OSMOTIC DRYING KINETICS OF CYLINDRICAL APPLE SLICES OF DIFFERENT SIZES

The objective of this study was to evaluate the osmotic drying kinetics of cylindrical slices of apples as influenced by particle size at different concentrations of sucrose solutions and different temperatures. Osmotic drying was carried out, with cut apple cylinders of three different sizes (12, 17 and 20mm 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. The solution to fruit volume ratio was kept greater than 60. A central composite rotatable design (CCRD) was used with dive levels of sucrose concentrations (34–63°Brix) and five temperatures (34–66°C). Kinetic parameters included weight loss, moisture loss, solids gain, rates of moisture loss and solids gain generally increased with increasing treatment time, temperature and concentration of osmotic solution, and decreased with an increase in sample size. The parameter “osmotic drying time to achieve a given moisture loss” obviously showed the opposite. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.     

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

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.     

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.     

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k_m and k_s) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

Osmotic Dehydration of Apple Cylinders: I. Conventional Batch Processing Conditions

Osmotic drying was carried out, with cylindrical samples of apple cut to a diameter-to-length ratio of 1:1, in a well-agitated large tank containing the osmotic solution at the desired temperature. The solution-to-fruit volume ratio was kept greater than 30. A modified central composite rotatable design (CCRD) was used with five levels of sucrose concentrations (34-63°Brix) and five temperatures (34-66°C). Kinetic parameters weight reduction (WR), moisture loss (ML), solids gain (SG) were considered. A polynomial regression model was developed to relate moisture loss and solids gain to process variables. A conventional diffusion model involving a finite cylinder was also used for moisture loss and solids gain, and the associated diffusion coefficients were computed. The calculated moisture diffusivity ranged from 8.20 × 10^-10 to 24.26 × 10^-10 m²/s and the solute diffusivity ranged from 7.82 × 10^-10 to 37.24 × 10^-10 m²/s. Suitable ranges of main parameters were identified for OD kinetics further study.     

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k _m and k _s ) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

Osmotic Dehydration of Apple Cylinders: I. Conventional Batch Processing Conditions

Osmotic drying was carried out, with cylindrical samples of apple cut to a diameter-to-length ratio of 1:1, in a well-agitated large tank containing the osmotic solution at the desired temperature. The solution-to-fruit volume ratio was kept greater than 30. A modified central composite rotatable design (CCRD) was used with five levels of sucrose concentrations (34–63°Brix) and five temperatures (34–66°C). Kinetic parameters weight reduction (WR), moisture loss (ML), solids gain (SG) were considered. A polynomial regression model was developed to relate moisture loss and solids gain to process variables. A conventional diffusion model involving a finite cylinder was also used for moisture loss and solids gain, and the associated diffusion coefficients were computed. The calculated moisture diffusivity ranged from 8.20 × 10^?10 to 24.26 × 10^?10 m²/s and the solute diffusivity ranged from 7.82 × 10^?10 to 37.24 × 10^?10 m²/s. Suitable ranges of main parameters were identified for OD kinetics further study.     

Ultrasound-Assisted Osmotic Dehydration of Strawberries: Effect of Pretreatment Time and Ultrasonic Frequency

Pretreatment of fruits prior to drying has shown success in reducing drying time and costs. In this work, ultrasound-assisted osmotic dehydration has been implemented as a method to increase water diffusivity and reduce drying time in strawberries. Strawberry halves were immersed in distilled water and in two different concentrations of sucrose solutions while pretreatment time and ultrasonic frequency levels were varied to determine their effect on drying time, water loss, and soluble solids gain. A microscopic analysis was carried out to evaluate the formation of microchannels and other changes to the fruit tissue structure. Greater sucrose concentration used in ultrasound-assisted osmotic dehydration resulted in greater water loss with greatest loss observed for the strawberry halves pretreated for 45 min in a 50% w/w sucrose solution. The pretreatment carried out for 30 min employing an osmotic solution of 50% w/w of sucrose resulted in the highest drying rate among the pretreatments. Osmotic dehydration used alone during pretreatment increased total processing time, whereas osmotic dehydration combined with ultrasonic energy during pretreatment reduced total processing time and increased effective water diffusivity. Cell distortion and breakdown were observed not only in pretreatments employing ultrasound-assisted osmotic dehydration but in conventional osmotic dehydration. Formation of microchannels through ultrasonic application and effects of osmotic pressure differential were considered to be largely responsible for reducing drying time for strawberry halves.     

Enthalpy-Entropy Compensation for Water Loss of Vegetable Tissues during Air Drying

Transition state theory was used to study enthalpy-entropy compensation for water loss during air drying of potato and apple slices. Slices of either potato or apple of 4-mm thickness, 40 mm diameter and air drying temperatures of 323, 333, 343, and 353 K were employed in the experiments. Moisture content and internal potato and apple slice temperatures were recorded during the drying runs. Water loss during drying was described by the unsteady-state Fick's equation and moisture diffusivity evolution was established by applying the method of the slopes. Thus, the experimental drying curve was compared to the theoretical diffusion curve, and the slope of the two curves were estimated at the same moisture content to in order give the corresponding value of diffusivity. During drying, the moisture diffusivity reached a maximum value as the water content of potato and apple slice was around 1 kg water/kg dry solid, regardless of the air temperature. The isokinetic temperature was found to be 320.2 and 312.8 K for potato and apple tissues, respectively. These values were greater than the experimental harmonic mean temperature, which was found to be 307.4 and 308.3 K for potato and apple tissues, respectively. Thus, it was concluded that the water loss process is enthalpy controlled.     

INFLUENCE OF SOLUTE TEMPERATURE AND CONCENTRATION ON THE COMBINED OSMOTIC AND AIR DRYING

The effect of solute concentration (15-45 % sugars) and temperature (10-40 °C) on the osmotic dehydration of apple was investigated. Cylindrical samples of apple were immersed in glucose or sucrose solutions and the water loss, the volume of solids and the porosity were measured as a function of time. Water loss was proportional to the square root of time, while the volume of solids decreased and the porosity increased with time. Sugar gain and water loss decreased the compression stress of the apple samples. Osmotic pre-treatment reduced the shrinkage and the porosity of apple solids during air-drying, compared to the no-treated samples. The results of this investigation are useful in the design of efficient osmotic dehydration processes, and in the evaluation of texture of dehydration of products.     

KINETICS OF OSMOTIC DEHYDRATION OF APPLES WITH PECTIN COATINGS

The solids gain and water loss during osmotic dehydration of coated apples were found to be dependend on concentration of pectin solution (0.5-4%) and drying time of coated apples (0-40 min). The coated apples were found to have generally a smaller the solids gain and higher or the same water loss than uncoated samples. For analysed low methyled pectin solutions and drying time the best for coating the apples before osmotic dehydration considering the highest water loss and the smallest solids gain was 2% pectin solution and 10 min drying time.     

TEMPERATURE AND CONCENTRATION EFFECTS OF OSMOTIC MEDIA ON OD PROFILES OF SWEET POTATO CUBES

A study was conducted to evaluate the effects of temperature and sucrose osmotic solution concentrations on osmotic dehydration profiles of sweet potato (Ipomea batata) cubes (3.5 cm sides). Two temperatures (26 and 50°C) and three concentrations (30:100, 50:100 and 70:100 w/w) were studied for various exposition times, up to 168 hours. Main influence was observed at higher temperature (50°C) due the fact that water loss (WL) and solids gains (SG) are faster and more intense. At 26°C no appreciable change in solids concentration was observed at distances deeper than 0.5 cm from the cubes surfaces even at 168 hours. At 50°C all the layers are affected even at shorter times (8 hours).     

Convective drying of osmotically dehydrated apples described by three-dimensional numerical solution of the diffusion equation with analysis of water effective diffusivity spatial distribution

Abstract

This study presents two liquid diffusion models to represent the convective drying of apple, osmotically dehydrated in sucrose solution, cut into parallelepiped-shaped pieces. Model 1 considered water diffusivity and the volume of the slices with constant values. Model 2 considered water effective diffusivity and the dimensions of the slices as variable. The numerical solution of the three-dimensional diffusion equation in Cartesian coordinates was obtained through the finite volume method, with a fully implicit formulation and boundary condition of the third kind. Process parameters were estimated by an optimizer using experimental data. A spatial distribution analysis was carried out for water effective diffusivity and moisture content in the apple slices. The results showed that the concentration of the osmotic solution used in the pretreatment influenced the drying process and that the mathematical model that considered a variable diffusivity and shrinkage was more suitable to describe the experimental data.     

EFFECT OF OSMOTIC AGENT ON OSMOTIC DEHYDRATION OF FRUITS

ABSTRACT

Mass transfer phenomena were investigated during osmotic dehydration of apple, banana and kiwi in glucose and sucrose osmotic solution. A complete set of experiments was performed for a wide range of temperature, sample size, speed of agitation, osmotic agent concentration and immersion time. An empirical model, based on a first order kinetic equation, was fitted satisfactorily to experimental data. Furthermore, the effect of solute molecular weight on mass transfer phenomena during the osmotic treatment was evaluated. The results showed that low molecular weight solute (glucose) leads to higher water loss and solids uptake than high molecular weight solute (sucrose), of osmodehydrated fruits under the same solution concentration.     

EFFECT OF OSMOTIC AGENT ON OSMOTIC DEHYDRATION OF FRUITS

Mass transfer phenomena were investigated during osmotic dehydration of apple, banana and kiwi in glucose and sucrose osmotic solution. A complete set of experiments was performed for a wide range of temperature, sample size, speed of agitation, osmotic agent concentration and immersion time. An empirical model, based on a first order kinetic equation, was fitted satisfactorily to experimental data. Furthermore, the effect of solute molecular weight on mass transfer phenomena during the osmotic treatment was evaluated. The results showed that low molecular weight solute (glucose) leads to higher water loss and solids uptake than high molecular weight solute (sucrose), of osmodehydrated fruits under the same solution concentration.     

Optimization of Osmotic Dehydration of Kiwifruit

Mass transfer rates were quantitatively investigated during osmotic dehydration of kiwifruit slices using response surface methodology with the sucrose concentration (20-80%, w/w), temperature of sucrose solution (15-75°C), osmotic time (60-420 min), and slice thickness (2-10 mm) as the independent process variables. Quadratic regression equations are obtained to describe the effects of independent process variables on the water loss (WL), sucrose gain (SG), and ascorbic acid loss (AAL). It was found that all factors had significant effect on the WL during osmotic dehydration of kiwifruit. Effects of temperature, time, and slice thickness were more pronounced on SG than the effect of concentration of sucrose solution. The osmotic solution temperature was the most significant factor affecting the AAL, followed by slice thickness and duration of treatment. The optimal conditions for osmotic dehydration were: 60% sucrose concentration, 30-40°C osmotic temperature, 150 min osmotic time, and 8 mm slice thickness.     

OSMOTIC DEHYDRATION KINETICS OF BLUEBERRIES

Abstract

The kinetics of moisture loss and solids gain during osmotic dehydration of blueberries under different conditions of temperature (37°C - 60°C), concentration of the sucrose solution (47°Brix - 70°Brix) and contact time between fruit and sucrose solution (0.5 h - 5.5 h) were studied, and modeled based on Fick's law of unsteady state diffusion. The study showed that all factors influenced moisture loss and solids gain (p<0.001), both generally increasing with temperature (T) and sucrose concentration (C). Based on the diffusion model, the calculated effective moisture diffusivity (D_m) ranged from 1.98 × 10^?10 to 5.10 × 10^?10 m²/s and the effective solids diffusivity (D_s) ranged from 2.54 × 10^?11 to 2.22 × 10^?10 m²/s. Both D_m and D_s showed increasing trends with temperature and sucrose concentration, and could be modeled as quadratic functions of T and C.     

Control of Polyphenol Oxidase Activity in Banana Slices During Osmotic Dehydration

The purpose of this study was to determine the effect of sodium metabisulfite and 4-hexylresorcinol on polyphenol oxidase (PPO) activity change in banana slices during osmotic dehydration in sucrose syrup.

From a previous study only three osmotic dehydration conditions were selected as the most adequate due to color change but also where PPO residual activity was still present after osmotic drying: T1 (60°Bx, 50°C, pH 6), T2 (60°Bx, 60°C, pH 8), and T3 (70°Bx, 50°C, pH 8). The level of sodium metabisulfite was varied from 100 to 1000 ppm and 4-hexylresorcinol from 10 to 100 ppm. The inner and outer parts of the banana slice were used for PPO activity determination at 302 nm with 4 methyl-catechol as a substrate.

During osmotic dehydration of a banana slice, PPO activity tended to decrease and residual enzymatic activity was still detected. Sodium metabisulfite concentration up to 200 ppm and 4-hexylresorcinol up to 50 ppm had no effect on PPO residual activity after 4 h of osmotic dehydration. Depending on the process conditions, a higher concentration of additives than these were necessary to control PPO activity during osmotic dehydration of banana slices.     

INFLUENCE OF SOLUTE TEMPERATURE AND CONCENTRATION ON THE COMBINED OSMOTIC AND AIR DRYING

ABSTRACT

The effect of solute concentration (15–45 % sugars) and temperature (10–40 °C) on the osmotic dehydration of apple was investigated. Cylindrical samples of apple were immersed in glucose or sucrose solutions and the water loss, the volume of solids and the porosity were measured as a function of time. Water loss was proportional to the square root of time, while the volume of solids decreased and the porosity increased with time. Sugar gain and water loss decreased the compression stress of the apple samples. Osmotic pre-treatment reduced the shrinkage and the porosity of apple solids during air-drying, compared to the no-treated samples. The results of this investigation are useful in the design of efficient osmotic dehydration processes, and in the evaluation of texture of dehydration of products.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.