Estimation of Effective Moisture Diffusivity of Okra for Microwave Drying

The effect of microwave output power and sample amount on effective moisture diffusivity were investigated using microwave drying technique on round okra (Hibiscus esculentus L.). The various microwave output powers ranging from 180 to 900 W were used for the determination of effective moisture diffusivity for constant sample amount of 100 g okra. To examine the effect of sample amount on effective moisture diffusivity, the samples in the range of 25–100 g were dried at constant microwave output power of 360 W. By increasing the microwave output powers and decreasing the sample amounts, the effective moisture diffusivity values ranged from 20.52 × 10^?10 to 86.17 × 10^?10 and 34.87 × 10^?10 to 11.91 × 10^?9 m²/s^?1, respectively. The modeling studies were performed to illustrate the relationship between the ratio of the microwave output power to sample amount and effective moisture diffusivity. The relationship between drying constant and effective moisture diffusivity was also estimated.     

THIN-LAYER DRYING CHARACTERISTICS AND MODELING OF CELERY LEAVES UNDERGOING MICROWAVE TREATMENT

The effect of microwave drying on moisture content, moisture ratio, drying rate, drying time, and effective moisture diffusivity of celery leaves (Apium graveolens) was investigated. By increasing the microwave output power from 180 to 900 W the drying time decreased from 34 to 8 min; by increasing the sample amount from 25 to 100 g, the drying time increased from 25 to 49 min. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among the models proposed, the semi-empirical Midilli et al. model gave a better fit for all drying conditions applied. The activation energy was calculated using an exponential expression based on the Arrhenius equation. The relationship between the drying rate constant and effective moisture diffusivity was also estimated and gave a linear relationship.     

Microwave Heat Treatment of Spinach: Drying Kinetics and Effective Moisture Diffusivity

The effect of microwave drying technique on moisture content, moisture ratio, drying rate, drying time, effective moisture diffusivity, and porosity of spinach (Spinacia oleracea L.) were investigated. By increasing the microwave output powers (180–900 W) and the sample amounts (25–100 g), the drying time decreased from 18 to 3.5 min and increased from 7.7 to 25 min, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among of the models proposed, Page's model gave a better fit for all drying conditions applied. The activation energy was calculated using an exponential expression based on Arrhenius equation. The relationship between the drying rate constant and effective moisture diffusivity was also estimated and gave a linear relationship.     

Estimation of Effective Moisture Diffusivity of Okra for Microwave Drying

The effect of microwave output power and sample amount on effective moisture diffusivity were investigated using microwave drying technique on round okra (Hibiscus esculentus L.). The various microwave output powers ranging from 180 to 900 W were used for the determination of effective moisture diffusivity for constant sample amount of 100 g okra. To examine the effect of sample amount on effective moisture diffusivity, the samples in the range of 25-100 g were dried at constant microwave output power of 360 W. By increasing the microwave output powers and decreasing the sample amounts, the effective moisture diffusivity values ranged from 20.52 × 10^-10 to 86.17 × 10^-10 and 34.87 × 10^-10 to 11.91 × 10^-9 m²/s^-1, respectively. The modeling studies were performed to illustrate the relationship between the ratio of the microwave output power to sample amount and effective moisture diffusivity. The relationship between drying constant and effective moisture diffusivity was also estimated.     

Effective Diffusivities and Energy Consumption of Whole Fruit Chinese Jujube (Zizyphus jujuba Miller) in Microwave Drying

Microwave drying of whole fruit Chinese jujube was performed at 45, 90, and 135 W. Ten commonly used mathematical models of thin-layer drying were compared. The Midilli model was best in describing drying time dependency of product moisture ratios. The initial drying rate and drying rate constant are linearly proportional to microwave power level. An effective diffusivity model was presented and validated with the Renka-Cline algorithm. The model has very high predictive precision, suggested by the relative percentage error of 3.734% on average between the model and the Renka-Cline algorithm. The effective diffusivity was proved to be a linear function of microwave power level and a quadratic function of moisture content. Energy consumption in microwave drying of Chinese jujube decreased as microwave power increased from 45 to 135 W, but 90 W was adequate for high-quality products with less energy consumption.     

Dielectric Properties and Optimization of Parameters for Microwave Drying of Petroleum Coke Using Response Surface Methodology

The dielectric properties of petroleum coke at five temperatures between 20 to 100 ^° C, covering different moisture content levels at 2.45 GHz, were measured using an open-ended coaxial probe dielectric measurement system. The effects of drying temperature, duration of drying, and sample mass on the moisture content and dehydration rate of petroleum coke was assessed utilizing the response surface methodology. The dielectric constant, loss factor, and loss tangent were all found to increase nearly linearly with increase in moisture content. Three predictive empirical models were developed to relate the dielectric constant, loss factor, and loss tangent of petroleum coke as a linear function of moisture content from 3–10%. An increase in temperature between 20 to 100 ^° C was found to increase the dielectric properties. The penetration depth was observed to increase linearly with decrease in moisture content in the range of 3 to 10%. A predictive empirical model was developed to calculate penetration depth for petroleum coke. Two mathematical models were established and analyzed using RSM to describe the relationship between the microwave drying conditions and the responses, moisture content, and dehydration rate. Statistical analysis with response surface regression showed that microwave drying temperature, duration of drying, and sample mass were significantly related to moisture content and dehydration rate. Based on the RSM analysis, the optimum process conditions were estimated to be a microwave drying temperature of 75 ^° C, drying duration of 10 sec, and sample mass of 60 g, with the resultant moisture content being 0.34 at a dehydration rate of 2.94 g/min.     

Microwave,Vacuum, and Air Drying Characteristics of Collard Leaves

Collard leaves (Brassica oleracea L. var. acephala) with an initial moisture content of 6.65 on percentage dry basis (%db) were dried by three different drying methods: microwave, air, and vacuum. Samples of fresh leaves, 25 g each, were dried until their moisture was down to 0.1 on a dry basis. The following drying levels were used in each of the drying processes: 350, 500, 650, 750, 850, and 1000 W for microwave drying; 50, 75, 100, 125, 150, and 175°C for air drying; and 0.4, 50, and 100 mmHg at 50 and 75°C for vacuum drying, respectively. Drying times ranged between 2.5 to 7.5 min, 8 to 210 min, and 35 to 195 min for microwave, air, and vacuum drying, respectively. The data obtained compared well with a thin-layer drying model. Microwave drying at 750 W provided optimal results with respect to drying time, color, and ascorbic acid content (vitamin C).     

Optimization of Microwave-Vacuum Drying of Button Mushrooms Using Response-Surface Methodology

Button mushrooms (Agaricus bisporous) were dried in a microwave-vacuum dryer up to a final moisture content of around 6% (d.b.). The effect of microwave power level (115 to 285 W), system pressure (6.5 to 23.5 kPa), and slice thickness (6 to 14 mm) on drying efficiency and some quality attributes (color, texture, rehydration ratio, and sensory attributes) of dehydrated mushrooms were analyzed by means of response surface methodology. A rotatable central composite design was used to develop models for the responses.Analysis of variance showed that a second-order polynomial model predicted well the experimental data. The system pressure strongly affected color, hardness, rehydration ratio, and sensory attributes of dehydrated mushrooms. A lower pressure during drying resulted in better quality products. Optimum drying conditions of 202 W microwave power level, 6.5 kPa pressure, and 7.7 mm slice thickness were established for microwave vacuum drying of button mushrooms. Separate validation experiment was conducted at the derived optimum conditions to verify the predictions and adequacy of the models.     

Drying kinetics and effective moisture diffusivity of purslane undergoing microwave heat treatment

The effects of microwave drying on moisture content, moisture ratio, drying time and effective moisture diffusivity of purslane leaves (Portulaca oleracea L.) were investigated. By increasing the microwave output power (180–900W) and the sample amounts (25–100 g), the drying time decreased from 43 to 12.5 minutes and increased from 27 to 54 minutes, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among the models proposed, the semi-empirical Midilli et al. model gave a better fit for all drying conditions applied. By increasing the microwave output power and decreasing the sample amount, the effective moisture diffusivity values ranged from 5.913×10⁻¹¹ to 1.872×10⁻¹⁰ m²/s and from 9.889×10⁻¹¹ to 3.292×10⁻¹¹ m²/s, respectively. The activation energy was calculated using an exponential expression based on the Arrhenius equation.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

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

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

ABSTRACT

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

Effect of Calcium Ion and Microwave Power on Structural and Quality Changes in Drying of Apple Slices

Effects of impregnated calcium ion and microwave (MW) power on texture, rehydration, shrinkage, color, and other selected indexes of freeze-dried apple slices were investigated. Apple slices were dried by freeze drying and microwave freeze drying (MFD) separately. Vacuum impregnated (VI) and non-vacuum-impregnated apple slices were dried at various microwave power levels. Non-vacuum-impregnated apple slices were found to preserve their mechanical structure better than the VI-treated ones. Microwave application resulted in decreased protopectin fraction as well as total pectin content. As microwave power varied from 1.2 to 2.0 W/g, the total pectin content decreased from 0.810 to 0.521 (expressed as g galacturonic acid/100 g fresh sample). The effect of microwave power in decreasing the drying time was significant. The color of MFD apple slices varied to a minor extent with MW power level over the range of parameters studied.     

Microwave Drying Kinetics of a Thin-Layer Liquorice Root

Liquorice root (LR) (Glycyrrize glabra) is known as a sweetener and medicine plant. Drying kinetics of LR with initial moisture content of 49.5% (wet basis (w.b)) were experimentally investigated in a microwave drying system. The drying experiments were carried out at different drying temperatures (40, 45, 50, and 55°C) and microwave power levels (250, 500 and 750 W). Several models from literature were selected to fit the experimental data. The fit quality of models was evaluated using the coefficient of determination (R²), sum square error (SSE), and root mean square error (RMSE). A new model has been proposed for LR drying in the microwave drying. This new model best describes the experimental data for LRs. The activation energy was calculated to be 46.807 kJ/mol and effective diffusivity ranged from 2.9 × 10^?9 to 5.41 × 10^?9 m²/s, depending on drying temperatures at constant microwave power level.     

Modeling Microwave Vacuum Drying Kinetics and Moisture Diffusivity of Carrot Slices

Carrot slices of 3.5 mm thickness were dried in a laboratory microwave vacuum dryer at five different microwave power density levels of 2, 4.66, 7.33, 10, and 12.66 W/g and at three vacuum chamber pressure levels of 6.66, 19.98, and 33.3 kPa to 4–6% d.b. moisture content. Inside the dryer the sample holding plate was rotated with the speed of 4 rpm for uniform microwaves application. The drying rates were increased with the increase in microwave power density at all pressure levels and the Page model was found to be the most suitable model to predict the drying behavior of carrot slices at all process conditions. The Page model drying rate constant (k, min^?1) showed high correlation with microwave power density at constant pressure by a power law equation and showed a logarithmic relationship with the microwave power density and pressure. Similar to the drying rate constant, the average moisture diffusivity at constant pressure was found to be function of microwave power density by power law equation as well as was also dependent on the power density and pressure by a logarithmic relationship.     

Characterization and Process Optimization of Microwave Drying of Plaster of Paris

The changes in the characteristics of plaster of Paris (pop) during drying operation under microwave irradiation conditions, namely surface morphology, effective moisture diffusivity, and absorption of microwave, were studied. The drying characteristics and kinetics of the process during microwave drying of plaster were studied for rectangular-faced cuboids (80 × 70 × 15, L × B × H in mm) through various drying parameters like microwave power input, initial moisture content, and drying time. Further, the experimental data on moisture ratio of plaster for different operating conditions were obtained and the optimization of the microwave drying process parameters was performed with response surface methodology (RSM) by considering all the above-said independent variables. Based on the RSM analysis, the optimum values of the process variables were obtained as: initial moisture content (A) 60%; microwave power input (B) 180 W; and drying time (C) 480 S.     

Microwave drying of wastewater sludge: Experimental and modeling study

This study investigates experimentally and using mathematical modeling the microwave drying of wastewater sludge with determination of moisture diffusivity at different drying conditions. The drying behavior was observed at different power levels (480, 840, and 1,080 W) and different initial masses (90, 120, and 150 g). The observed drying kinetics were divided into three parts: a short adaptation period, a long constant drying rate period, and a falling drying rate period. The maximum drying rate was observed during the constant rate period. Mainly, the results show that the drying rate decreases with the initial mass increase (from 0.45 kg·kg^?1·min^?1 for 90 g to 0.25 kg·kg^?1·min^?1 for 150 g) and increases with an increase in power level (from 0.15 kg·kg^?1·min^?1 at 480 W to 0.45 kg·kg^?1·min^?1 at 1,080 W). The measurement of the sample dimensions shows that shrinkage can occur and, depending on the drying conditions, it ranged between 0.42 and 0.37 of the sample initial volume. Presenting a more accurate solution of the diffusion model by incorporating shrinkage and finite dimensions of the sample is the novelty of this study. The drying conditions influenced the diffusion coefficient, which ranged from 1.53 × 10^?7 to 7.67 × 10^?7 m²s^?1. Similar to the drying rate, the diffusion coefficient was directly proportional to the power level and inversely proportional to the initial mass. Activation energy was determined using an Arrhenius relationship of the diffusion coefficient as a function of the ratio initial mass to the power level.     

Characteristics of Chard Leaves during Microwave,Convective, and Combined Microwave-Convective Drying

Chard leaves (Beta vulgaris L. var. cicla), which weighs 25 g with a moisture of 9.35 (db), were dried using three different drying methods, microwave, convective, and combined microwave-convective. Drying continued until leaf moisture fell down to 0.1 (db). Drying periods lasted 5–9.5, 22–195, and 1.5–7.5 min for microwave, convective, and combined microwave-convective drying, respectively, depending on the drying level. In this study, measured values were compared with predicted values obtained from Page's semi-empirical equation. Optimum drying period, color, and energy consumption were obtained for combined microwave and convective drying. The optimum combination level was 500 W microwave applications at 75°C.     

Optimizing Drying Conditions for Vacuum-Assisted Microwave Drying of Green Peas (Pisum sativum L.)

Green peas were dried in a vacuum-assisted microwave drying system. The effects of microwave power levels (100–300 W) and system vacuum (50–400 mm Hg) on drying parameters (viz. drying efficiency and drying time) and some quality attributes (viz. linear shrinkage, apparent density, green color, rehydration, and sensory attributes) of dehydrated peas were analyzed by means of response surface methodology. A face-centered central composite design was used to develop models for the responses. Analysis of variance showed that a second-order polynomial model predicted well the experimental data. The system microwave power level strongly affected quality attributes of dehydrated peas and drying parameters. A higher vacuum during drying resulted in a better quality product. Microwave power of 237.31 W and a 360.22 mm Hg vaccum were found to be optimum drying conditions for vacuum-assisted microwave drying of green peas.     

Microwave-Assisted Thin Layer Drying of Wheat

Drying characteristics of Canada Western Red Spring (CWRS) wheat were studied using a domestic microwave convective oven. The effects of microwave power level, grain bed thickness, and initial grain moisture on the drying kinetics were investigated. Wheat samples with initial moisture levels of 0.18 to 0.29 kg water/kg of dry matter were dried for different drying periods of 180 to 360 s. The moisture loss data were recorded at regular short intervals. Then moisture loss data were fitted to various models (Page equation, modified drying equation, and Midilli equation) to study the drying kinetics of wheat. The results showed that wheat moisture loss increased with increasing microwave power level. A mathematical model was developed by coupling mass and energy balances, resulting in a system of non-linear equations. The predicted moisture loss data from the developed model were compared by fitting to experimental microwave data that were in good agreement.     

Optimization of Microwave Drying Biomass Material of Stem Granules from Waste Tobacco Using Response Surface Methodology

Tobacco stems, an underutilized waste in the tobacco industry, can be transformed into a viable product through the preparation of biomass material of stem granules. Response surface methodology (RSM) was used to optimize microwave drying of stem granules from waste tobacco. The effects of microwave power (35–40 kW), moisture content (25–35%), material thickness (30–50 mm), and drying time (90–150 s) on filling power and yield of stem granules were studied. Gas chromatography–mass spectroscopy (GC-MS) and scanning electron microscopy (SEM) were used to determine volatile components and microstructures of stem granules dried under optimal conditions. The filling power and yield of stem granules could be adequately fitted to a quadratic model (R ² = 0.951) and a two-factor interaction model (R ² = 0.887), respectively. The optimal conditions for microwave drying of stem granules were 35 kW, 30%, 30 mm, and 150 s. When prepared under optimal conditions, the filling power and yield of stem granules were 7.94 cm³/g and 64.06%, respectively, which differed by only 4.53 and 3.50% from model predictions. The quadratic and two-factor interaction models provided a reasonably accurate (<5% error) assessment of optimal conditions for microwave drying of biomass material of stem granules from waste tobacco stems.