Intermittent Microwave Convective Drying Research Articles

Investigation of the Effects of Energy-Efficient Drying Techniques and Extraction Methods on the Bioactive and Functional Activity of Banana Inflorescence

Plant-derived foods with therapeutic potential have strong connection with both the pharmaceutical and nutraceutical industries. The effectiveness of these therapeutic properties is heavily influenced by the thermal treatment during drying and extraction methods. Traditional convective drying is a very energy incentive and lengthy process. Although some advanced and hybrid drying methods have been developed, these have not been applied in drying of banana inflorescence. In this study, we investigated the effects of freeze-drying (FD) and intermittent microwave convective drying (IMCD), as well as traditional convective oven drying (CD), on the polyphenol profile of banana inflorescence when extracted using the energy-efficient Accelerated Solvent Extraction method (ASE). Our findings revealed that the freeze-dried banana inflorescence powder exhibited the highest extraction of bioactive compounds when using 75% methanol at 100 °C as a solvent. It recovered 2906.3 ± 20.83 mg/100 g of the phenolic compounds and 63.12 ± 0.25% antioxidant activity under the optimal extraction conditions. While IMCD was found to be the second-best drying method in terms of preserving bioactive compounds, its operational time and cost were significantly lower compared to freeze-drying. Furthermore, our study confirmed the presence of medicinal compounds such as gallic acid, protocatechuic acid, caffeic acid, coumaric acid, catechin, ferulic acid, kaempferol, and quercetin in banana inflorescence. The development of innovative functional foods and pharmaceutical ingredients through green extraction methods and optimal drying conditions holds significant potential to save energy in the process, enhance human health, and promote environmental sustainability and circular economy processes. These efforts align with supporting Sustainable Development Goals (SDGs) 3 and 12.

Effect of intermittent microwave convective drying on quality characteristics of persimmon fruit

The present study investigates the quality parameters of dried persimmon (Diospyros kaki) fruit slice by the application of intermittent microwave convective drying (IMCD) method. For the experiment four microwave power 200, 300, 400 and 500 W and three pulse ratio cycle (PR) 1:5, 1:10 and 1:15 were used. Total of 12 experimental runs were conducted with different combinations of selected microwave power and PR to reduce the moisture content of persimmons fruit slice from 90±2% (w.b.) to a safer limit of around 9.86±1.62% (w.b.). Out of all the experimental runs the sample dried at microwave power of 500 W and PR of 1:5 found to have shorter drying time of 66 min and the longer drying time of 208 min was observed for the sample dried at 200 W and PR of 1:15. The functional properties such as total phenolic content (TPC), antioxidant activity and total flavonoid content (TFC) decreased as drying progressed whereas the physical quality parameter change in color was increased with increase in drying time. The functional properties were fitted with first and second order kinetic model, and second order kinetic model found to predict the data with higher accuracy when compared with first order kinetic model. The change in color followed zero order kinetic model with parameter ranging from 0.105 to 0.323min−1. Overall, persimmon fruit slice dried at IMCD condition with power 500 W and PR 1:5 had the highest functional property with lower change in color.

Study on Intermittent Microwave Convective Drying Characteristics and Flow Field of Porous Media Food

Numerical simulations were carried out for moist, porous media, intermittent microwave convective drying (IMCD) using a multiphase flow model in porous media subdomains coupled with a forced-convection heat-transfer model in an external hot air subdomain. The models were solved by using COMSOL Multiphysics was applied at the pulse ratio (PR) of 3. Based on drying characteristics of porous media and the distribution of the evaporation interface, IMCD was compared with convection drying (CD). Drying uniformity K, velocity difference, temperature difference, and humidity difference were introduced to evaluate the performance of three models with different inlets and outlet wall curvature. The numerical results show that as the moisture content of slices was reduced to 3 kg/kg, the drying rate in IMCD was 0.0166–0.02 m/s higher than that in CD, and the total drying time of the former was 81.35% shorter than that of the latter. In the late drying stage of IMCD, the core of the sample still had a high vapor concentration and temperature, which led to the evaporation interface remaining on the surface. The vapor evaporated from the slices can diffuse rapidly to the outside, which is why IMCD is superior to traditional convection drying. Through the comprehensive analysis of the models with different inlet and outlet wall curvatures, the drying uniformity K of the type III was the highest, reaching 89.28%. Optimizing flow-field distribution can improve uniform of airflow distribution.

Effect of drying techniques on quality and sensory properties of tropical fruits

SummaryDrying is a popular food preservation technology that reduces food waste and promotes food preservation and food security. Though drying reduces microbial growth and increases shelf life of fruits, it alters sensory properties both favourably and unfavourably depending on the nature of the drying process applied and significantly influences the final product quality. However, there is currently no review article on the impact of the drying process on the sensorial quality attributes of tropical fruits. This review attempts to describe the relative merits and challenges of some widely used advanced drying techniques with a particular focus on intermittent microwave convective drying (IMCD) techniques which offer high sensory properties with excellent drying efficiency. The paper aims to provide an insight into the effect of these drying techniques on the sensory qualities of tropical fruits to identify the best drying method for future food applications.

Investigation of nutritional quality evolution of papaya during intermittent microwave convective drying

Papaya is a highly popular tropical fruit with its unique aroma, taste, and excellent nutritional characteristics. Drying is widely applied as a preservation technique to reduce post-harvest losses and to increase the shelf life. However, traditional convective drying is a time-consuming process, and directly affects to the final quality of the dried product. Intermittent microwave convective drying (IMCD) is an advanced drying method, which has great potential to improve drying performance and energy efficiency. However, the impact of this innovative drying method the quality of dried papaya has not been adequately investigated. In this study, an extensive investigation was carried out to determine the effects of IMCD conditions on drying performance and some quality attributes of papaya samples. Ascorbic acid content, total polyphenol content, color, and water activity of IMCD dried papaya samples were determined using standard methods. For IMCD process conditions, combinations of different power loads (0.5–1.5 W g−1), intermittency ratios (4–6) and air temperatures (50–70 °C) were considered. The results indicate that drying times were significantly reduced in IMCD while the color change tended to increase with higher microwave power. The total polyphenol content was recorded to increase in some conditions while ascorbic acid remained high in most of the IMCD conditions. The water activity of all dried samples was within product storage stability and safety levels. The optimal condition for the IMCD of papaya was a microwave power load of 1.038 W g−1, intermittency ratio of 5.22 and hot air temperature of 50 °C. The optimized drying time was 235 min, color change ΔE = 12.34, water activity was 0.53, retention of ascorbic acid was 64% and total polyphenol content was 4% higher than the fresh sample. Empirical relationships for drying time, color, total polyphenol content, ascorbic acid content, and water activity in terms of IMCD process variables of papaya drying were developed for the first time. Predicted and experimental values closely matched, validating the proposed models.

Effect of intermittent microwave convective drying on physicochemical properties of dragon fruit.

The study was carried out to investigate the effect of Intermittent microwave convective drying (IMCD) on the overall quality of dried dragon fruit in terms of total phenolic content, color change, and rehydration ratio. Three levels of microwave power (200-600W) and a temperature of 60°C for hot air were applied alternately throughout the process with three levels of pulse ratio such as 1:10, 1:20, and 1:40, respectively. The total phenolic content of the dragon fruit slice obtained by IMCD was ranged between 5.750 and 6.575mg GAE/g dry weight. Within the experimental range of process variables under IMCD conditions, the drying efficiency, color change, and rehydration ratio of the dried dragon fruit slices were 15.287-51.930%, 18.643-24.847, and 1.908-3.239, respectively. The Weibull model scale (α) parameter was found to vary between - , while the shape (β) parameter was found to vary between . The Weibull model parameters were shown to decrease with increasing microwave power at constant pulse ratio. The IMCD method produced a dried dragon fruit slices with reduced color changes and higher total phenolic content and rehydration ratio values. This investigation would contribute to the development of effective drying techniques for increased food quality and product consistency in the drying of diverse fruits and vegetables.

Drying kinetics and properties evolution of apple slices under convective and intermittent-MW drying

Intermittent microwave-convective drying (IMCD) is a relatively advanced drying technology that is used to overcome the shortcomings of microwave, convective, and microwave-convective drying. No research has been done on the evolution of mechanical properties during IMCD of apple. This research aims to shed new light on the effect of heating patterns on the evolution of the mechanical properties of the apple slice. In this research, comparisons of drying and rehydration kinetics of conventional drying IMCD derived samples have also been carried out in light of their mechanical properties changes. The result shows significant differences in drying and rehydration kinetics between IMCD dried and convective dried food materials. To remove the same amount of moisture, convective heating takes about 10 times more time compared with IMCD. Moreover, better rehydration capacity and appearance have been found in IMCD dried samples. Eventually, IMCD offers an energy-efficient quick drying option along with retaining higher quality than conventional drying.

Investigation of the Effect of Drying Conditions on Phytochemical Content and Antioxidant Activity in Pineapple (Ananas comosus)

Pineapple, with its attractive sensorial and nutritional characteristics, is the most traded tropical fruit worldwide. However, due to its high-water content (85–92 w.b %), it is highly perishable, and therefore, postharvest loss of this fruit is a global concern. Drying is an important preservation technique to increase the self-life and to preserve the essential vitamins, minerals, fiber, carbohydrate, and antioxidant properties. In this study, the impact of drying conditions on the phytochemical properties of pineapple was investigated. Experiments have been conducted using different process variables in convective drying (CD) and intermittent microwave convective drying (IMCD) to monitor the changes in total phenolic (TPC), total flavonoid content (TFC), and their antioxidant activity. Samples were dried at temperatures of 50 °C, 60 °C, and 70 °C. Results show that highest TPC was observed in IMCD dried sample at 60 °C compared to CD and fresh samples. However, significant decrease was observed in TFC in both IMCD and CD samples at different drying conditions. IMCD dried sample exhibit strongest antioxidant activity compared to CD and fresh samples. Antioxidant activity was highest in IMCD dried sample at 60 °C compared to CD samples and fresh samples. This is due to disruption of hydrophilic regions of cell such as vacuoles and apoplasts that triggers the release of bound phenolics with oxidative and hydrolytic enzymes that would destroy the antioxidants in fruits. However, high temperature at 60 °C deactivates these enzymes and prevents the loss of phenolic acids, leading to the increase of total phenolic content, thus exhibiting stronger antioxidant activity.

A mathematical model for predicting the transport process and quality changes during intermittent microwave convective drying

Intermittent microwave convective drying (IMCD) is an advanced drying method where volumetric heating of samples drives the drying process. Understanding of the physical effects of IMCD on simultaneous heating and mass transfer as well as quality changes during IMCD is essential to predict accurately drying processes and quality attributes of end products. However, there is a lack of studies in this particular interest area. The aim of this research was to develop an IMCD model coupled with quality degradation kinetics by integrating a simultaneous heat and mass transfer model with Maxwell’s equations for microwave heating and the chemical reaction kinetics model. The simulated results were compared with experimental results and a good agreement was observed. As it was found that power ratio (PR) had a vital role in altering quality attributes, different PR and drying conditions were considered to investigate the effects of IMCD on the drying kinetics. The simulated results showed that the model was capable of predicting accurately moisture and temperature distributions along with heath beneficial compounds, such as total phenolic content (TPC) and ascorbic acid (AA) as well as colour changes during IMCD processing. About 70% of AA was degraded during IMCD drying using PR of 1/3. However, losses were reduced when PR was reduced to 1/4 or 1/5. Likewise, TPC degraded significantly during the early stages (first 60 min) of IMCD processing but stabilised at later stages.

Modelling of simultaneous heat and mass transfer considering the spatial distribution of air velocity during intermittent microwave convective drying

Drying is a complex simultaneous heat and mass transfer process where water is migrated from the sample to the drying air through continuous evaporation. The rate of evaporation can be accelerated by the intermittent application of microwave energy with convective drying, which is called intermittent microwave convective drying (IMCD). However, theoretical aspects of IMCD process is not clearly understood yet, which is required to obtain a better -quality product and for designing energy efficient IMCD drying systems. Therefore, several researchers have attempted to develop a theoretical modelling framework for IMCD. However, most of the existing models do not consider the spatial distribution of air velocity around the product, which is important as it significantly affects the heat and mass transfer coefficient throughout the sample and eventually alters the drying kinetics. This paper aims to address this research gap by developing an IMCD modelling framework through integrating a simultaneous heat and mass transfer model with a computational Fluid Dynamics (CFD) model (drying air flow) under the condition of volumetric heating. A 3D model was developed, and the simulated results were validated with the experimental results. It was found that the integration of fluid flow (CFD) with an IMCD model significantly affected the drying kinetics as heat and mass transfer coefficients spatially varied throughout the sample. Drying models may thus overpredict the drying kinetics like moisture content if the spatial distribution of air velocity is not taken into account. It was also found that the nonuniform microwave power absorption caused an inhomogeneous distribution of the sample temperature. The new findings and knowledge from this study will lead to the development of more accurate drying models.

Sensitivity analysis of intermittent microwave convective drying based on multiphase porous media models

In this study fundamental models were used by considering heat and mass multiphase transport in a porous medium which was coupled with the intermittent microwave convective combination drying process. The models were validated by experimental data sourced from references, and independent analysis was conducted to select a suitable mesh and time step. Sensitivity analysis was performed using different empirical formula-type input parameters. The water activity and the thermal conductivity mainly influenced on the temperature of the sample, and had no effect on the simulation outputs of moisture content; relative permeability of gas and the capillary diffusivity mainly had an impact on the moisture content of the sample; the specific heat, the binary diffusion coefficient and the dielectric loss factor affected both temperature and moisture content at the same time; the relative permeability of water and the dielectric constant both had no effect on the simulation results. In addition, a sample with a lower thermal conductivity, a higher capillary diffusivity or a higher binary diffusion coefficient tended to form a drier interior as well as a moister surface; a sample with a higher thermal conductivity or a higher specific heat tended to form a more uniform temperature; a sample with a higher capillary diffusivity tended to form a more uniform moisture content.

Nutritional quality of heat-sensitive food materials in intermittent microwave convective drying.

BackgroundThe retention of health promoting components in nutrient-rich dried food is significantly affected by the dehydration method. Theoretical and experimental investigations reported in the literature have demonstrated that intermittent microwave convective drying (IMCD) can effectively improve the drying performance. However, the impact of this advanced drying method on the quality food has not been adequately investigated.DesignA programmable NN-SD691S Panasonic inverter microwave oven (1100 W, 2450 MHz) was employed for the experiments. The microwave power level was set at 100 W and ran for 20 seconds at different power ratios and the constant hot air conditions was set to a temperature of 60°C and 0.86 m/s air velocity.ObjectiveIn this study, natural bioactive compounds (ascorbic acid and total polyphenol), water activity, colour and microstructure modifications which can occur in IMCD were investigated, taking kiwifruit as a sample.Results and DiscussionThe microwave (MW) power ratio (PR) had significant impact on different quality attributes of dried samples. The results demonstrate that applying optimum level MW power and intermittency could be an appropriate strategy to significantly improve the preservation of nutrient contents, microstructure and colour of the dried sample. The IMCD at PR 1:4 was found to be the ideal drying condition with the highest ascorbic acid retention (3.944 mg/g DM), lowest colour change (ΔERGB = 43.86) and a porous microstructure. However, the total polyphenol content was better maintained (3.701 mg GAE/g DM) at higher microwave density (PR 1:3). All samples attained a desirable level of water activity which is unsusceptible for microorganism growth and reproduction.ConclusionOverall, IMCD significantly improved the drying performance and product quality compared to traditional convective drying.

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.

Applicability of fruit blanching and intermittent microwave-convective belt drying to industrial peel waste of different mango cultivars for the recovery of functional coproducts

To exploit the full potential of industrial mango peel waste (MPW0) as a starting material for the recovery of bioactive and functional coproducts despite its seasonality, it must be processed efficiently into a storable dried byproduct of uniform high quality, irrespective of impacts of different cultivars and ripeness. The focus of this feasibility study, performed on the pilot plant scale, was on technological options for three key process stages in the manufacture of dried mango peel (DMP) as a possible bulk commodity: fruit blanching, peel drying, and DMP packaging. Depending on cultivars, hot-water blanching of fruit (65–85°C) before peeling proved inappropriate for peel color retention, but could even induce enzymatic browning until peel drying, and thus losses of bioactive compounds due to oxidative polymerization and insolubilization. Hence, this process step should be limited to brief washing of the fruit at maximally 65°C to reduce microbial load. Intermittent microwave-convective drying (IMWC) of MPW0 in a continuous belt dryer at high peel throughput was feasible at ambient air temperatures. Finalizing initial IMWC of MPW0 by convective drying (CD, 80°C) in the falling-rate period avoided local charring. Alternatively, MPW0 had to be washed before IMWC to remove adherent pulp, but at the expense of soluble solids and β-carotene losses. Unlike the carotenoids, alk(en)ylresorcinols were hardly affected by IMWC. By washing MPW0, quality defects compared to CD-dried peel in terms of antioxidant capacity, water-holding capacity, and yield and quality of pectin were compensated. Adequate preprocessing plus IMWC was thus deemed to be a promising option, especially for biorefinery concepts including biogas production and congeneration. The minimal moisture barrier properties needed for flexible intermediate bulk containers were estimated from an 11-month shelf-life test of IMWC-dried peel in two climate zones.

Investigation of intermittent microwave convective drying (IMCD) of food materials by a coupled 3D electromagnetics and multiphase model

ABSTRACTIntermittent microwave convective drying (IMCD) improves energy efficiency and the product quality during drying of agricultural products. However, the physical mechanism of heat and mass transfer involved in IMCD is poorly understood due to lack of a comprehensive and realistic mathematical model of this process. A multiphase porous media model considering coupled electromagnetics and multiphase transport phenomena in porous media can potentially provide fundamental details of underlying mechanisms of IMCD. The aim of this study is to develop a mathematical model for IMCD considering electromagnetics using Maxwell’s equations coupled with multiphase porous media in 3D and validate the model against experimental results. The results show that the temperature distribution is uneven in the material, which redistributes during the tempering period. The water and vapor fluxes showed asymmetric profile along the diameter of the sample due to the non-uniformity of microwave heating. A clear understanding of these transport mechanisms in IMCD will lead to the development of appropriate drying process for improved food quality, energy efficiency, and optimization of the IMCD process.

Modelling of intermittent microwave convective drying: parameter sensitivity

Abstract The reliability of the predictions of a mathematical model is a prerequisite to its utilization. A multiphase porous media model of intermittent microwave convective drying is developed based on the literature. The model considers the liquid water, gas and solid matrix inside of food. The model is simulated by COMSOL software. Its sensitivity parameter is analysed by changing the parameter values by ±20%, with the exception of several parameters. The sensitivity analysis of the process of the microwave power level shows that each parameter: ambient temperature, effective gas diffusivity, and evaporation rate constant, has significant effects on the process. However, the surface mass, heat transfer coefficient, relative and intrinsic permeability of the gas, and capillary diffusivity of water do not have a considerable effect. The evaporation rate constant has minimal parameter sensitivity with a ±20% value change, until it is changed 10-fold. In all results, the temperature and vapour pressure curves show the same trends as the moisture content curve. However, the water saturation at the medium surface and in the centre show different results. Vapour transfer is the major mass transfer phenomenon that affects the drying process.

Multiphase transfer model for intermittent microwave-convective drying of food: Considering shrinkage and pore evolution

Intermittent microwave convective (IMCD) drying is an advanced drying technology that improves both energy efficiency and food quality during drying. Although many experimental studies on IMCD have been conducted, there is no complete multiphase porous media model describing the physics of the process available in the literature. A multiphase porous media model considering liquid water, gases and the solid matrix of food during drying can provide in-depth understanding of IMCD process. Currently there is no IMCD model that have taken shrinkage and pore evolution during drying into consideration. In this study, first a multiphase porous media model with shrinkage (IMCD2) has been developed for IMCD. Then the model has been compared with IMCD model without shrinkage (IMCD1). Simulated temperature, moisture content, density, porosity from IMCD2 are then validated against experimental data. The profile of vapour pressures and evaporation during IMCD are also presented and discussed.

Microwave, convective, and intermittent microwave-convective drying of pulsed vacuum osmodehydrated pumpkin slices

The drying of pumpkin slices (Cucurbita moschata Duch.) by microwave, convective, and intermittent microwave–convective techniques was studied. The samples were pretreated by blanching followed by pulsed vacuum osmotic dehydration. The microwave output power was 780 W. Air temperatures were 30 °C and 60 °C and air velocities were 2.22 and 4.44 m s−1. The final moisture content was set at 0.08 kg [H2O] kg−1 [dry matter]. The drying kinetics were obtained and fitted with the Fick's model and the Page's empirical correlation. Color, water activity, shrinkage, and rehydration were evaluated. Shorter drying times, higher diffusivity and better color quality were obtained by employing microwave. Convective experiments carried out to higher water activity and lower shrinkage. For describing the drying behavior, the better adjustment was reached by the Page's empirical correlation. Practical applications The pumpkin (Cucurbita moschata Duch.) is well known as an important source of carotenoids, vitamins and minerals. After the harvest, techniques are required to assist the preservation of fruits, once they present high moisture contents. Microwave drying is an alternative method to preserve temperature-sensitive compounds and to economize energy and time. Microwave energy combined with other drying methods can improve the drying efficiency as well as the quality of food products which is far better than that achievable by the application of each method alone. This study investigates the efficacy of convective, microwave and intermittent microwave–convective drying techniques for osmodehydrated pumpkin slices. The mathematical models provide information about the processes. It was also evaluated the changes on physical characteristics of the product after drying.

Real-time color change monitoring of apple slices using image processing during intermittent microwave convective drying.

An intermittent microwave convective drying method combined with a real-time computer vision technique was employed to detect the effect of drying parameters on color properties of apple slices. The experiments were performed at air temperature of 40 to 80℃, air velocities of 1-2 m/s, microwave powers of 200-600 W, and pulse ratios (PRs) of 2-6. Drying rate and drying time varied from 0.014 to 0.000001 min-1 and 27 to 244 min, respectively. The normalized lightness values had ascending and descending parabolic trends with decrease in product moisture content. With descending dimensionless moisture content, redness, yellowness, color change, hue angle, and chroma were enlarged. The normalized redness values changed from -4 to 3. Models relating drying parameters with drying time, drying rate, and lightness were obtained and found to be significant (P < 0.01). Results indicated that microwave power and PRs had more influence on lightness and color change than other parameters.

Multiphase porous media model for intermittent microwave convective drying (IMCD) of food

Intermittent microwave convective (IMCD) drying is an advanced drying technology that improves both energy efficiency and food quality during the drying of food materials. Despite numerous experimental studies available for IMCD, there is no complete multiphase porous media model available to describe the process. A multiphase porous media model considering liquid water, gases and the solid matrix inside the food during drying can provide in depth understanding of IMCD. In this article, firstly a multiphase porous media model was developed for IMCD. Then the model is validated against experimental data by comparing moisture content and temperature distributions after each heating and tempering periods. The profile of vapour pressures and evaporation during IMCD are presented and discussed. The relative contribution of water and vapour fluxes due to gas pressure and diffusion demonstrated that the fluxes due are relatively higher in IMCD compared to convection drying and this makes the IMCD faster.