SHRINKAGE OF APPLE DISKS DURING DRYING BY WARM AIR CONVECTION AND FREEZE DRYING

Volumetric and thickness shrinkage evaluated by direct measurement and n-heptane displacement were determined during convective and freeze drying of Golden delicious apples. For convective drying, the influence of blanching and diameter/thickness ratio of the apple disks used were analysed at different levels of moisture content under constant conditions. It was found that shrinkage of dried samples, both by convection and by freeze-drying, is anisotropic to a level which depends on sample geometry (ratio diameter/thickness) used. Blanching did not affect shrinkage results.

Based on results obtained a new model to predict bulk density of materials during drying is proposed, showing a better fit to experimental data than previous models reported in the literature. This model was further used to predict changes in apple porosity during drying.     

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.     

DENSITIES,SHRINKAGE AND POROSITY OF SOME VEGETABLES DURING AIR DRYING

Abstract

Bulk density, particle density, shrinkage and porosity were experimentally determined at various moisture content during air drying for apple, carrot and potato cubes. A simple mathematical model was used to predict the above properties versus material moisture content. Four parameters were incorporated in the model: enclosed water density, dry solids' density, bulk density of dry solids, and volume-shrinkage coefficient. The model was fitted to experimental data satisfactorily, and the parameters were estimated. The influence of varying drying conditions was also investigated.     

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.     

EVALUATION AND MODELING OF TWO-STAGE OSMO-CONVECTIVE DRYING OF APPLE SLICES

ABSTRACT

Two-stage drying kinetics of cylindrical pieces of apples were evaluated by subjecting test samples first to various osmotic treatments and then to convective air drying to complete the drying process. Osmotic drying was carried out with cut apple cylinders of three different sizes (12, 17 and 20 mm diameter), all with a length to diameter ratio of 1 : 1, in a well agitated large tank containing the osmotic solution at the desired temperature. Solution to fruit volume ratio was kept greater than 60. After the osmotic treatment, apple slices were further dried in a cabinet drier at an average temperature 58°C. A central composite rotatable design (CCRD) with five levels of sucrose concentrations (34–63°Brix) and five temperatures (34–66°C) was used for osmotic treatment. Half-drying time and solids gain time were used as measures of rate of drying and associated diffusion coefficients for moisture loss and solids gain were evaluated. Half-drying time decreased with an increase in temperature or concentration, or a decrease in sample size. Diffusion coefficients were lower for smaller samples, and were higher for migration of moisture as compared to solids. For a given level of moisture removal, air drying times were shorter than osmotic drying times. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.     

DRYING RELATED PROPERTIES OF APPLE

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 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.     

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.     

SNIPPETS ON DRYING

Abstract:

Potato v. Irga was subjected to blanching, and thereafter was dried by convection, puff-drying, and freeze-drying. Microstructure of raw, blanched, and dried tissue was analyzed under the light microscope using computer image analysis. It was found that the tissue of the investigated variety is built up from cells much smaller than those described in literature for other cultivars. Blanching caused starch gelatinization and increase of cross-sectional area of cells. There was no evidence of broken cell walls. Convective drying resulted in cell shrinkage and some breaking of cell walls. It was estimated that some 12% of cells lost integrity during drying. Limited disintegration of the tissue caused by convective drying is attributed to small size of cells, large contact area, and high cohesive forces between cells. Puff-drying damaged the tissue much more than the convective drying. The most devastating to tissue microstructure was freeze-drying, probably freezing per se.     

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.     

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.     

A NON DESTRUCTIVE METHOD FOR QUANTIFYING THE WOOD DRYING QUALITY USED TO DETERMINE THE INTRA AND INTER SPECIES VARIABILITY

ABSTRACT

A non destructive method for surface strain measurements is proposed to quantify the wood drying quality during convective drying. This method uses a visible laser scan micrometer intercepting needles maintained vertical at the board surface with a special device, and then gives shrinkage values at several surface points. The results analysis is related to heal and mass transfer phenomena.

Experiments were made on softwoods and hardwoods either with superheated steam or with moist air. Results show that stages of shrinkage agree with classical periods of transfer. Besides, shrinkage results from a compromise between global shrinkage of the board section and local effects bounded with drying conditions. We define two criteria for the drying quality. One compares experimental average shrinkage and free shrinkage, and the other quantifies the differences of shrinkage values between several surface points. Both criteria have to be associated in order to analyse the species behavior in terms of checking during the second drying period.     

SOME ASPECTS OF MODELLING CONVECTIVE CORN DRYING

ABSTRACT

Specific heat claim of dewatering at large scale convective corn driers could be reduced to about 3000 kf/kg by means of an accurate modelling of the drying process. The main inadequecy of the models used in these days is the ignorance of change caused by interactions between some process parameters and material features during drying process.

In this study dehydration of seven Pioneer type hybrids was tested at model and large scale driers. By an empirical resolution of specific heat and bulk density functions of shelled corn furthermore by establishing a drying speed equation depending on the main grain- and process-features an appropriate result was provided.     

AIR DRYING OF ONION: SOME THEORETICAL CONSIDERATIONS

Abstract

The equation for the air drying of solids during a constant drying-rate period was modified to formulate a new model for describing the initial phase of onion drying, with volume shrinkage of dried particle taken into account. The model was fitted to experimental data satisfactory, and the parameters were estimated. It was shown that drying shrinkage of the onion particle, caused that initial phase of the falling drying-rate period is controlled by the external water transfer resistance. It was recommended that the Neuman-type, rather than Dirichlet-type, boundary conditions should be used for calculation of the effective diffusivity during the first phase of the falling rate period of drying of the onion.     

DRYING KINETICS OF A MULTICOMPONENT MIXTURE OF ORGANIC SOLVENTS

ABSTRACT

Drying kinetics of volatile organic solvents have been examined during the drying process of a pharmaceutical coating containing a multicomponent mixture of ethyl acetate, n-heptane, propanol-2, and toluene. A complete set of experiments was performed in two drying apparatuses, a laboratory air-dryer and an oven dryer, for a wide range of drying temperature, air velocity, initial coating thickness and drying time. An empirical kinetic model is used to predict the concentration of each organic solvent in the mixture during the drying process. The results show that both drying conditions and sample characteristics affect significantly the drying rate of solvents.     

FRUIT AND VEGETABLE POWDERS PRODUCTION TECHNOLOGY ON THE BASES OF SPRAY AND CONVECTIVE DRYING METHODS

ABSTRACT

The peculiarities of fruit and vegetable powders production technology based on spray & convective drying methods are shown.

The experimental data related to the drying kinetics of apple juice droplets as well as the production flow diagrams and the performance of a spray dryer intended for the treatment of hygroscopic pasty products are presented.     

Computer vision for bulk volume estimation of apple slices during drying

The accuracy of imaging for bulk volume estimation of apple slices in the process of drying at temperatures from 40 to 80°C was investigated and compared with physical caliper measurements. The initial hypothesis was to estimate bulk volume of cylindrically shaped apple slices from diameter and thickness imaging. Imaging of diameter showed strong agreement with caliper measurements throughout the entire drying process, however imaging of thickness was not accurate due to the irregular shrinkage and bending phenomena. A linear model reflecting the relationship between diameter and bulk volume was developed and validated at temperatures of 40, 60, and 80°C. It showed good correlation between imaging diameter and bulk volume changes in the range of moisture contents from 9.1 to 0.25?g/g. The relative percentage error of the prediction model was approximately 6.45%, which suggests that imaging can be used as a robust tool for bulk volume estimation of apple slices during convective drying.     

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.     

CHANGES OF RHEOLOGICAL PROPERTIES OF APPLE TISSUE UNDERGOING CONVECTIVE DRYING

ABSTRACT

The aim of this work was to investigate rheological properties of apple cubes undergoing convective drying. Compression-relaxation test was used to follow changes of properties under investigation.

Raw apple appeared to be very heterogeneous material from the rheological point of view. The relationship between stress and strain was concave downwards and the concavity increased until water content of 2·5 g/g d.m. was reached. Then the relationship begun to straighten and for dry material developed stress was linearly dependent on strain. The resistance of deformation decreased with decreasing water content. Analysis of relaxation showed that the rate of relaxation increased with decreasing water content and the unrelaxed stress was smaller the lower was the water content.

Apple cubes undergoing drying are pictured as material composed of three compartments with different rheological properties. The outer layer is dry, inelastic and difficult to deform. Underneath is moist material with no turgor and relatively easy to compress, and the core with properties of raw apples. During drying the second layer grows in expense of the core and porous structure with many voids filled with air is formed. Share of each compartment in the volume of apple cube undergoing drying affects average rheological properties of the material.     

SPRAY DRYING HANDBOOK

ABSTRACT

A mathematical model is presented to predict the drying rates and energy consumption for the process of impingement steam drying of paper during the constant-rate drying period. Preliminary experiments have verified the validity of the simulation model. The predicted results show consumption may be as low as 640 kJ/kg water evaporated while hot air Yankee dryers typically consume 5020 to 7100 kJ/kg water evaporated     

HYGROTHERMAL AND MECHANICAL BEHAVIOUR OF ABRASIVE AGGLOMERATE. APPLICATION TO RECTIFICATION WHEELS DRYING.

ABSTRACT

The distribution of the water content and temperature have been modelised and numerically investigated within the body of unbaked rectification wheels during convective drying. The obtained profiles have been applied to mechanical stress distribution simulations, fitting process parameters in order to avoid crack formation. The thermophysical, cinetical and mechanical properties of the abrasive agglomerate were determined experimentally.

The transport phenomena within the medium have been described by a classical water vapour diffusion and thermal conduction model with convective boundary conditions. The water vapour diffusivity and thermal conductivity were obtained respectively by water content profile analysis during 1-D isothermal diffusional water vapor transfer and by the flash method. The heat and mass transfer coefficients at the boundaries were calculated on the basis of [itterature correlations. The set of coupled non-linear differential equations was discretized in the space domain by the finite-volume method and integrated in the time domain by Runge-Kutta-Fehlberg procedure widi ACSL package.

A finite-element variational formulation with elasto-plastic behaviour laws has been used to assess the mechanical strain and stress distribution resulting from the pre-calculated water content and temperature profiles. The simulations were carried out with the industrial Abaqus package. The shrinkage coefficient was measured by means of a laser beam displacement detector on samples dried by micro-wave radiation. The elasticity modulus, tangential plastic modulus, compression and tension failure stress were determined by compression and bending tests on hygro thermally conditioned samples.