Effective Moisture Diffusion Coefficient Research Articles

Effect of ultrasonic energy density on moisture transfer during ultrasound enhanced vacuum drying of honey

The purpose of this work is to explore the water content, distribution and stage changes of honey during ultrasound enhanced vacuum drying process. Ultrasound enhanced vacuum drying of honey was carried out at ultrasonic energy density levels of 0, 0.4, 0.8, 1.2, 1.6 W/g, respectively, and the drying kinetics model was established. The transverse relaxation time T2 inversion spectra of honey at different ultrasonic energy densities during drying process were measured and the changing characteristics of internal moisture state were analyzed by using the low-field nuclear magnetic resonance (LF-NMR). The results showed that the process of honey during ultrasound enhanced vacuum drying could mainly be divided into two periods, namely accelerating rate period and falling rate period. The range of effective moisture diffusion coefficient (Deff) was from 0.7879 × 10−7 to 2.1850 × 10−7 m2/s and the Deff values increased with the rise of ultrasonic energy density. According to the nonlinear fitting based on 9 dynamic models, two-term exponential model was determined by the optimal model, and the values of R2, χ2 and RMSE were 0.9991, 0.001 and 0.0091, respectively. The LF-NMR results showed that immobilized water was the main moisture in honey, which was 96.47%. As the drying proceeded, the relaxation time of immobilized water (T22) and free water (T23) decreased, which indicated that their mobility reduced gradually. The relaxation time of bound water (T21) showed little changes. The amplitude of immobilized water (A22) and free water (A23) decreased significantly at the initial stage of drying, and the amplitude of bound water (A21) decreased significantly at the last stage. Magnetic resonance imaging results revealed that the moisture distribution in honey was uneven and the brightness of images started to darken with the increase of drying time. Therefore, the results of this work demonstrate that NMR is a promising method to measure and quantify residual water content, water distribution and state of water of honey in real time during the drying process.

Heat and mass transfer enhancement during foam-mat drying process of lime juice: Impact of convective hot air temperature

Foam-mat drying has been recently welcomed as an effective technique since it lacks deficiencies that plague conventional methods including poor water absorption and long process time. In this study, effect of hot air temperature (50, 60 and 70 °C) on quality properties of lime juice was investigated during foam-mat drying. The process was also simulated to determine the effect of temperature on the uniformity of moisture and temperature distributions during drying. Increase in temperature improved powder flowability. With a decrease in temperature, water absorption index increased by 17.74%. Moreover, effective moisture diffusion coefficient increased as a result of a rise in temperature, which, in turn, increased the drying rate by 20.23%. Increase in temperature also resulted in a 1.80% increase in the total color change and 6.60%, 1.18% and 1.35% decrease in chroma index, hue angle and browning index, respectively. A correlation coefficient of 0.903 was achieved between experimental and simulated data. Uniform distributions of temperature and moisture during drying resulted in an improvement in the qualitative properties of the final product regarding the uniformity of the powder's moisture content, improvement of water absorption properties and, finally, increase in the product's quality.

Effect of Microwave Vacuum Drying on the Drying Characteristics, Color, Microstructure, and Antioxidant Activity of Green Coffee Beans

The aim of this study is to investigate the effect of microwave vacuum drying (MVD) on the drying characteristics and quality attributes of green coffee beans. We specifically focused on the effective moisture diffusion coefficient (Deff), surface temperature, glass transition temperature (Tg), water state, and microstructure. The kinetics of color changes during drying, total phenolic content (TPC), and antioxidant activity (DPPH, FRAP, and ABTS) were also characterized. Microwave power during MVD affected the porosity of coffee beans, their color, TPC, and antioxidant activity. The Allometric 1 model was the most suitable for simulating surface temperature rise kinetics. Thermal processing of green coffee beans resulted in increased b*, L*, ΔE, and TPC values, and greater antioxidant capacity. These findings may provide a theoretical reference for the technical improvement, mechanisms of flavor compound formation, and quality control of dried green coffee beans.

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

ABSTRACTIn 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.

Influence of foam thickness on production of lime juice powder during foam-mat drying: Experimental and numerical investigation

The present study aimed at investigating the effect of foam thickness (0.004, 0.005, 0.006 m) on lime juice foam-mat drying at 50 °C and with ovalbumin (4% concentration) as the foaming agent. The process was also simulated in order to examine the effect of foam thickness on temperature and moisture distributions. The model was validated by comparing experimental data with model data. The increase in foam thickness led to a 3.11% decrease in water solubility index (WSI) and a 14.47% increase in water absorption index (WAI). By increasing foam thickness from 0.004 to 0.006 m, the drying rate decreased by 33.33%. Furthermore, the effective moisture diffusion coefficient (Deff) values ranged between 8.980 × 10−9 and 1.138 × 10−8 m2/s. The increase in foam thickness decreased total color change, chroma index, and browning index by 4.45, 3.88, and 2.19%, respectively. There was a good correspondence between experimental and simulation data. The correlation coefficient obtained by comparing experimental with predicted data was >0.91. The uniform distribution of temperature and moisture during the foam-mat drying process enhanced the qualitative properties of the final product, which in turn resulted in higher quality lime juice powder.

Drying kinetics of tobacco strips at different air temperatures and relative humidities

The effects of air temperature and relative humidity (RH) on the drying kinetics of two kinds of flue-cured tobacco strips (C3F and B3F) were studied using a thermogravimetric device in this work. The drying experiments were carried out with drying air temperatures of 60, 70, 80, and 90 °C and RH values of 0, 10, 20, 30, and 40% at a constant airflow. Taking the effect of RH into consideration, a modified Arrhenius-type equation of diffusivity was proposed. In addition, comparing five thin-layer drying models and five equilibrium moisture content models to describe the drying kinetics and the desorption isotherms of tobacco strips, high coefficients of determination (R2) and low reduced Chi-square (χ2) and residual sum of squares (RSS) values indicated that the Logarithmic model and the Modified Oswin model appeared to be the most suitable for predicting the moisture ratio and the moisture desorption relationship of tobacco strips. The effective moisture diffusion coefficient under different temperatures and RHs ranged from 1.68 × 10−11 to 6.81 × 10−11 m2 s−1 for C3F and from 1.62 × 10−11 to 6.68 × 10−11 m2 s−1 for B3F. A modified Arrhenius-type equation with an RH term was found to be effective for describing the drying behavior of two flue-cured tobacco strips, and the activation energy (Ea) values were 34.6 and 35.2 kJ mol−1 for C3F and B3F, respectively.

Vacuum drying of apples (cv. Golden Delicious): drying characteristics, thermodynamic properties, and mass transfer parameters

In this work, apples of cv. Golden Delicious were cut into slices that were 5 and 7 mm thick and then vacuum dried at 50, 60 and 70 °C and pressure of 0.02 bar. The thin layer model drying kinetics was studied, and mass transfer properties, specifically effective moisture diffusivity and convective mass transfer coefficient, were evaluated using the Fick’s equation of diffusion. Also, thermodynamic parameters of the process, i.e. enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG), were determined. Colour properties were evaluated as one of the important indicators of food quality and marketability. Determination of mass transfer parameters and thermodynamic properties of vacuum dried apple slices has not been discussed much in the literature. In conclusion, the Nadi’s model fitted best the observed data that represent the drying process. Thermodynamic properties were determined based on the dependence of the drying constant of the Henderson and Pabis model on temperature, and it was concluded that the variation in drying kinetics depends on the energy contribution of the surrounding environment. The enthalpy and entropy diminished, while the Gibbs free energy increased with the increase of the temperature of drying; therefore, it was possible to verify that variation in the diffusion process in the apple during drying depends on energetic contributions of the environment. The obtained results showed that diffusivity increased for 69%, while the mass transfer coefficient increase was even higher, 75%, at the variation of temperature of 20 °C. The increase in the dimensionless Biot number was 20%.

Thin-Layer Drying Characteristics of Microwave-Dried Radix Eremut (Eremurus Spectabilis)

The drying characteristics of radix eremut ( Eremurus spectabilis) were investigated using microwave drying method. The drying curves of radix eremut locate in the falling rate regime and time required for drying reduced by raising microwave powers ranging from 160 to 640 W. The experimentally obtained data were fitted to six models . The most accurate models for determining the thin layer drying characteristic of radix eremut were determined to be the Midilli and Page models as the best. Effective moisture diffusion coefficients (D eff ) were computed utilizing Fick’s second law equation and the values of D eff found to be in range of 2.300 x 10 -9 to 1.690 x 10 -8 m 2 .s -1 . Revised Arrhenius equation was implemented to identify the activation energy and its value was calculated as 148.82 W.g -1 . The energy efficiency raised with raising microwave power and moisture ratio.

Low temperature hot air drying of potato cubes subjected to osmotic dehydration and intermittent microwave: drying kinetics, energy consumption and product quality indexes

Hot-air drying is a slow energy-extensive process. Use of intermittent microwave (IM) in hot-air (HA) drying of food products is characterized with advantages including reduced process time, energy saving, and improved final quality. In this study, the effect of IM-HA drying following an osmotic dehydration (OD) pretreatment was analyzed on qualitative and quantitative properties of the output (i.e. effective moisture diffusion coefficient (Deff), shrinkage, bulk density, rehydration and energy consumption). Temperature and airflow velocity were fixed at 40°C and 1 m/s, respectively. The process variables included sucrose solution concentration at five levels (0 or control, 10, 30, 50 and 70 w/w%), microwave output power at four levels (0 or control, 360, 600 and 900 W), and pulse ratio at four levels (1, 2, 3 and 4). Use of osmotic dehydration in combination with IM-HA drying reduced the drying time by up to about 54%. Increasing the osmotic solution concentration to 30% and using higher pulse ratios increased the Deff. The lowest shrinkage and bulk density as well as the highest rehydration belonged to the 900 W microwave power and pulse ratio of 4. The lowest energy consumption was observed when using the 900 W power level, showing 63.27% less consumption than the HA drying method.

Multi-stage continuous and intermittent microwave drying of quince fruit coupled with osmotic dehydration and low temperature hot air drying

Abstract In recent years, using intermittent microwave (IM) to dry foodstuffs has been taken into consideration as one of the new drying methods in food industry. The aim of this research was to dry cubic pieces of pre-treated “quince” fruit by sucrose osmotic solution using IM – hot air (HA) drying at a low temperature (40 °C) in order to investigate the effects of this process on improving the dried product quality. The variables of the process included sucrose osmotic solution in 5 concentration levels of 0 (control), 10, 30, 50, and 70% (w/w) and microwave at 4 powers of 0 (control), 360, 600, and 900 W, with 4 pulse ratios of 1, 2, 3, and 4. Findings indicated that compared to control samples, the samples pre-treated by osmotic solution had lower effective moisture diffusion coefficient (Deff). However, Deff increased through increase in power and pulse ratio of the microwave. Increasing the concentration of the osmotic solution, power, and pulse ratio led to significant decreases in shrinkage. Due to high shrinkage, the quince samples dried by continuous microwave – HA method showed higher bulk density in comparison to the samples dried by IM – HA. In addition, samples dried by IM with low power showed the highest rehydration followed by those dried by IM with high power; however, the control samples dried merely by HA had the lowest rehydration. Moreover, a significant decrease in specific energy consumption was noticed through increasing the microwave power. Drying by IM – HA at the power of 900 W and the pulse ratio of 4 had the lowest specific energy consumption, while drying with only HA had the highest specific energy consumption.

The experimental new hybrid solar dryer and hot water storage system of thin layer coffee bean dehumidification

The thin layer coffee beans dehumidification and hot water storage were investigated in solar greenhouse dryer simultaneously. The thermal energy was stored for use in the absence of sunlight. The conditions were studied the flow rate of water circulating, model of installation for solar collector assistance, the area ratio of product dehumidification to solar hot water producing (Ad:Ac), flow pattern of water circulation and capacity tank of water circulating in the system. The experiments were found the flow pattern of water circulation within the force flow 0.20 kg/s-mˆ2, tray products temperature (45 deg. C) at (Ad:Ac) 1:1, capacity tank of water circulating (60 L). The thermal energy can be used and stored in the form of hot water and reused at a time without sunlight as well. The initial coffee beans moisture content was dropped from 55 to below 12 (%w.b.) in 12 h drying time. The optimum of mathematical equations for thin layer coffee beans drying based on Midilli mathematical model i.e. (MR = a exp(-ktˆn) – bt); k = 0.03838, n = 1.56771, a = 1.03046, b = −0.00477, R2 = 0.9896, RMSE = 0.0420. Moreover, the effective moisture diffusivity coefficient of 9.754 × 10ˆ-11 mˆ2/s.

Influence of microwave vacuum drying on glass transition temperature, gelatinization temperature, physical and chemical qualities of lotus seeds.

This study investigated the effects of microwave power density on effective moisture diffusion coefficient (Deff), glass transition temperature (Tg), gelatinization temperature (TP), physical and chemical qualities of lotus seeds during microwave vacuum drying. Deff increased by 42% and 127% at 15W/g and 20W/g, respectively, when compared with 10W/g. TP was negatively correlated with the relaxation times of T21 and T22, while Tg was negatively correlated with the relative areas A22. The rates of change of color were observed to be divided roughly into two periods, consisting of a rapid change caused by enzymatic browning and a slow change caused by non-enzymatic browning. An equation is provided to illustrate the relationship of k1 and k2 of Peleg's model depending on power density during rehydration kinetics. The samples at 20W/g exhibited the higher content of amino acid (540.19mg/100gd.b.) while lower starch (17.53g/100gd.b.).

Application and Simulation of Distribution Function in Grape Drying Process

In order to investigate the influence factors of Weibull distribution function and its application in drying, the drying process of grape slices in different microwave vacuum drying power (330, 460, 590W) and vacuum degree (-50, -70, -90kPa) was taken as the research object, and the Weibull distribution function was used to simulate and analyze its drying kinetic curves, the results showed that the Weibull distribution function was able to simulate microwave vacuum drying process of grape slice well: scaling parameterαwas related with microwave power and vacuum degree, and decreased with the rise of them; the influence of drying variable on the form parameterβwas relatively low. The estimated effective moisture diffusivity coefficient in the dry process was obtained by calculating, its value increased with the rise of microwave power and vacuum degree in the scope from 3.45078× 10 to 6.74613× 10m/s; through Arrhenius equation, the activation energy of drying was calculated respectively 1.34701, 1.49099 and 1.57108 W/g under the vacuum degree of -50, -70 and -90kPa. This study has provided the technical basis for the application of Weibull distribution function in microwave vacuum drying technology of grape slices.

Infrared thin layer drying of saffron (Crocus sativus L.) stigmas: Mass transfer parameters and quality assessment

Saffron is the most precious and expensive agricultural product. A dehydration treatment is necessary to convert Crocus sativus L. stigmas into saffron spice. To the best of our knowledge, no information on mass transfer parameters of saffron stigmas is available in the literature. This study aimed at investigating the moisture transfer parameters and quality attributes of saffron stigmas under infrared treatment at different temperatures (60, 70, …, 110°C). It was observed that the dehydration process of the samples occurred in a short accelerating rate period at the start followed by a falling rate period. The effective moisture diffusivity and convective mass transfer coefficient were determined by using the Dincer and Dost model. The diffusivity values varied from 1.1103×10−10m2·s−1 to 4.1397×10−10m2·s−1 and mass transfer coefficient varied in the range of 2.6433×10−7–8.7203×10−7m·s−1. The activation energy was obtained to be 27.86kJ·mol−1. The quality assessment results showed that the total crocin content increased, when the temperature increased up to 90°C but, in higher temperatures, the amount of crocin decreased slightly. The total safranal content of the samples decreased slightly when drying temperature increased from 60°C to 70°C and then continuously increased up to 110°C. Also, the amount of picrocrocin increased from 83.1 to 93.3 as the drying temperature increased from 60°C to 100°C.

Process of Producing Parboiled Rice with Different Colors by Fluidized Bed Drying Technique Including Tempering

Steamer is utilized to gelatinize rice starch. High pressure or long steaming time is conventionally applied to obtain the dark brown color of the product. A new alternative method to produce dark brown parboiled rice was proposed in this work. High temperature fluidized bed drying technique including tempering was therefore explored to determine the operating condition to meet the requirement of light and dark brown parboiled rice along with high head rice yield. In addition, the couple of heat and mass transfer model was developed to determine the effective moisture diffusion coefficient, the temperature and moisture distributions within a grain kernel during drying. The effective diffusion coefficient was well correlated with grain temperature by Arrhenius equation. The drying temperature and moisture content after drying caused the drop of head rice yield. When the parboiled paddy at the intermediate moisture contents of 22 and 27% d.b. was tempered, the head rice quality was improved while the parboiled rice color was browner. To obtain high drying capacity, high head rice yield, and light brown color, the parboiled paddy should be dried at a maximum allowable temperature of 150 °C and tempered for 30 min. The tempering time should be extended to 60 min for the dark brown parboiled rice.

Convective drying of hawthorn fruit (Crataegus spp.): Effect of experimental parameters on drying kinetics, color, shrinkage, and rehydration capacity

Thin layer drying characteristics and physicochemical properties of hawthorn fruit (Crataegus spp.) were investigated using a convective dryer at air temperatures 50, 60 and 70°C and air velocities of 0.5, 0.9 and 1.3m/s. The drying process of hawthorn took place in the falling rate period, and the drying time decreased with increasing air temperature and velocity. The experimental data obtained during the drying process were fitted to eleven different mathematical models. The Midilli et al.’s model was found to be the best appropriate model for explaining the drying behavior of hawthorn fruit. Effective moisture diffusion coefficients (Deff) were calculated by Fick’s diffusion model and their values varied from 2.34×10−10m2/s to 2.09×10−9m2/s. An Arrhenius-type equation was applied to determine the activation energies. While the shrinkage decreased, the rehydration ratio increased with increasing air temperature and air velocity.

Influence of temperature on vacuum drying characteristics, functional properties and micro structure of Aloe vera (Aloe barbadensis Miller) gel

Aloe vera possesses therapeutic, antioxidant and some other functional properties. These properties may be affected by processing operations. The present study investigated the influence of operating temperature on vacuum drying characteristics, functional properties and the inner solid structure of the fresh aloe vera gel. The gel was dried at a constant pressure of 720 mm Hg in the drying chamber at varying temperature of 30–60°C. The experimental data of moisture ratio of Aloe vera were used to fit different models and the effective moisture diffusion coefficients and activation energy were also calculated. The Page model was found to be the best fit to experimental data. The functional properties like water retention capacity, fat absorption capacity, and swelling of the dried product were studied and found to be decreased with increasing operating temperature. The damage to the inner solid structure was more pronounced at higher temperatures because of faster mass transfer through the pores of the solid. The best quality product was obtained when the temperature was maintained at 30°C.

مدل سازی سینتیک افت رطوبت قطعات سیب زمینی پیش تیمار شده با اولتراسوند و مایکروویو در طی فرایند سرخ کردن عمیق

مدل سازی فرایند سرخ کردن در جهت آگاهی از روابط بین متغییرهای مختلف، می تواند گامی مناسب در جهت بهبود کیفیت محصول باشد. در این پژوهش، اثر پیش تیمار اولتراسوند تحت فرکانس های 28 و 40 کیلوهرتز به مدت 15 دقیقه و پیش تیمار مایکروویو با توان های 3 و 6 وات بر گرم به مدت 10 دقیقه بر میزان رطوبت سیب زمینی سرخ شده در درجه حرارت های 150، 170 و 190 درجه سانتی گراد به مدت 60، 120، 180 و 240 ثانیه مورد بررسی قرار گرفت. برازش داده های تجربی افت رطوبت نمونه ها در طی فرآیند سرخ کردن با استفاده از شش مدل تجربی پیشنهادی و قانون فیک انجام شد. نتایج نشان داد که هر دو پیش تیمار اولتراسوند و مایکروویو در کلیه سطوح مورد استفاده، میزان رطوبت نهایی نمونه ها را به طور معنی داری در سطح احتمال 5 درصد کاهش دادند. همچنین، با افزایش دما، میزان رطوبت نمونه های سیب زمینی به شکل غیرمعنی دار در سطح احتمال 5 درصد کاهش یافت. از طرف دیگر، با افزایش زمان فرایند، میزان رطوبت نمونه ها به طور معنی دار و بصورت نمایی کاهش پیدا کرد. همچنین، مدل های پیشنهادی در این پژوهش، با داشتن ضریب تبیین بالا (2R( و ریشه میانگین مربعات خطا (RMSE) پایین، به خوبی داده های تجربی را برازش نمودند. ضریب انتشار مؤثر رطوبت برای نمونه های مختلف در دامنه 8-10×57/3 الی 8-10×08/11 متر مربع بر ثانیه به دست آمد. اعمال پیش تیمارهای اولتراسوند و مایکروویو موجب افزایش ضریب انتشار مؤثر رطوبت و کاهش انرژی فعال سازی نمونه ها شدند.

Influence of Blanching Pretreatment on the Drying Characteristics of Cherry Tomato and Mathematical Modeling

Abstract The hot air drying with blanching pretreatment may provide a practical method for the production of dried cherry tomatoes. The influences of drying temperature (50°C, 60°C, 70°C and 80°C) and blanching on the drying kinetics of cherry tomatoes were studied. Nine mathematical models were evaluated and the determination of coefficient (R 2), chi-square (χ 2) and root mean square errors (RMSE) were compared. The Logarithmic model gave best results with R 2 of 0.9992 and 0.9995 for fresh and blanched cherry tomatoes, respectively. The values of effective moisture diffusivity coefficient D eff varied in the range of 1.7281 × 10−9 to 4.6306 × 10−9 m2 s−1 for the fresh cherry tomatoes, while 2.1034 × 10−9 to 6.6487 × 10−9 m2 s−1 for the blanched samples. The values of activation energy were 31.99 and 36.21 kJ mol−1 for the raw and blanched cherry tomatoes, respectively. Furthermore, the effect of temperature and blanching on color change of cherry tomatoes was measured.

Numerical modeling of heat and mass transfer during convective drying of cylindrical quince slices

A numerical model for non-steady heat and mass transfer during convective drying of cylindrical quince slices, with axis parallel to the air flow, is developed. The model is based on the numerical solution of the coupled one-dimensional heat and mass transport equations, assuming moisture transport due to Fick’s diffusion, with an effective moisture diffusion coefficient derived by fitting the analytical solution of the Fick’s law to experimentally derived drying curves, on the basis of an Arrhenius-type temperature dependence. The necessary convective heat and mass transfer coefficients are obtained from CFD calculations of the turbulent flow field around the slices using a commercial CFD package. A new correlation of the Nusselt number, as a function of Prandtl and Reynolds numbers is proposed for the specific geometric flow configuration. The model is validated against experimental data for different air stream velocities (1 and 2m/s) and temperatures (40, 50 and 60°C). The model was found to be robust, computationally efficient and able to capture with sufficient accuracy the time evolution of the temperature and the moisture loss, with a minimum need for experimental adjustment, and hence, is considered suitable from an engineering point of view.