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.     

MOISTURE DIFFUSIVITY BEHAVIOR DURING FREEZE DRYING UNDER MICROWAVE HEATING POWER APPLICATION

The necessary criteria for the design of a freeze-drying operation, where radiant and microwave heating are combined, are analyzed in this work. Microwave heating presents indubitable advantages for enhancing drying kinetics. Modifications of different value characteristics must be also considered. Different freeze-drying strategies using MW heating are analyzed taking into account kinetics and diffusivity behavior compared to a constant diffusivity model. A microwave on-off cycled strategy with simultaneous up-down modification of pressure was found an acceptable power regulation system when magnetrons, whose field strength exceeds the breakdown voltage of gas, are used. In this case, chamber pressure becomes a convenient control parameter to avoid plasma discharge and subsequent melting of product. Different evolutions of specific power inputs per gram were proposed depending on the size and disposition of the product.     

MOISTURE DIFFUSIVITY BEHAVIOR DURING FREEZE DRYING UNDER MICROWAVE HEATING POWER APPLICATION

The necessary criteria for the design of a freeze-drying operation, where radiant and microwave heating are combined, are analyzed in this work. Microwave heating presents indubitable advantages for enhancing drying kinetics. Modifications of different value characteristics must be also considered. Different freeze-drying strategies using MW heating are analyzed taking into account kinetics and diffusivity behavior compared to a constant diffusivity model. A microwave on-off cycled strategy with simultaneous up-down modification of pressure was found an acceptable power regulation system when magnetrons, whose field strength exceeds the breakdown voltage of gas, are used. In this case, chamber pressure becomes a convenient control parameter to avoid plasma discharge and subsequent melting of product. Different evolutions of specific power inputs per gram were proposed depending on the size and disposition of the product.     

A SIMPLIFIED MODEL FOR DETERMINATION OF MOISTURE DIFFUSIVITY OF DATE FROM EXPERIMENTAL DRYING CURVES

ABSTRACT

Experimental drying curves for Tunisia Deglet Nour dates were obtained in a laboratory dryer under different drying conditions The air temperature was varied from 30 to 69^°C, relative humidity from 11.6 to 47.1 % and air velocity from 0.9 to 2.7 m/s. A numerical method to obtain a solution of a diffusion equation in which the diffusivity depends upon temperature and moisture content has been proposed to investigate the moisture movement in a date by assuming the sample to be a homogenous infinite cylinder. To rind the fitting moisture and temperature dependent diffusivity, the calculated drying curves are compared with the observed drying curves and an empirical equation for the moisture diffusivity of the date has presented as a function of temperature and moisture. It has been shown that the moisture distribution in the date during drying can be obtained by using the empirical equation presented.     

A SIMPLIFIED MODEL FOR DETERMINATION OF MOISTURE DIFFUSIVITY OF DATE FROM EXPERIMENTAL DRYING CURVES

Experimental drying curves for Tunisia Deglet Nour dates were obtained in a laboratory dryer under different drying conditions The air temperature was varied from 30 to 69^°C, relative humidity from 11.6 to 47.1 % and air velocity from 0.9 to 2.7 m/s. A numerical method to obtain a solution of a diffusion equation in which the diffusivity depends upon temperature and moisture content has been proposed to investigate the moisture movement in a date by assuming the sample to be a homogenous infinite cylinder. To rind the fitting moisture and temperature dependent diffusivity, the calculated drying curves are compared with the observed drying curves and an empirical equation for the moisture diffusivity of the date has presented as a function of temperature and moisture. It has been shown that the moisture distribution in the date during drying can be obtained by using the empirical equation presented.     

DETERMINATION OF MOISTURE DIFFUSIVITY OF PINE NUT SEEDS

The mechanism of drying and the diffusion of water in fresh pine nut seeds (2.5-2.62mm diameter) were successfully interpreted and modeled by using Fick's law. The initial moisture content of fresh pine nut seed was 23-24% and drying temperatures (45-60°C) were varied, but the drying air was kept at constant velocity and humidity.

The effective diffusivity was estimated from drying rate curves and expressed by an Arhenius relation. In addition, the absorption, and desorption-isotherms for fresh pine nuts were estimated at 20°C at water activity of 0.15-1.00. The isotherms of the fresh pine nuts were estimated in experimental runs.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

ABSTRACT

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

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.     

SIMULATION OF THE HEAT AND MOISTURE TRANSFER PROCESS DURING DRYING IN DEEP GRAIN BEDS

ABSTRACT

Grain drying is a simultaneous heat and moisture transfer problem. The modelling of such a problem is of significance in understanding and controlling the drying process. In the present study, a mathematical model for coupled heat and moisture transfer problem is presented. The model consists of four partial differential equations for mass balance, heat balance, heat transfer and drying rate. A simple finite difference method is used to solve the equations. The method shows good flexibility in choosing time and space steps which enable the simulation of long term grain drying/cooling processes. A deep barley bed is used as an example of grain beds in the current simulation. The results are verified against experimental data taken from literature. The analysis of the effects of operating conditions on the temperature and moisture content within the bed is also carried out     

SIMULATION OF THE HEAT AND MOISTURE TRANSFER PROCESS DURING DRYING IN DEEP GRAIN BEDS

Grain drying is a simultaneous heat and moisture transfer problem. The modelling of such a problem is of significance in understanding and controlling the drying process. In the present study, a mathematical model for coupled heat and moisture transfer problem is presented. The model consists of four partial differential equations for mass balance, heat balance, heat transfer and drying rate. A simple finite difference method is used to solve the equations. The method shows good flexibility in choosing time and space steps which enable the simulation of long term grain drying/cooling processes. A deep barley bed is used as an example of grain beds in the current simulation. The results are verified against experimental data taken from literature. The analysis of the effects of operating conditions on the temperature and moisture content within the bed is also carried out     

WATER TRANSFER IN POTATO DURING AIR DRYING

Drying data (moisture vs time) were obtained on slabs of potato in an air dryer operated at 70°C and an air velocity of 1.6 m/s. The considerable differences between the experimental data and the receeding front: and Fickian diffusion models for moisture profiles were found. The temperature in the potato sample increased with increasing drying time. This increase which results in starch gelation might be the main reason in the formation of moisture profile peaks.     

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

ABSTRACT

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 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.     

WATER TRANSFER IN POTATO DURING AIR DRYING

ABSTRACT

Drying data (moisture vs time) were obtained on slabs of potato in an air dryer operated at 70°C and an air velocity of 1.6 m/s. The considerable differences between the experimental data and the receeding front: and Fickian diffusion models for moisture profiles were found. The temperature in the potato sample increased with increasing drying time. This increase which results in starch gelation might be the main reason in the formation of moisture profile peaks.     

REGULAR REGIME ANALYSIS AND MOISTURE DIFFUSIVITY IN WOOD

The Regular Regime analysis, introduced by Schoeber in 1976, enables the determination of concentration dependent diffusivities from a single desorption experiment. A study on theoretical and practical aspects of this analysis is presented here. The theoretical background of the method is treated. Efforts have been made to simplify the calculation procedures by compacting and clarifying some complicated relationships. The calculation has become simple, straightforward and easy to use in practice. A stepwise procedure is given.

The Regular Regime analysis has been applied to wood specimens (Norway spruce). The moisture diffusivities are compared with those obtained from a Stationary Flux method. The observed differences are discussed. Also attention is paid to some problems in the application of the Regular Regime analysis.     

REGULAR REGIME ANALYSIS AND MOISTURE DIFFUSIVITY IN WOOD

ABSTRACT

The Regular Regime analysis, introduced by Schoeber in 1976, enables the determination of concentration dependent diffusivities from a single desorption experiment. A study on theoretical and practical aspects of this analysis is presented here. The theoretical background of the method is treated. Efforts have been made to simplify the calculation procedures by compacting and clarifying some complicated relationships. The calculation has become simple, straightforward and easy to use in practice. A stepwise procedure is given.

The Regular Regime analysis has been applied to wood specimens (Norway spruce). The moisture diffusivities are compared with those obtained from a Stationary Flux method. The observed differences are discussed. Also attention is paid to some problems in the application of the Regular Regime analysis.     

DETERMINATION OF DRYING CONSTANTS AND THEIR DEPENDENCE ON DRYING AIR PARAMETERS FOR THIN LAYER ONION DRYING

An experimental through dryer with continuous weighing system was developed for determining the drying rate constants for thin layer drying of onion (Allium cepa L, variety: Agnfound White- 1). Drying characteristics of onion were examined using ambient heated air for a temperature range of 50-80°C and air flow velocity range of 0.25-1.00 m/s. The influence of drying variables on drying rate constant was explained suitably hy Arrhenius type equation. This was also compared with Power model. The models were tested with data produced in an experimental through dryer, using nonlinear regression analysis. The Arrhenius type model predicted the moisture ratios well at the experimental drying conditions.     

DENSITIES, SHRINKAGE AND POROSITY OF SOME VEGETABLES DURING AIR DRYING

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.