modelled by using Fick's law. Solids content of the initial concentrate (5–15% w/w), and drying temperature (60^° – 100^° C) were varied but the drying air was kept at constant velocity and humidity.

The effective moisture diffusivity was estimated from the drying rate curves and expressed by an Arrhenius relation. Further, it was observed that case hardening has a large effect on the diffusion process causing the effective diffusional distance and the rate of moisture accumulation in the hardened crust to vary with the moisture content, according to a sorption controlled mechanism.