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.     

微波干燥锰碳合金球的响应曲面法优化

为了研究微波干燥锰碳合金球新工艺过程,确定各因素对脱水率的影响情况,得到优化工艺参数,文中采用响应曲面法中心组合设计对工艺过程进行设计并分析了干燥温度、干燥时间、物料质量及其交互作用对相对脱水率的影响,建立了关于相对脱水率的数学模型。方差分析表明,干燥温度、干燥时间对相对脱水率有较显著影响,建立的数学模型拟合度良好,获得优化工艺参数:干燥温度90℃,干燥时间176 s,物料质量150.61 g,相对脱水率达到93.96%。实验验证表明,实际值与模型预测值相差0.71%,干燥后锰碳合金球的含水质量分数为0.26%,达到炼钢用锰碳合金球的水分指标。研究表明:模型预测结果可靠有效,应用响应曲面法优化得到微波干燥锰合金球的工艺条件合理可行,为工业化应用提供必要的工艺参数。     

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).     

湿法炼锌泥的微波干燥实验

目前回转窑干燥物料的方法主要存在干燥时间长、环境污染严重、效率低等缺点,为了解决这一问题,开展了微波干燥湿法炼锌泥的实验研究,采用实验室自制的微波干燥设备,考察微波功率、干燥时间以及物料质量对样品脱水率的影响。研究结果表明:在一定条件下,脱水率随干燥时间延长而增加,随着微波功率的增加而提高,随物料质量增加而降低。实验在微波功率600W、物料质量40g、微波加热时间3min时,湿法炼锌泥的脱水率达到99.57%;FT-IR分析可知,干燥后波数1625cm^-1和3300cm^-1附近水分子的伸缩振动峰基本消失,说明微波干燥后物料基本不含水分;与常规干燥进行比较,微波干燥湿法炼锌泥具有干燥时间短、脱水效率高、清洁无污染的优点,是一种高效节能的新型干燥方法。     

Drying of Fresh and Rewetted Shelled Corn in Microwave Fields

ABSTRACT

Drying of freshly harvested and artificially rewetted yellow dent shelled corn was studied experimentally under microwave conditions. The dehydration rate of rewetted corn was found to be higher than that of freshly harvested corn; the deviation in the dehydration rates was found to be a power law function of the moisture content. under similar conditions, freshly harvested corn has a lower drying rate relative to the rewetted samples. The difference between these drying rates varies by a power law relationship with kernel moisture content.     

IMPROVING ENERGY EFFICIENCY IN COMBINED MICROWAVE-CONVECITVE DRYING

ABSTRACT

Convective-microwave drying has been recognized as a convenient option for drying of valuable materials. However, the advantages of the method must be carefully evaluated in order to establish the limits for full scale operating conditions because, as demonstrated in this work, the reduction in the absolute drying time as a function of the applied microwave power is devalued by an increase in the unabsorbed microwave energy, decreasing the overall energy efficiency of the drying process.

Experimental study of the energy efficiency of combined microwave-convective drying of agar gel and Gelidium seaweeds was carried out in a laboratory scale microwave-convective dryer by continuously following the absorption of microwave energy by the sample as well as the change of the sample mass during the drying process, under different operating conditions. Several drying strategies based on the on-off application of the microwave power were proposed and evaluated from the point of view of both drying kinetics and energy efficiency in order to program and rationalize the use of microwave energy in the combined microwave-convective drying process.     

Optimization of microwave freeze drying strategy of mushrooms (Agaricus bisporus) based on porosity change behavior

ABSTRACT

For dehydrated foods, porosity is a crucial parameter which affects mass and heat transfers, and is related to the product quality. It is important to monitor porosity change behavior to optimize the drying process. To achieve faster drying along with high product quality, microwave freeze drying (MFD) was applied to mushrooms dehydration processing. The relationship between porosity (include total, open-, and closed-pore porosity) and dried mushroom qualities was studied, and a suitable microwave loading strategy was obtained. It was found that lots of open pores could transform to closed pores while the moisture content (MC) was below 0.25?±?0.05?g/g (db), and the closed-pore porosity was arrived at a relatively stable level at moisture content below 0.17?±?0.03?g/g (db). Both total porosity and closed-pore porosity had a significant influence (P?<?0.01) on texture of MFD mushroom, and open-pore porosity had a significant effect (P?<?0.01) on rehydration ratio of MFD mushroom. According to the porosity change behavior of mushrooms, a dynamic microwave loading strategy can be used to reduce drying time and keep product quality during MFD process.     

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.     

Modeling of Muti-layer Far-Infrared Dryer

Abstract

The heat and mass transfer models applicable to biological products at each element in a far-infrared fixed-bed dryer were established. The model was validated by comparing the simulation results with experimental data of black mushrooms (Lentinus edodes) in terms of moisture content and drying rate. The very good agreement suggests that the models for heat transfer and drying rate of biological products in the far-infrared fixed-dryer can be obtained by establishing the equations of radiation and dehydration in interspace elements. An unevenness of change in moisture content and drying rate on the same horizontal layer in the dryer was found. Thus it is necessary to take some measures to solve this nonuniformity in drying if the proposed dryer is employed.     

HEAT TREATMENT OF CHOPPED ALFALFA IN ROTARY DRUM DRYERS

This paper deals with the heat treatment of alfalfa chops during the high temperature dehydration process. It outlines the dryer characteristics, difficulties and potential errors in measuring temperatures in the dryer, computation techniques, the relationships between moisture and temperature during drying, and the potential effect of dehydration on the destruction of the insect Hessian Fly due to elevated temperatures.

From the analysis based on an existing computer model for dehydration of alfalfa chops, and the available field data, it is shown that the dried chops will attain a temperature of 90°C or higher when the input temperatures are between 500°C and 800°C. These conditions apply to the drying of wet alfalfa (moisture content more than 55 percent wet basis). The plant material loses a large portion of its moisture in the first few seconds in the dryer. The rapid release of moisture may cause the rupture or detachment of particles such as eggs, larva, pupa, and insects from the plant material. These small particles are exposed to an intense heat and rapid dehydration.     

Energetic Analysis of Drying Biological Materials with High Moisture Content by Using Microwave Energy

One of the most important parameters in the drying process is the efficiency of energy transfer. Although it is well known that the absorbed component of the referred energy in the dehydration process under microwave conditions is theoretically lower—especially during the falling rate period of drying—by an order of magnitude than that of convective drying, the real efficiency of energy transfer in the microwave field depends on numerous—partly unknown—factors. In this study, the most significant relations between the energy transfer efficiency and the drying parameters were investigated. During the experiments, two power levels were used to study the effects of domain variables in apple, potato, and onion dehydration processes. The results showed that variations in microwave power level played an important role in overall drying kinetics.

Based on the recorded data, we determined the typical energy efficiency factors for microwave drying of apple, potato, and onion. The specific energy consumption was calculated to compare the energy efficiency of a single measurement series. It was evident that by applying higher power levels, the energy utilization was more efficient. Furthermore, we theoretically proved and experimentally showed the nonthermal effects of the microwave energy.     

Numerical Investigation on Dielectric Material Assisted Microwave Freeze-Drying of Aqueous Mannitol Solution

Abstract

The dielectric material assisted microwave freeze-drying was investigated theoretically in this study. A coupled heat and mass transfer model was developed considering distributions of the temperature, ice saturation and vapor mass concentration inside the material being dried, as well as the vapor sublimation-desublimation in the frozen region. The effects of temperature and saturation on the effective conductivities were analyzed based on heat and mass flux equations. The model was solved numerically by the variable time-step finite-deference technique with two movable boundaries in an initially unsaturated porous sphere frozen from an aqueous solution of mannitol. The sintered silicon carbide (SiC) was selected as the dielectric material. The results show that dielectric material can significantly enhance microwave freeze-drying process. For case of the dielectric field strength, E = 4000 V/m under typical operating conditions, the drying time is 2081 s, 30.1% shorter and 47.2% longer, respectively, than those for E = 2000V/m and E = 6000 V/m. The heat and mass transfer mechanisms during the drying process were discussed.     

Quality optimisation of combined osmotic dehydration and microwave assisted air drying of pineapple using constant power emission

Combination of osmotic dehydration with microwave assisted air drying offers increased flexibility for process control and product quality. Osmotic dehydration (55°Brix solution at 40 °C for 90 min) combined with microwave assisted air drying (MWAD) was tested on smooth cayenne pineapples. The influence of the four most relevant processing parameters (osmotic treatment time, microwave power, air temperature and air velocity) was studied using a 2⁴ circumscribed central composite experimental design. The product quality was evaluated in terms of charred appearance at the surface, moisture content, soluble solids content, water activity, firmness, colour and volume. Microwave power and air temperature were the two most important processing parameters that influenced the quality of the dehydrated pineapple, with the parameters most affected by the operating conditions being water content and percentage of charred pieces. Only in the latter was a significant quadratic effect found, all others were approximately linear. There was also a significant interactive effect between microwave power and air temperature affecting the percentage of charred pieces. Model predictions using a quadratic surface for water content and % charred pieces were validated with an additional experiment. Quadratic models were used to indicate optimum drying conditions for various targets.     

DRYING RELATED PROPERTIES OF APPLE

ABSTRACT

Drying related properties of apple are evaluated for various different drying methods (namely, convective, vacuum, microwave, osmotic and freeze drying), and their corresponding process conditions. The examined properties are drying kinetics, equilibrium material moisture content, density, porosity, color and viscoelastic characteristics. The effect of various process factors on these properties is described through particular mathematical models. The model parameters are estimated by fitting the corresponding model equations on a wide range of experimental data. Drying kinetics is greatly affected by the characteristic particle size and drying air temperature for convective drying, while for the case of microwave drying they are affected by the vacuum pressure and the emitted radiation power. Equilibrium material moisture content is affected by the temperature and the humidity of the surrounding air, while the osmotic pretreatment shifts the sorption isotherms to higher water activity levels. The quality properties examined, are significantly affected by the drying method. More specifically, osmotic dehydration decreases the porosity of the final product, while it prevents color deterioration and enchances the viscous nature of dehydrated apple. Freeze-dried apples develop the highest porosity, have the most elastic structure and the lowest rate of color deterioration.     

Application of Artificial Neural Networks (ANNs) in Drying Technology: A Comprehensive Review

Inspired by the functional behavior of the biological nervous system of the human brain, the artificial neural network (ANN) has found many applications as a superior tool to model complex, dynamic, highly nonlinear, and ill-defined scientific and engineering problems. For this reason, ANNs are employed extensively in drying applications because of their favorable characteristics, such as efficiency, generalization, and simplicity. This article presents a comprehensive review of numerous significant applications of the ANN technique to solve problems of nonlinear function approximation, pattern detection, data interpretation, optimization, simulation, diagnosis, control, data sorting, clustering, and noise reduction in drying technology. We summarize the use of the ANN approach in modeling various dehydration methods; e.g., batch convective thin-layer drying, fluidized bed drying, osmotic dehydration, osmotic-convective drying, infrared, microwave, infrared- and microwave-assisted drying processes, spray drying, freeze drying, rotary drying, renewable drying, deep bed drying, spout bed drying, industrial drying, and several miscellaneous applications. Generally, ANNs have been used in drying technology for modeling, predicting, and optimization of heat and mass transfer, thermodynamic performance parameters, and quality indicators as well as physiochemical properties of dried products. Moreover, a limited number of researchers have focused on control of drying systems to achieve desired product quality by online manipulating of the drying conditions using previously trained ANNs. Opportunities and limitations of the ANN technique for drying process simulation, optimization, and control are outlined to guide future R&D in this area.     

A kinetic study of microwave and fluidized-bed drying of a Chinese lignite

The drying kinetics of Chinese lignite in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave were investigated. The changes in the mass as a function of drying time were measured under various drying conditions. The variations of moisture ratio with time were used to test ten different thin-layer empirical drying models given in the literature. In studying the consistency of all the models, some statistical tests, such as χ², residual sum of squares (RSS) and F-value were also used as well as coefficient of determination R². In nitrogen fluidized-bed and superheated steam fluidized-bed, the Midilli–Kucuk model best described the lignite drying process. Drying data in microwave were best described by the Page model, indicative of a difference in kinetics between the two drying methods. This difference was attributed to different heat transfer mechanisms under conventional and microwave drying conditions. The effects of drying parameters in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying on the constants and coefficients of the selected models were studied by multiple regression analysis. The apparent diffusion coefficient of moisture in samples was obtained from the kinetics data and the apparent activation energies under nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying were found to be rather similar.     

Optimization of Heat Pump–Assisted Intermittent Drying

In the present study, heat pump–assisted drying of salak fruit was optimized by dividing the dehydration process into three distinct phases, namely, the initial, intermittent, and final stages. Drying variables considered for the optimization were the intermittent duration (X ₁), intermittent ratio (X ₂), and intermittent cycle (X ₃); the response variables studied were the total drying time (Y ₁), total heating time during intermittent drying (Y ₂), total heating time after intermittent drying (Y ₃), total color change (Y ₄), ascorbic acid content (Y ₅), and total phenolic content (Y ₆). Response surface methodology was used to determine the best combination of the drying variables that could provide the shortest drying period and premium product quality. Experimental results showed that all of the response variables were improved under the optimized intermittent drying conditions compared to the conventional method using constant drying conditions. The optimized heat pump–assisted intermittent drying reduced the drying time by 36% and improved phytochemicals retention with ascorbic acid and total phenolic content recorded at 18.4 ± 1.8 mg ascorbic acid/100 g dw and 43.3 ± 2.2 mg gallic acid equivalent (GAE)/g dw, respectively. The color change of the final product was minimum with a ΔE* value of 7.26 ± 2.03.     

微波干燥八水氢氧化锶的研究

提出了利用微波干燥八水氢氧化锶的新工艺,探讨了主要影响因素对样品相对脱水率的影响,其优化干燥条件为：物料质量50g、物料厚度5mm、微波干燥时间3min、微波功率950W,此时八水氢氧化锶的相对脱水率达到99．95％;与常规电加热干燥相比,微波干燥时间仅为常规干燥时间的1／40,而且微波干燥的最大相对脱水率提高了5．35％。     

Energy and drying time optimization of convective drying: Taguchi and LBM methods

In this study, a novel method for numerical simulation of drying is proposed and the process is optimized by Taguchi method. A 2D numerical solution is performed to analyze coupled heat and mass transfer occurring during drying of a rectangular moist object. The dryer section and the moist object are conjugately simulated where the coupled heat and mass transfer equations are solved together. The lattice Boltzmann method is employed to solve hydrodynamic, heat, and mass transfer equations. This study applied the Taguchi method to determine optimum conditions for drying so as to minimize the drying time and energy consumption. The control factors included temperature, air velocity, and thickness ratio (the moist object thickness to channel width). The following optimal conditions were obtained: temperature (T?=?60?°C), velocity (V?=?0.1 m/s), and thickness ratio (TR =0.1). The results of numerical solution are then compared to the measured data available in the literature, presenting a reasonable agreement.     

The effects of microwave heating on the kinetics of isothermal dehydration of equilibrium swollen poly(acrylic‐co‐methacrylic acid) hydrogel

The isothermal dehydration of poly(acrylic‐co‐methacrylic acid) (PAM) hydrogel under microwave heating (MWH) was investigated. The isothermal kinetics curves of the PAM hydrogel dehydration at temperature range from 293 K to 333 K were recorded. Based on the differential isoconversion method it was concluded that the microwave dehydration of poly(acrylic‐co‐methacrylic acid) hydrogel is an elementary kinetics process. Applying the model‐fitting method it was established that the kinetics of microwave isothermal hydrogel dehydration can be described by the kinetics model of the phase‐boundary controlled process (contracting area). The values of the kinetics parameters (activation energy (E_a) and preexponential factor (lnA)) of the dehydration process under microwave heating are lower than the values for conventional heating (CH). The established influence of MWH on the kinetics of hydrogel dehydration is explained with a specific activation mechanism of water molecules for dehydration and with the increase in the value of the energy of the ground level of the resonant oscillator of water molecule (v = 837 cm^?1) due to the absorption of microwave energy. POLYM. ENG. SCI., 56:87–96, 2016. © 2015 Society of Plastics Engineers