Effect of Blanching by Ohmic Heating on the Osmotic Dehydration Behavior of Apple Cubes

The effect of blanching by ohmic heating (OH) on the damage to apple tissues and subsequent osmotic dehydration kinetics was investigated. Apple cubes were heated ohmically to various blanching scales. Heating temperature and duration were, respectively, 60–95 ± 2°C and 0–6 min. After cooling, the treated samples were put into sucrose solutions (70 °B) for the osmotic dehydration (OD). The equilibrium state of osmotic dehydration was estimated using the Azuara model. Ohmic heating leads, even for short treatments, to significant changes in the cellular structure of apples and to the enhancement of mass transfer during osmotic dehydration.     

On the Development of Osmotically Dehydrated Seabuckthorn Fruits: Pretreatments,Osmotic Dehydration,Postdrying Techniques,and Nutritional Quality

Osmotic dehydration of whole seabuckthorn berries, followed by convective or vacuum drying, was investigated. First, different pretreatments were applied to the fruits in order to accelerate the rate of osmotic dehydration: immersion in liquid nitrogen, steam blanching, or freeze cycles. Immersion in liquid nitrogen was found to be the best pretreatment to maximize dehydration rate and to increase sugar gain during osmotic dehydration. An evaluation of moisture loss and sugar gain kinetics during osmotic dehydration of seabuckthorn fruits pretreated with liquid nitrogen, followed by vacuum or hot-air drying, was then performed. Loss of nutritional compounds due to processing was also measured. Sugar intake and partial dehydration of seabuckthorn samples increased with osmosis time and reached an equilibrium value after 4 h treatment. The finish drying methods (vacuum or convective) applied after OD showed a marked impact on the remaining moisture content of seabuckthorn samples. Concentration of some nutritional compounds was, however, dramatically reduced after the combined osmotic dehydration/drying processes.     

Effect of Pulsed Electric Field and Osmotic Dehydration Pretreatment on the Convective Drying of Carrot Tissue

The influence of pulsed electric field (PEF) and subsequent centrifugal osmotic dehydration (OD) on the convective drying behavior of carrot is investigated. The PEF was carried out at an intensity of E = 0.60 kV/cm and a treatment duration of t _PEF  = 50 ms. The following centrifugal OD was performed in a sucrose solution of 65% (w/w) at 40°C for 0, 1, 2, or 4 h under 2400 × g. The drying was performed after the centrifugal OD for temperatures 40–60°C and at constant air rate (6 m³/h).

With the increase of OD duration the air drying time is reduced spectacularly. The dimensionless moisture ratio Xr = 0.1 is reached for PEF-untreated carrots after 370 min of air drying at 60°C in absence of centrifugal OD against 90 min of air drying after the 240 min of centrifugal OD. The PEF treatment reduces additionally the air drying time. The total time of dehydration operations can be shortened when OD time is optimized. For instance, the minimal time required to dehydrate untreated carrots until Xr = 0.1 is 260 min (120 min of OD at 40°C and 140 min of drying at 60°C). It is reduced to 230 min with PEF-treated carrots.

The moisture effective diffusivity D _eff is calculated for the convective air drying based on Fick's law. The centrifugal OD pretreatment increases drastically the value of D _eff . For instance, 4 h of centrifugal OD permitted increasing the value of D _eff from 0.93 · 10^?9 to 3.85 · 10^?9 m²/s for untreated carrots and from 1.17 · 10^?9 to 5.10 · 10^?9 m²/s for PEF-treated carrots.     

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.     

Comparison of two alternatives of combined drying to process blueberries (O'Neal): Evaluation of the final quality

In this work, we examined and compared two combined alternatives for the drying of blueberries (O’Neal). Pretreatments of osmotic dehydration (60°Brix sucrose solution at 40°C for 6 h) and hot air drying (HAD) (60°C, 2.5 m/s for 90 min) were performed to reach the same water content. Pretreated blueberries were then dried by microwave at different microwave output power values: 562.5, 622.5, and 750 W. The combined drying processes were also compared with HAD alone (control). The effects of the processes over blueberries were studied in terms of decrease in water content, drying rate (DR), mechanical properties (firmness and stiffness), optical properties (L*, a*, and hue angle (h)), antioxidant capacity, and rehydration capacity. The hot air–microwave drying decreased the process time and presented a high drying rate compared with the osmotic dehydration–microwave processes and the control drying. In terms of quality, the antioxidant and rehydration capacities were the most affected. The results showed that the best drying method to obtain the desired final product was the hot air–microwave drying (750 W).     

The Development of Ginger Drying Using Tray Drying,Heat Pump–Dehumidified Drying,and Mixed-Mode Solar Drying

Mature ginger was pretreated by soaking in citric acid prior to drying in a single layer in a tray and heat pump dehumidified dryer at three temperatures of 40, 50, and 60°C and in a mixed-mode solar dryer at 62.82°C and a radiation intensity of 678 W/m². The drying data were applied to the modified Page model. Diffusivities were also determined using the drying data. Quality evaluation by color values, reabsorption, and 6-gingerol content showed best quality for ginger with no predrying treatment and dried at 40°C in a heat pump–dehumidified dryer. At drying temperature of 60 to 62.82°C, no pretreated dried ginger from mixed-mode solar dryer provided the shortest drying time and retained 6-gingerol as high as heat pump–dehumidified dryer.     

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.     

Effect of High Pressure Pretreatments on Structural and Dehydration Characteristics of Aloe Vera (Aloe barbadensis Miller) Cubes

The effect of high-pressure (HP) pretreatments (300–500 MPa for 5–15 min) on dehydration characteristics of aloe vera cubes (AVC), dried at 50–70°C and air velocity of 1–2 m/s, was studied. Pretreatments resulted in higher drying rates, leading to a reduction in drying time compared to untreated ones. The drying rate and drying time were most significantly affected (p < 0.05) by the drying temperature, followed by air velocity, pressure level, and dwell time. HPP enhanced the firmness of AVC with a maximum of up to 21% for the sample treated at 500 MPa for 15 min. Microstructural analysis using scanning electron microscopy (SEM) indicated non-uniform structures in the pretreated and dried AVC samples. Semi-quantitative elemental detection (EDS) confirmed the presence of a considerable amount (14%) of calcium in aloe vera.     

Effect of Osmotic Dehydration with Pulsed Vacuum on Hot-Air Drying Kinetics and Quality Attributes of Cherry Tomatoes

The effects of osmotic dehydration (OD) with or without pulsed vacuum (PV) on hot-air drying kinetics and quality attributes of cherry tomatoes were investigated. Both OD and PVOD pre-treatments were performed for 3 h at 50°C in 50 and 70^o Brix sucrose solutions with a solution-to-fruit mass ratio of 4:1, and PVOD was applied for 15 min before OD at atmospheric pressure. Samples were further dried at air temperature of 70°C. Effective moisture diffusivity (D _eff) of osmotically dehydrated samples increased gradually while the D_eff curve of fresh samples had a plateau stage during hot air drying. Lower glass transition temperature, T_g, values of osmotically dehydrated samples indicated that they needed a lower storage temperature. Both OD and PVOD pre-treatments had advantages in shortening drying cycles and improving quality of products. Compared with air drying, osmo-air drying decreased the total drying time, color change, and hardness of dried samples by 32.26%, 18.11%, and 88.21%, respectively, and increased volume ratio and vitamin C retention rate by 72.31% and 125.82%. As compared with OD, PVOD decreased color change and hardness by 28.48% and 45.17%, increased volume ratio and vitamin C retention rate by 27.41% and 17.77%, but there was no significant difference shown in drying time. Therefore, osmotic pre-treatment can shorten the total dehydration time, and improve the general quality of dried cherry tomatoes.     

Analysis of Chestnut Cellular Tissue during Osmotic Dehydration,Air Drying,and Rehydration Processes

The analysis of changes in microstructure is very important in order to understand heat and mass transfer processes in biological systems and to evaluate physical properties and the quality of fresh and processed food materials. In this study, chestnut (Castanea sativa M.) samples were submitted to several processes such as osmotic dehydration by immersion in sucrose solution (60% w/w) at 25°C, drying with hot air at 65°C, and rehydration by contact with water at 25°C. The surface cellular tissue of fresh and processed chestnut at different moisture contents was characterized by optical microscopy by means of size (surface area, perimeter, Feret's diameter) and shape (roundness, compactness, and elongation) parameters. The results indicated that during the osmotic dehydration the changes in microstructure can be considered practically negligible and during drying the size of chestnut cells decreased and shape parameters changed (decreasing roundness and compactness and increasing elongation). Finally, during rehydration the cells recovered their size by swelling rapidly (30 min is enough to recover the size of the cells of fresh tissue), achieving at longer times greater sizes than those measured in the fresh chestnut; nevertheless, the shape parameters are not completely recovered (lower compactness and roundness values).     

Control of Solids Uptake by Convective Drying Prior to Osmotic Processing of Foods

Abstract

Solute impregnation constitutes a major drawback to the commercial use of osmotic dehydration of food and several methods to control solute uptake have been proposed. In this work the effect of a convective drying step before osmotic treatment in minimizing solute uptake by the material is analyzed. Results indicate that the initial convective step is a useful way to control solute uptake of apple without increasing total operation time. Reduction of solute uptake using an initial convective step of 30 min varied between 75 and 85% with respect to the samples submitted to a single osmotic treatment with the same duration. It was determined that an initial convective treatment of 30–60 min does not modify total operating time and provides an efficient control of solute gain by the material. This combined dehydration process may be an attractive alternative for producing a new generation of vegetable snacks.     

Optimization of Fluidized Bed Drying Process of Green Peas Using Response Surface Methodology

The fluidized bed drying process of green peas was optimized using the response surface methodology for the process variables: drying air temperature (60–100°C), tempering time (0–60 min), pretreatment, and mass per unit area (6.3–9.5 g/cm²). The green peas were pretreated by pricking, hot water blanching, or chemical blanching. Product quality parameters such as rehydration ratio, color, texture, and appearance were determined and analyzed. Second-order polynomial equations, containing all the process variables, were used to model the measured process and product qualities. Rehydration ratio was influenced mostly by pretreatment followed by tempering time, temperature, and mass per unit area. Pretreatment and mass per unit area significantly affected color and texture. Higher levels of temperature and lower levels of tempering time and mass per unit area increased the rehydration ratio. The optimum process conditions were derived by using the contour plots on the rehydration ratio and sensory scores generated by the second-order polynomials. Optimum conditions of 79.4°C drying air temperature, 35.8-min tempering time, pretreatment of the once pricked peas with chemical blanching in a solution of 2.5% NaCl and 0.1% NaHCO₃, and mass per unit area of 6.8 g/cm² were recommended for the fluidized bed drying of green peas. At these conditions the rehydration ratio was 3.49.     

Optimization of Osmotic Dehydration of Potato Cubes Under Pulsed Microwave Vacuum Environment in Ternary Solution

Optimization of the process parameters for osmotic dehydration of 12.2-mm potato cubes was carried out using response surface methodology. The experiments were conducted using a central composite rotatable design (CCRD) with four factors, viz. sucrose concentration (27.5–42.5% w/w), salt concentration (7.5–12.5% w/w), total osmosis time (26.25–68.75 min), and microwave power density for the initial 4 min (0.375–1.125 W/g of total weight of solution and potato cubes) at two levels each to take into account the individual and interaction effects of the factors. A sample-to-solution ratio of 1:10 and pressure of 0.16 kPa for the initial 4 min were kept constant throughout all of the experiments. It was found that the linear effects of all factors on the water loss (WL) and solids gain (SG) were highly significant. The optimum condition was found at a sucrose concentration of 36.35%, salt concentration of 12.50%, osmosis time of 68.72 min, and microwave power density of 0.38 W/g for the initial 4 min, with a WL of 37.26% initial weight and SG of 8.74% initial weight. The drying of potato cubes was carried out using hot air, microwave–vacuum, and osmotic microwave–vacuum drying methods. It was found that potato cubes dried by combined osmotic microwave–vacuum had better sensory qualities.     

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.     

Optimization of Foaming Parameters and Investigating the Effects of Drying Temperature on the Foam-Mat Drying of Shrimp (Penaeus indicus)

In this study, foaming conditions of shrimp (Penaeus indicus) were optimized using response surface methodology (RSM) and the effect of drying temperature on drying behavior was investigated. The experiments were conducted according to face-centered central composite design for three independent variables: xanthan gum concentration (0.1–0.4% w/w), whipping time (2–6 min), and water : shrimp ratio (2:1–6:1 w/w) to minimize the foam density (FD) and the drainage volume (DV) as responses. Statistical analysis of results showed that linear terms of the models were significant (p < 0.01) except the linear term of whipping time in DV. Xanthan gum concentration 0.19% (w/w), water : shrimp ratio 4.5:1 (w/w), and whipping time 5.89 min were found to be the optimum foaming conditions. The effect of different drying temperatures (45, 60, 75, and 90°C) on drying behavior of optimized foam was then evaluated. The drying air temperature had a considerable effect on drying time and drying rate. As the temperature increased from 45 to 90°C, the drying time decreased to 90 min. Drying rate curves showed that foam-mat drying of shrimp principally occurred in the constant rate period. Different mathematical models were tested with the drying behavior of shrimp foam in the dryer. According to the results, the Weibull distribution model is superior to the other models for explaining the drying behavior. Effective moisture diffusivity was calculated and was between 1.114 × 10^?8 and 3.951 × 10^?8 m²/s within the studied temperature range. An Arrhenius relation with an activation energy value of 26.89 kJ/mol expressed the effect of temperature on diffusivity.     

Sorption Properties of Vacuum-Dried Strawberries

This work demonstrates the influence of changes in parameters of vacuum drying (temperature and pressure) on the sorption properties of dried strawberries. Fruits were dried at 50 and 70°C under pressures of 4 and 16 kPa. Vacuum drying was also conducted during the first 4 h at 70°C and then the temperature was decreased to 50°C at a pressure of 4 kPa. The other combination included increasing the pressure after the first 4 h from 4 to 16 kPa at a drying temperature of 70°C. Sorption isotherms were determined in the dried strawberries. It was shown that with increasing drying temperatures, there was a notable deterioration in the capacity for absorbing water vapor by the vacuum-dried fruit. On the other hand, the pressure at which vacuum drying proceeded did not significantly affect water vapor absorption. Changing the parameters of vacuum drying—that is, temperature in the range of 50–70°C and pressure in the range of 4–16 kPa—affected the shape and structure of the resultant dried strawberries. The combination of vacuum drying with convective drying also influenced the shape and structure of the dried fruit.     

Three-stage hybrid osmotic–intermittent microwave–convective drying of apple at low temperature and short time

ABSTRACT

In recent years, intermittent microwave coupled with hot air-drying has been used increasingly, thanks to considerable improvements observed in drying properties. The aim of this study was to investigate the effect of process of drying apple pretreated osmotically with sucrose solution at five concentrations of 0 (control), 10, 30, 50, and 70% (w/w), using intermittent microwave at four power levels of 0 (control), 360, 600, and 900?W, four pulse ratios of 1, 2, 3, and 4, and convective hot air (40°C) on drying kinetics, effective moisture diffusion coefficient, shrinkage, bulk density, rehydration ratio, and energy consumption. Results showed that the three-stage hybrid osmotic–intermittent microwave–convective drying of apple at low temperature yielded higher drying rates (with 41.5% decrease in drying time) and improved quality of final product. The effective moisture diffusion coefficient increased with an increase in power, pulse ratio, and the concentration of osmotic solution. Furthermore, shrinkage, bulk density, and energy consumption of the samples decreased with an increase in power, pulse ratio, and the concentration of osmotic solution. In summary, the use of intermittent microwave coupled with forced convection of hot air (at low temperature) in drying of apple pretreated by sucrose osmotic solution led to products with improved properties in terms of both quality and quantity.     

The Effects of Predrying Treatments and Different Drying Methods on Phytochemical Compound Retention and Drying Characteristics of Moringa Leaves (Moringa oleifera Lam.)

The most appropriate maturity stage of Moringa oleifera leaves was selected for drying based on phytochemical content, including quercetin and kaempferol. Desorption isotherms were developed and were best fit by the modified Henderson model. Prior to drying, samples were left untreated, blanched in boiling water, and blanched in NaHCO₃/MgO. The leaves were dried by hot air tray drying (TD) and heat pump–dehumidified drying air (HPD) at air temperatures of 40, 50, and 60°C. Alternatively, leaves were subject to microwave drying (MWD) at 150, 450, and 900 W and to freeze drying (FD). The moisture versus time data were fitted to five drying models. In general, a three-parameter model gave the best fit. The drying constant was related to the drying temperature or microwave power using an Arrhenius model. Effective moisture diffusivity (D _eff) increased with higher drying temperature, higher microwave power, or blanching treatments. Structural changes in the leaves after drying and upon rehydration were observed by scanning electron microscopy (SEM). Leaves blanched and dried using HPD at 50°C and fresh and dried using FD showed a partial breakdown of the tissue structure upon rehydration. HPD and blanching reduced the drying time by 8.3% and increased quercetin and kaempferol levels by 42.1 and 51.4%, respectively, compared to TD at 50°C. MWD provided the quickest drying followed by HPD and TD, respectively. HPD drying of M. oleifera after blanching resulted in relatively greater quality compared to TD and MWD.     

Effect of high pressure pretreatment on drying kinetics and oleoresin extraction from ginger

The present study reports for the first time the effect of high pressure pretreatment (100–400?MPa, 10?min) on drying kinetics of ginger and its oleoresin extraction. High pressure pretreated samples were dried, powdered and solvent extracted. The increase in drying temperature (55–85°C) increased the moisture diffusivity (2.03–4.87?×?10^?9?m²/s) but resulted in decrease in 6-gingerol (53.98%) and oleoresin yield (57.31%). However, high pressure pretreatment followed by dehydration (55°C) resulted in higher moisture diffusivity (2.84–6.09?×?10^?9?m²/s) as well as enhanced extraction yield of 6-gingerol (34.05%) and oleoresin (28.29%).     

Effect of Various Pretreatments on the Quality of Vacuum-Fried Carrot Chips

Carrot slices were subjected to the following four different pretreatments prior to vacuum frying: (1) blanching, (2) blanching and air drying, (3) blanching and osmotic dehydration, (4) blanching, osmotic dehydration, followed by freezing. The effects of these pretreatments on the physicochemical properties and fat distribution in vacuum-fried carrot chips were also investigated. There were significant differences in the total yield, amounts of carotenes, vitamin C, and the color values of carrot chips following different pretreatments (P < 0.05). Pretreatment significantly affected the water content, fat content, and water activity of carrot chips (P < 0.05), while there were no significant differences in the breaking force of carrot chips treated with different pretreatments (P > 0.05). Spearman correlation analysis showed a high positive correlation between the fat content of carrot chips and the initial water content of carrot slices. The fat distribution pattern depended on the initial water content and the structure of the material left by water evaporation.