Drying Characteristics of Pulsed Electric Field-Treated Carrot

This study was designed to establish the interaction effect of various components of pulsed electric field (PEF) system on drying characteristics of carrot, a vegetable rich in carotenoid. Carrots were treated in PEF system of 0.5, 1, and 1.5 kV/cm electric field strength, 0.5 and 1 μF capacitance, and pulse numbers of 10, 30, and 50. The samples were subsequently dried at 70°C until reaching an acceptable moisture content level. Drying rates were significantly (P < 0.05) influenced by PEF treatment. Variation in drying constant was not significantly influenced by the interaction effect of electric field components but by the energy per pulse. The Henderson and Perry model was fitted to the drying curve and it gave correlation coefficients ranging between 0.86 and 0.96.     

The effect of pulsed electric field on drying kinetics,color, and microstructure of carrot

The aim of the study was to investigate the impact of pulsed electric field (PEF) treatment on the convective drying kinetics of a carrot and color and microstructure changes of the dried product. Samples were treated by PEF with the specific energy input equal to 5.63, 8 and 80 kJ · kg^?1. After PEF treatment, thermal conductivity and electrical conductivity were measured. Drying time of the PEF-treated samples was reduced up to 8.2% (W_s = 8 kJ · kg^?1, 5 kV · cm^?1; 10 pulses) in comparison to intact tissue. Statistical analysis showed that Midilli et al.’s model was considered to describe the kinetics of the process the most precisely. Pulsed electric field treatment increased the effective water diffusion coefficient up to 16.7%. Moreover, PEF treatment and drying caused the alteration of the sample color. After drying, the lightness and chroma were higher or unchanged in comparison to the intact tissue. The dried PEF-treated samples exhibited significantly higher redness (higher value of a^* parameter) in comparison to the untreated dried samples. Moreover, the visual inspection of scanning electron microscope images revealed that PEF pretreatment performed at high electric field intensity (5 kV · cm^?1, regardless of pulse number) provoked the material to form greater cavities during drying in comparison to the untreated material.     

Convective air,microwave, and combined drying of potato pre-treated by pulsed electric fields

Abstract

The effects of pulsed electric field (PEF) treatment on drying of potato have been investigated. The convective air (CA) (T?=?50?°C), microwave (MW), and combined (CA?+?MW or MW?+?CA) drying was applied. The drying curves, progression of temperature inside samples, microstructure, and capillary imbibition capacity of untreated (U) and PEF treated samples were compared. The PEF treatment noticeably affected the CA and MW drying characteristics and structure of dried samples. It reflected impact of electroporation on the heat and mass transfer processes during the different drying periods. The different progressions of temperatures inside U and PEF samples for the MW mode of drying were revealed. The highest and smallest rehydrations were observed for the U samples for individual CA and MW modes of drying, respectively. The observed behavior was explained accounting for the absence of starch gelatinization for the CA mode of drying and presence of it for the MW mode of drying.     

Drying Kinetics of Apple Tissue Treated by Pulsed Electric Field

The aim of this work was to study the influence of pulsed electric field (PEF) on the drying kinetics of apple tissue. Therefore, mathematical models that are commonly used in the literature were applied to describe the process. PEF treatment of the samples was carried out at an intensity of E = 5–10 kV/cm and 10–50 pulse numbers. Subsequently, the apples were convectively dried at 70°C and air velocity of 2 m/s. Based on electrical conductivity measurement, the cell disintegration index Z _p was computed. Midilli et al.'s(Drying Technology, Vol. 20, pp. 1503–1513, 2001) model was evaluated as the most adequate to describe the moisture transfer in PEF-treated and intact samples. PEF pretreatment induced a reduction in drying time of up to 12% when 10 kV/cm and 50 pulses were applied. For instance, after 60 min of drying, the dimensionless moisture ratio for PEF-treated (10 kV/cm, 50 pulses) samples was 0.18 compared to 0.26 for the untreated apples. The effective moisture diffusivity, calculated on the basis of the Fick's second law, was 1.04 × 10^?9 m/s for intact samples and from 1.09 × 10^?9 to 1.25 × 10^?9 m²/s for PEF-treated samples at 10 pulses at 5 kV/cm and 50 pulses at 10 kV/cm, respectively.     

Pulsed Electric Field Pretreatment for Osmotic Dehydration of Apple Tissue: Experimental and Mathematical Modeling Studies

The aim of this study was to analyze the influence of pulsed electric field pretreatment (PEF) on the osmotic dehydration of apple tissue. Osmotic dehydration was carried out in sucrose solution at 40°C and 100 rpm in a water-bath shaker. PEF pretreatment was performed using varying field strength of 5 and 10 kV/cm and 10 and 50 pulses. On the basis of electric conductivity measurement, the cell disintegration index was calculated. The course of osmotic dehydration was described by means of water loss, solid gain, weight reduction, and water content changes. Moreover, the course of the process was described by different mathematical models that are commonly used in the literature. PEF application before osmotic dehydration significantly increased water loss after 60 minutes of the process. In turn, no significant differences were found in the case of solid gain. The highest osmotic dehydration efficiency ratio (WL/SG) was noticed for samples treated by PEF at the electric field strength of 5 kV/cm and 10 pulses. The statistical analysis of mathematical modeling of the process showed the equations utilized generally exhibit a good fit to the experimental data.     

A Comparative Study on the Effect of Chemical,Microwave, and Pulsed Electric Pretreatments on Convective Drying and Quality of Raisins

In order to discourage the use of chemicals in raisin processing, the effect of microwave and pulsed electric field (PEF) pretreatments on the drying rate and other quality parameters like color, total soluble solids, bulk density, appearance, and market quality were compared with that of chemically pretreated raisins dried at 65°C. The untreated and pretreated samples had a statistically significant difference in drying rate (P < 0.05). The drying rate of chemically pretreated raisins was the highest when compared to others. The results showed that the PEF and microwave-treated samples had a significantly high Total Soluble Solids (TSS), along with good appearance and market quality.     

Comparison of Water Blanching and High Hydrostatic Pressure Effects on Drying Kinetics and Quality of Potato

Abstract

Drying kinetics and quality parameters of potato cubes were evaluated as affected by high pressure processing and hot water blanching. The potato cubes in 1% citric acid solution as immersion medium were pressure treated at 400 MPa for 15 min. Hot water blanching was conducted in boiling water for 3 min. Drying kinetics and quality parameters (i.e., rehydrability, texture, color and apparent density) were assessed for the high pressure–treated and water-blanched samples and for dehydrated and rehydrated samples. Drying rates were found to be higher (p < 0.05) in the initial period of drying for the pressure treated samples. The Page model was found to better fit drying data of the thermally treated samples, and the two-terms model better described the drying behavior of high pressure–treated samples. High pressure–treated samples had a similar rehydrability to thermally treated samples. It was found that pressure–treated samples had a hardness value close to that of fresh samples, whereas thermal treatment resulted in a softer texture. After rehydration, samples of both treatments returned their texture before drying. The total color difference for the thermally blanched samples was higher (p < 0.05) than for pressure–treated samples before drying and after drying. High pressure–treated and dried potato cubes had a color close to that of fresh potato cubes. High pressure–treated and air-dried samples were found to have higher (p < 0.05) apparent density than thermally treated samples.     

Optimization of pulsed electric field pretreatment parameters for preserving the quality of Raphanus sativus

Pulsed electric field (PEF) pretreatment can increase the drying rate of produce, but preserving product quality while minimizing energy consumption and maintaining food quality is a significant challenge. The goal of this study was to determine optimal PEF parameters for pretreatment of Raphanus sativus (radish) prior to the drying process. The effects of pulse intensity, treatment time, and pulse number on the drying rate, vitamin C (Vc) content, and ascorbic acid oxidase activity of R. sativus were characterized. Optimal PEF pulse parameter values were determined through quadratic orthogonal regression tests followed by multi-objective nonlinear optimization. The optimal PEF pulse parameters for pretreatment of R. sativus were: pulse intensity, 1446 V · cm^?1; reaction time, 28 μs; and pulse number, 87. This study provides reference values to guide application of PEF pretreatment in R. sativus processing.     

Effect of Static High Electric Field Pre-Treatment on Microwave-Assisted Drying of Potato Slices

Dehydration of fruits and vegetables affects their physical, biochemical, and organoleptic properties. In the present study, the effect of static high electric field intensity of 4 kV/cm on drying kinetics of microwave-convective and hot air drying systems was evaluated. The studied electric field intensity had minimal to no effect on the drying kinetics of the potato slices. Less to no curling was observed for high electric field pre-treated samples compared to microwave-convective and hot-air-dried samples. This characteristic was attributed to membrane permeabilization induced by application of a high electric field.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

ABSTRACT

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

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.     

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.     

Drying kinetics and quality of carrots subjected to microwave-assisted drying preceded by combined pulsed electric field and ultrasound treatment

Abstract

In the study the impact of combined pulsed electric field and ultrasound pretreatment on microwave-assisted air drying kinetics and quality of dried carrot was evaluated. Pretreatment of samples has been realized through sequential application of PEF followed by US or vice versa. Sonication was performed using either the contact or immersive method whereas PEF treatment was carried out at 5?kV·cm^?1 and 10 pulses. After pretreatment the samples were subjected to microwave-assisted convective drying. Combined pretreatment reduced drying time by 27–49%. The highest retention of carotenoids (93.7%) was noted for samples subjected to PEF treatment followed by contact sonication. Optical properties of pretreated carrots were similar to reference samples (without pretreatment) and total color change ranged from 2.8 to 5.8. Application of immersive sonication, regardless of the sequence, resulted in the highest rehydration capacity and the highest loss of soluble solids. Despite samples subjected to combined pretreatment exhibited the most porous structure, the hygroscopic properties remained almost unchanged in comparison to the reference sample.     

Constant Rate Expressing of Juice from Biological Tissue Enhanced by Pulsed Electric Field

Abstract

Pulsed electric field (PEF) is an innovative technology, which can be successfully combined with solid/liquid expressing of juice from biological materials. The fresh juicy plants contain the juice in closed cells. The application of PEF ensures the electropermeabilization of cellular membranes, which facilitates the expulsion of liquid from the interior of cells. This article describes the influence of PEF on the kinetics of juice extraction from the layer of sugar beet particles expressed at a constant rate. Experimental study was carried out using a laboratory filter-press cell connected to a PEF treatment system. The PEF was applied by two different modes: as a pretreatment operation before pressing and as a treatment during pressing. The application of PEF to nonpressurized cake leads to increase of energy consumption and higher applied voltage. The PEF treatment of excessively pressurized cakes enhances the juice yield; however, the expressing of juice is significantly delayed. The best result is obtained when the sugar beet tissue is treated with PEF at 1.5–5 bars. The influence of PEF parameters on the juice yield was also studied. The intensity of PEF was varied from 0 to 1000 V/cm and the number of pulses was varied between 0 and 1000. It was established that the optimal parameters of PEF after the cake pressurization at 5 bar were in the next range: intensity of PEF E = 500 V/cm and duration of PEF application 0.03–0.05 s.     

Aerated Apple Leathers: Effect of Microstructure on Drying and Mechanical Properties

Fruit leathers are restructured products made from drying fruit purees until a leathery consistence is achieved. Aerated apple leathers (AAL) were created in order to incorporate air into the fruit structure and decrease the caloric density. AAL were prepared using gelatin as a foaming agent. Effects of gelatin concentration (0, 0.5, 1.0, and 1.5%) and whipping time (3, 5, 7, and 9 min) on microstructure, drying behavior, and mechanical properties of AAL were studied. Increasing the gelatin content and whipping time increased the gas hold-up up to 42.3% and decreased the mean bubble size of the foam. The time of drying decreased as the aeration level was increased. Samples with 1.5% gelatin and seven minutes’ whipping presented the highest drying rate, taking 2.8 h of drying to reach a moisture content of 0.13 kg H₂O/kg dry basis. The drying behavior of samples was satisfactorily modeled with a semi-empirical relationship. Mechanical properties showed differences among aerated products elaborated with different whipping times and gelatin contents. Aeration conferred unique characteristics to apple leathers in terms of appearance and mechanical properties.     

Drying of Potato Tissue Pretreated by Ohmic Heating

The influence of the ohmic heating pretreatment on convective drying of potato tissue was investigated. The degree of material damage Z under the different modes of electrical treatment is studied and the electroporation phenomena is assumed to be the main reason of material disintegration. The relations between Z, electric field strength E, energy input W, and temperature of ohmic heating T are analyzed. Two different damage stages, short and long, are revealed in this study. It is shown that ohmic treatment at relatively low fields (E < 100 V/cm) results in incomplete damage of material, but allows to obtain material with high disintegration degree Z ≈ 0.7 at E > 70 V/cm. The electric field strength E and the total electric energy input W influence considerably the drying rate. The highest drying rates are always observed for more energy consuming freeze-thawed pretreatment, but the AC pretreatment seems to be promising for enhancing the air drying processes.     

EHD-ENHANCED DRYING WITH WIRE ELECTRODE

ABSTRACT

The enhancement of drying rate by electrostatic field (corona wind) has been experimentally evaluated in this study. The samples used are 3 mm and 6 mm glass beads saturated with water. Two sets of experiments were conducted, with and without cross-flow, at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of drying enhancement using electric field (with or without cross-flow). Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. For EHD drying without cross-flow, the enhancement in drying rate increases with the applied voltage. With the presence of cross-flow, the enhancement in drying rate is nearly independent of the applied voltage and is close to the result obtained by drying with cross-flow alone.     

Scanning Electron Microscopic Study of Microstructure of Gala Apples During Hot Air Drying

Microscopic changes that occur in plant food materials during drying significantly influence the macroscopic properties and quality factors of the dried food materials. It is critical to study the microstructure to understand the underlying cellular mechanisms to improve the performance of food drying techniques. However, there is limited research on such microstructural changes of plant food material during drying. In this work, Gala apple parenchyma tissue samples were studied using a scanning electron microscope for gradual microstructural changes as affected by temperature, time, and moisture content during hot air drying at two drying temperatures: 57 and 70°C. For fresh samples, the average cellular parameter values were as follows: cell area, 20,000 µm²; ferret diameter, 160 µm; perimeter, 600 µm; roundness, 0.76; elongation, 1.45; and compactness, 0.84. During drying, a higher degree of cell shrinkage was observed with cell wall warping and an increase in intercellular space. However, no significant cell wall breakage was observed. The overall reductions in cell area, ferret diameter, and perimeter were about 60, 40, and 30%. The cell roundness and elongation showed overall increments of about 5% and the compactness remained unchanged. Throughout the drying cycle, cellular deformations were mainly influenced by the moisture content. During the initial and intermediate stages of drying, cellular deformations were also positively influenced by the drying temperature and the effect was reversed at the final stages of drying, which provides clues regarding case hardening of the material.     

Control of Sludge-to-Wall Adhesion by Applying a Polarized Electric Field

An application of a polarized electric field to reduce adhesion of a biological sludge to the dryer wall was studied experimentally by contact drying of a sludge cake deposited on a heated metal plate serving as the cathode. It was found that the adhesion intensity is greatly reduced due to a water layer formed at the heated plate, when a fraction of water in the sludge migrates to the cathode by electro-osmosis, as well as by gas bubbles, which are generated by the electrochemical reaction. The voltage gradient from 4 to 5 V/cm was found optimal for drying 10-mm-thick sludge samples. Under these conditions, the electric energy consumption amounted to 3.87 kWh/t. An increase in cathode temperature reduced the sludge-adhering intensity and shortened drying time. The beneficial effect of an electric field on reducing sludge adherence decreases with increasing wall temperature.     

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.