Experimental and numerical investigation of heat and mass transfer during drying of Hayward kiwi fruits (Actinidia Deliciosa Planch)

In this paper we undertake an experimental and numerical study on heat and mass transfer analysis during drying of kiwi fruits. In the experimental part, the effects of various drying conditions in terms of air velocity, temperature and relative humidity on drying characteristics of kiwi fruits are investigated. In the numerical part, the external flow and temperature fields are studied using a commercial CFD package. From these fields, the local distributions of the surface convective heat transfer coefficients for the fruits are determined to predict the local convective mass transfer coefficients through the analogy between the thermal and concentration boundary layers (known as the Chilton–Colburn analogy). In addition, the time-dependent temperature and moisture distributions for different cases are obtained using the code developed to investigate heat and mass transfer aspects inside the fruits. Numerical results are then compared with experimental data and a considerably high agreement is obtained.     

Heat and mass transfer during the coffee drying process

For a better comprehension of heat and mass transfer during the coffee drying process and optimization of the industrial application transport coefficients and coffee properties were determined. Heat transfer coefficients were measured for different air velocities and were found to follow the known dimensionless equations for the flow surrounding a sphere. Thermal conductivities and effective diffusion coefficients were measured as a function of moisture content as well as volume and densities of the coffee beans. The mentioned properties depend directly on the humidity of the coffee beans rather than on the drying conditions. Sorption behaviour was investigated and temperature dependent parameters for the Guggenheimer–Anderson–deBoer-isotherm (GAB) were determined according to the Arrhenius relationship.     

Effect of temperature on drying characteristics and quality of cooked rice

A shorter drying time, together with acceptable product quality, is preferred in food processing. To accomplish this requirement, the present work was studied on the effect of temperature on drying kinetics and quality of cooked rice. The long grain-jasmine rice was chosen as a raw material. The experiments were carried out at temperatures of 50, 60, 80, 100 and 120 °C. Effective diffusion coefficient was determined experimentally using a method of slopes. An empirical equation was developed to describe the effective diffusion coefficient as a function of temperature and moisture content. The developed equation was utilized to predict moisture content as the drying progressed and the prediction agreed with the experimental result relatively well. For the quality, the drying temperature affected the color of cooked rice, but it insignificantly affected shrinkage and rehydration capability of dried cooked rice. As examined by scanning electron microscope, morphologies of cooked rice dried at temperatures were similar, but they were relatively different to that of freshly cooked rice, with slightly more porous material after processing.     

Effect of osmotic dehydration on dielectric properties,microwave vacuum drying kinetics and quality of mangosteen

Mangosteen (Garcinia mangostana Linn.) with and without osmotic dehydration (OD) in sucrose solution was dried by microwave vacuum drying at 1200, 1440 and 1680 W. Because of water loss (49.12–49.98 g 100 g^?1) and solid gain (9.31–11.62 g 100 g^?1) during OD, dielectric constant, loss factor and loss tangent of mangosteen were significantly increased (P ≤ 0.05) to 24.82–25.12, 11.52–14.18 and 0.47–0.50, respectively. With the decreased initial moisture content and the modified dielectric properties, drying time of osmotically dehydrated mangosteen was shorter than that of mangosteen without OD. Moreover, an increase in microwave power enhanced drying kinetics. With OD, Tg of dried mangosteen was increased from ?7.01, ?3.00 to 11.11–25.96 °C. Hardness and lightness (seedless part) were significantly increased (P ≤ 0.05). Structure of dried seedless mangosteen was well protected, resulting in the improved rehydration ability (P ≤ 0.05). Nonetheless, rehydration ability of the mangosteen containing seed was not improved (P > 0.05).     

Modelling of coupled heat and mass transfer during a contact baking process

A mathematical model of coupled heat and mass transfer of a contact baking process is developed. In the current model formulation, a local evaporation of water is described with a reaction-diffusion approach, where a simultaneous diffusion and evaporation of water takes place. The resulting coupled model equations (unsteady state heat transfer, liquid water and water vapour) were solved using the Finite Element Method (COMSOL Multi-physics® version 3.5). During the baking process, local temperatures and overall moisture loss were measured continuously. The model - predicting temperature, liquid water content in the product and water in the vapour phase - was calibrated and partially validated using data obtained during baking of a representative food model (a pancake batter) under controlled conditions on a specially designed experimental rig. The unknown parameters in the model equations were estimated using the standard least squares method by comparing the measured with the predicted temperature profile. Good agreement was achieved between model predictions and the experimental values.     

A novel approach to evaluate the temperature during drying of food products with negligible external resistance to mass transfer

The paper deals with modeling the convective drying process. A relevant and reliable mathematical model that captures the history and distribution of temperature is presented. The attention is focused on the simultaneous heat and mass transfer occurring during drying where dry and hot air flows about the food. In the present study, external resistance to mass transfer is considered negligible. As a result, the drying curve is almost independent of the boundary conditions, which means that drying is diffusion-controlled. The main connotation of present study regards to undertake analytical procedure to establish the novel model for practical applications. The results show that the temperature evolution can be evaluated from an advanced analytical solution in a quick and efficient manner. The model is validated with the literature experimental data obtained for carrot and mango slabs. A good agreement is obtained between the model predictions and the available experimental results.     

Influence of drying temperature on the wet-milling performance and the proteins solubility indexes of corn kernels

The effects of air drying temperature on the wet-milling performance and the proteins solubility indexes were investigated for corn kernels dried between 54 °C and 130 °C. It was observed that when the drying temperature increases, the starch yield drops significantly. The gluten recovered increased abruptly for drying temperatures up to 80 °C. The albumin, globulin and zein solubility indexes decreased continuously when corn drying temperatures increased. According to the temperatures used, the starch yield, the gluten recovered and the salt-soluble proteins solubility indexes were adjusted satisfactorily by using a two asymptotic logistic model. This model has the advantage of supplying information on the dynamic of the variation of described parameters. The solubility index of total salt-soluble proteins was shown to be a suitable indicator of the severity of the drying treatment in regard to the corn wet-milling performance.     

Drying kinetics and quality changes during drying of red pepper

A mathematical model is proposed to simulate the process of drying of individual pieces of red pepper under constant external conditions and to predict changes in some nutritional and organoleptic attributes of the product. The model was solved numerically to obtain moisture content and temperature as well as ascorbic acid and carotenoids concentration in the product as a function of time. A good agreement between predictions and experimental data at different drying temperatures was obtained.Water sorption isotherms of red pepper were determined in the range 20-50 °C and represented by two different sorption equations. Drying kinetics were represented by a diffusive model, the effective moisture diffusivity ranging from 5.01 to 8.32×10⁻¹⁰ m²/s with an activation energy of 23.35 kJ/mol. Degradation kinetics for ascorbic acid and total carotenoids were measured in the range 50-70 °C and modelled as first-order reactions. The rate constants increased with temperature and product moisture content. Average activation energies for carotenoids and vitamin C degradation were 50.1 and 26.9 kJ/mol, respectively.     

Drying kinetics of the individual layer of cocoa beans during heat pump drying

Investigation was carried out to study the drying kinetics of the individual layer of cocoa beans, namely the testa and cotyledon, during heat pump drying. The cocoa beans were dried in thin layer using dehumidified air at 28.2 °C, 40.4 °C and 56 °C. Moisture contents of the bean, testa and cotyledon were recorded during drying. Reduction in moisture content was relatively fast at the testa as compared to the cotyledon in the initial two hours of drying. Subsequent drying showed a crossover region where the moisture content of the testa was higher than the cotyledon. The final moisture content of the testa was found to be higher than the inner cotyledon at the end of drying. This showed that moisture migrated from the cotyledon (lower moisture content) to the testa (higher moisture content) during drying. Although such transfer mechanism seems contradict but this can be further explained by the mass transfer potential concept as postulated by Luikov where the transfer of moisture is due to moisture potential difference instead of moisture content difference. Product quality analyses showed percent retention of cocoa polyphenols ranged from 44% to 73% as compared to freeze dried sample. Bean hardness was found reasonably comparable to the commercial sample and increased with decreasing moisture content.     

Shrinkage, vitamin C degradation and aroma losses during infra-red drying of apple slices

The goal of this study was to determine the effect of drying temperature on the shrinkage, ascorbic acid (vitamin C) degradation and aroma retention of apples. Apple samples were found to shrink continuously until a water content value of 2 kg/kg d.b. The degradation of ascorbic acid followed a pseudo-first-order reaction and the degradation rate constant increased when temperature increased from 40 to 70 °C. The loss of aroma volatiles increased with temperature and drying time.     

Modelling thermal degradation of zearalenone in maize bread during baking

The thermal degradation of zearalenone (ZEA) was investigated using a crust-like model, representing maize bread, which was prepared with naturally contaminated maize flour. Model samples were heated under isothermal conditions at the temperature range of 100–250°C. No reduction was observed at 100°C. Thermal degradation rate constants (k) were calculated as 0.0017, 0.0143 and 0.0216?min^?1 for 150, 200 and 250°C, respectively. Maize bread baked at 250°C for 70?min was used to test the capability of model kinetic data for the prediction of ZEA reduction. The time–temperature history in the crust and crumb parts was recorded separately. Partial degradation of ZEA at each time interval was calculated by means of the corresponding k-values obtained by using the Arrhenius equation, and the total reduction occurring at the end of the entire baking process was predicted. The reduction in the crumb and crust of bread was also experimentally determined and found to be consistent with the predicted values. It was concluded that the kinetic constants determined by means of the crust-like model could be used to predict the ZEA reduction occurring during baking of maize bread.     

Far-infrared and microwave drying of peach

Little detailed information is available for the far-infrared and microwave drying characteristics on peach and far-infrared combined with microwave drying on other food products. Experiments were conducted to study microwave and far-infrared dehydration characteristics and two-stage drying process involving far-infrared following microwave drying on peach. As microwave drying power and infrared drying power increased, dehydration rate of peach increased and whole drying energy consumption decreased. Peach experienced two falling rate periods when dried with microwave drying or far-infrared drying, and the first falling rate period under moisture content of peach more than 1.7 (dry basic, d. b.), the second falling rate period under less than moisture content 1.7 (d. b.). The same water loss will consume more energy and the steeper curve of energy versus moisture content were obtained when the moisture content is less than 1.7 (d. b.). However, differed from microwave drying, an accelerating dehydration rate period existed in the initial period of far-infrared drying. The effects of infrared drying power, microwave drying power and exchanging moisture content at former far-infrared drying converting into latter microwave drying (three factors) on energy consumption rate and sensory quality (two indices) are significant. The interaction effect of infrared drying power and exchanging moisture content on two indices is significant. The effects of second-order of microwave drying power and of interaction between infrared drying power and microwave drying power on energy consumption rate were not significant. The effects of second-order of exchanging moisture content and of interaction between exchanging moisture content and microwave drying power on sensory quality were not significant.     

热带亚热带果蔬渗透脱水研究进展

从渗透脱水的影响因素、渗透脱水对热带亚热带果蔬的影响以及渗透脱水传质动力学三个方面综述了果蔬渗透脱水的研究进展,其中重点介绍了渗透脱水的影响因素,同时对果蔬渗透脱水技术的局限性及其应用前景进行了探讨。     

Methodology for enhancing drying rate and improving maize quality in a fluidised-bed dryer

A theoretical diffusion model for a single maize (Zea mays L.) kernel has been developed to describe moisture migration during drying and tempering stages. The effective diffusivity was determined experimentally at a range of temperatures between 90°C and 170°C with initial moisture contents of 37% and 43% dry-basis. The functional relationship between the diffusivity and temperature is analogous to a modified Arrhenius equation.Two-stage drying including tempering between stages provides not only faster removal of moisture, but also a reduced number of stress cracks when compared to single-stage drying. However, the colour of the maize appears darker and more intense with longer tempering periods. To maintain maize quality and to reduce energy consumption, the results from the experiments and simulations are limited to temperatures of less than 150°C; high-moisture maize should be dried to 23% dry-basis and then tempered for 40 min.The ambient-air velocity for cooling the tempered maize in a range between 0.075 and 0.375 m/s does not affect the maize quality as measured by the number of stress cracks and colour.     

Heat and mass transfer in combined convective and far-infrared drying of fruit leather

Combined convective and far-infrared drying is a challenging assignment due to complex relationship between heat and mass transfer. In this paper, heat and mass transfer of fruit leather drying with combination of hot air and far-infrared has been carried out. The heat and the mass transfer coefficients were analyzed by heat–mass analogy. It could be found that the ratio between heat and mass transfer coefficients for the combination technique could not be obtained from the heat–mass analogy classical model and a modification is needed. The modified correlations for predicting ratio of heat and mass transfer coefficients and the heat transfer coefficient in term of heat transfer Nusselt number are developed. The model could fit the experimental data quite well within ±10% deviation.     

Study of banana dehydration using sequential infrared radiation heating and freeze-drying

The drying and quality characteristics of banana slices processed with a sequential infrared radiation and freeze-drying (SIRFD) method were investigated. Cavendish bananas slices with 5 mm thickness were predehydrated using IR heating at each one of three radiation intensities, 3000, 4000, and 5000 W/m² or hot air at 62.8 °C. The predehydrated samples with 20% and 40% weight reductions obtained using 4000 W/m² IR intensity were then further dried using freeze-drying for various times to determine the effect of predehydration on the drying rate during freeze-drying. To improve the quality of dried banana chips, the banana slices were also treated with a dipping solution containing 10 g/l ascorbic acid and 10 g/l citric acid before the IR predehydration. Control samples were produced using regular freeze-drying without the predehydration. The quality characteristics of dried banana chips, including color, thickness shrinkage and crispness, were evaluated. The predehydration results showed that the drying rate of IR heating was significantly higher than the hot air drying and increased with the increase of IR intensity. For example, it took 10 and 38 min to achieve 40% weigh reduction by using IR at 4000 W/m² and hot air drying, respectively. However, the banana slices with IR predehydration dried slower during freeze-drying compared to the samples without predehydration, which was due to texture changes that occurred during the predehydration. Acid dipping improved product color and also reduced freeze-drying time compared to non-dipped samples. It has been concluded that SIRFD can be used for producing high crispy banana chips and additional acid dipping improved product color and reduced required freeze-drying time.     

Determination of effective moisture diffusivity and assessment of quality attributes of banana slices during drying

The influence of drying temperatures on the moisture diffusivity and quality attributes of the dried banana slices in terms of volatile compound, shrinkage, color, texture and microstructure were studied. Bananas with peel color index of 5 corresponding to yellow color with green tip were sliced into 3 mm thickness, dipped into ascorbic acid solution and dried at four temperatures of 70, 80, 90 and 100 °C. Drying rate of banana slices can be divided into two sub-drying periods, first and second falling rate periods. The effective diffusivity estimated by the optimization technique was found to decrease sharply with moisture content in the first falling rate period and changed slightly in the second falling rate period. High-temperature drying seems to provide lower loss of volatile compounds in the dried sample. Moreover, the dried banana was very porous, resulting in remarkably lower hardness value than that obtained from the low-temperature drying whilst the crispiness was not significantly different amongst the samples obtained at various drying temperatures. Although the textural property could be improved at high temperature, the product color was brown as manifested by the low L- and hue values in particular at the drying temperature of 100 °C.     

Mass transfer of water and volatile fatty acids in cocoa beans during drying

In order to elucidate the phenomena involved in the remnant acidity of cocoa beans dried artificially, the diffusivities of water, and volatile fatty acids (VFAs: acetic, propionic, butyric and iso-butyric acids) in cocoa beans during drying were evaluated. Experimental drying kinetics of the acids were conduced at 40–60 °C with and without shell. Samples were taken at different drying times for moisture and acids content evaluation. VFAs content was evaluated by GC in a methanolic extract, and moisture content by a vacuum oven. Mass diffusivity was estimated from the fitting of experimental kinetics to a theoretical model that takes into consideration the beans’ shape. Acetic, propionic and butyric acids diffusivities were significantly (p < 0.05) smaller than water diffusivities both with and without shell. VFAs diffusivities were between 1/6 and 1/22 diffusivities values for water. Iso-butyric acid diffusivity was not statistically significant but the value was smaller than for the other VFAs. The diffusivities of VFAs may explain the remnant acidity in artificially dried cocoa beans.     

Microwave and convective dehydration of ethanol treated and frozen apple – physical properties and drying kinetics

The objective of this study was to examine how the drying kinetics and physical properties of apples are affected by pre-treatment with 95% ethanol or freezing at –18 °C before microwave-assisted air dehydration at 50, 60 and 70 °C. Microwave heating was used to obtain these temperatures in the centre of the apple cubes. After dehydration the shrinkage and rehydration capacity were measured. The texture of dehydrated and rehydrated samples was analysed with a puncture test in a texture analyser. Samples were also analysed with confocal laser scanning microscopy to determine the correlation between physical and microstructural properties. Diffusivity in the different dehydration processes was calculated. Ethanol-treated apples showed both high rehydration and high effective rehydration capacity compared with the other samples. Freezing before dehydration increased the diffusivity and reduced the firmness of rehydrated apples compared with no pre-treatment.