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.     

TOWARD PREDICTION OF POROSITY IN FOODS DURING DRYING: A BRIEF REVIEW

Four generic trends of pore formation during drying are identified from the literature. The present prediction methods are mainly based on empirical correlations. It is common to correlate porosity with water content by quadratic, polynomial, or exponential forms of equations, which do not provide insight into the physics of the process. The glass transition theory is one of the proposed concepts to explain the process of shrinkage and collapse during drying. However, the glass transition theory does not hold true for all products. Other concepts, such as surface tension, structure, environment pressure, and mechanisms of moisture transport also play important roles in explaining the formation of pores. It is hypothesized that as capillary force is the main force responsible for collapse, so counterbalancing this force causes formation of pores and lower shrinkage.     

PREDICTION OF QUALITY CHANGES DURING OSMO-CONVECTIVE DRYING OF BLUEBERRIES USING NEURAL NETWORK MODELS FOR PROCESS OPTIMIZATION

Artificial neural network (ANN) models were used for predicting quality changes during osmo-convective drying of blueberries for process optimization. Osmotic drying usually involves treatment of fruits in an osmotic solution of predetermined concentration, temperature and time, and generally affects several associated quality factors such as color, texture, rehydration ratio as well as the finish drying time in a subsequent drier (usually air drying). Multi-layer neural network models with 3 inputs (concentration, osmotic temperature and contact time) were developed to predict 5 outputs: air drying time, color, texture, and rehydration ratio as well as a defined comprehensive index. The optimal configuration of neural network model was obtained by varying the main parameters of ANN: transfer function, learning rule, number of neurons and layers, and learning runs. The predictability of ANN models was compared with that of multiple regression models, confirming that ANN models had much better performance than conventional mathematical models. The prediction matrices and corresponding response curves for main processing properties under various osmotic dehydration conditions were used for searching the optimal processing conditions. The results indicated that it is feasible to use ANN for prediction and optimization of osmo-convective drying for blueberries.     

PREDICTION OF QUALITY CHANGES DURING OSMO-CONVECTIVE DRYING OF BLUEBERRIES USING NEURAL NETWORK MODELS FOR PROCESS OPTIMIZATION

Artificial neural network (ANN) models were used for predicting quality changes during osmo-convective drying of blueberries for process optimization. Osmotic drying usually involves treatment of fruits in an osmotic solution of predetermined concentration, temperature and time, and generally affects several associated quality factors such as color, texture, rehydration ratio as well as the finish drying time in a subsequent drier (usually air drying). Multi-layer neural network models with 3 inputs (concentration, osmotic temperature and contact time) were developed to predict 5 outputs: air drying time, color, texture, and rehydration ratio as well as a defined comprehensive index. The optimal configuration of neural network model was obtained by varying the main parameters of ANN: transfer function, learning rule, number of neurons and layers, and learning runs. The predictability of ANN models was compared with that of multiple regression models, confirming that ANN models had much better performance than conventional mathematical models. The prediction matrices and corresponding response curves for main processing properties under various osmotic dehydration conditions were used for searching the optimal processing conditions. The results indicated that it is feasible to use ANN for prediction and optimization of osmo-convective drying for blueberries.     

A MATHEMATICAL MODEL FOR THE CONTINUOUS VACUUM DRYING OF HIGHLY VISCOUS FOODSTUFFS

An improved mathematical model for the continuous vacuum drying of highly viscous and heat-sensitive food-stuffs was proposed. The process of continuous vacuum drying was presented as a moving boundary problem of moisture evaporation in cylindrical coordinates. Boundary condition of the first kind for the known functional dependence of the drying body surface temperature on time was considered. Finally, the appropriate system of differential equations was solved numerically and the values of drying rate, integral moisture content of the material, moving boundary position as well as temperature in any point of the material and at any moment of time were obtained. This procedure was applied to continuous vacuum drying of foods such as natural cheese and fresh fish meat paste.     

APPLICATION OF FUZZY RULES-BASED MODELS TO PREDICTION OF QUALITY DEGRADATION OF RICE AND MAIZE DURING HOT AIR DRYING

The concept of modelling using a rules -based approach is introduced and developed for die prediction of the quality degradation of grains during hot air drying. It is men validated on two typical examples of quality degradation during drying: the head kernel yield of paddy rice and the wet-milling quality of maize. The linguistic approach gives a representation of die experts understanding of phenomena and leads to a good kinetic model after identification of die parameters using genetic algorithms. This study also points out die robustness of die model prediction, even if noisy data are used, and its ability to predict transient behaviours under certain conditions.     

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.     

APPLICATION OF FUZZY RULES-BASED MODELS TO PREDICTION OF QUALITY DEGRADATION OF RICE AND MAIZE DURING HOT AIR DRYING

ABSTRACT

The concept of modelling using a rules -based approach is introduced and developed for die prediction of the quality degradation of grains during hot air drying. It is men validated on two typical examples of quality degradation during drying: the head kernel yield of paddy rice and the wet-milling quality of maize. The linguistic approach gives a representation of die experts understanding of phenomena and leads to a good kinetic model after identification of die parameters using genetic algorithms. This study also points out die robustness of die model prediction, even if noisy data are used, and its ability to predict transient behaviours under certain conditions.     

COLLAPSE OF STRUCTURE DURING DRYING OF CELERY

ABSTRACT

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.     

POSSIBILITY OF CLAY DAMAGE DURING DRYING

The convective drying of plate made of clay is considered in the paper. The model of drying with material coefficients dependent on the moisture content and the temperature is presented. The material is assumed to be brittle-elastic (linear elasticity with moisture dependent material constants). The strength of the material estimated by bending test, is moisture dependent. Numerical simulation of drying processes is done. Through the comparison of the stresses arising during drying with the strength of the material the possibility of dried body damage is shown. The range of constant drying conditions in which damage is expected is specified.     

POSSIBILITY OF CLAY DAMAGE DURING DRYING

The convective drying of plate made of clay is considered in the paper. The model of drying with material coefficients dependent on the moisture content and the temperature is presented. The material is assumed to be brittle-elastic (linear elasticity with moisture dependent material constants). The strength of the material estimated by bending test, is moisture dependent. Numerical simulation of drying processes is done. Through the comparison of the stresses arising during drying with the strength of the material the possibility of dried body damage is shown. The range of constant drying conditions in which damage is expected is specified.     

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.     

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.     

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.     

DAMAGE OF FABA BEAN SEEDS DURING DRYING

ABSTRACT

Experiments have been carried out to determine a relation between mechanical properties, microstructure and drying conditions of faba bean seeds and their damages during single-stage drying. Results showed that physical damages of beans strongly depend on drying parameters, i.e.drying time and temperature, air flow rate and variety of seeds. It was revealed that some seed parts: the hilum and embryonal axis play a special role in the cracking mechanism of seeds.     

CONTRACTION CHARAClFRlSTlCS OF MOLDED CERAMICS DURING DRYING

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.     

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.     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

DAMAGE OF FABA BEAN SEEDS DURING DRYING

Experiments have been carried out to determine a relation between mechanical properties, microstructure and drying conditions of faba bean seeds and their damages during single-stage drying. Results showed that physical damages of beans strongly depend on drying parameters, i.e.drying time and temperature, air flow rate and variety of seeds. It was revealed that some seed parts: the hilum and embryonal axis play a special role in the cracking mechanism of seeds.     

DRYING KINETICS OF Chrysanthemus cinerariaoefolium FLOWERS AND PTRETHRIN CONTENT DECAY DURING DRYING

ABSTRACT

The external mechanisms that control drying rate are basically fixed by the operating conditions of the dryer being the temperature of the drying air the most important one. The final content of thermosensitive compounds is also determined by this condition and this is why the final quality of the product and the operating conditions are inter-related. The example of this phenomenon is the processing of pyrethrum. The organic compounds extracted from the Chrysanthemus cinerariaefolium flowers are called pyrethrins, which is the most ancient known natural insecticide. In this work, results obtained from the experimental drying of Chrysanthemus cinerariaefolium flowers are presented. Also, the influence of the drying air temperature on the pyrethrin content is established. The experimental data were used to derive a mathematical model that describes the drying rate and degradation kinetics.