Microwave-assisted Hot Air Research Articles

Effect of drying and extrusion processing on physical and nutritional characteristics of bilberry press cake extrudates

Mild drying and extrusion processing of side streams from berry juice production can enable retention of valuable compounds in the food chain and reduce waste production. The aim of this study was to evaluate the applicability of using hot air (HA) and microwave assisted hot air (MWHA) drying combined with extrusion for conversion of bilberry press cake into value-added extruded food products. Bilberry press cake was dried at 40 °C by HA and MWHA drying to a moisture content of 17 g/100 g. A twin screw extruder (average feed rate 72 g/min, temperature profile 135-128-89-69 °C) was used to extrude products containing organic wholegrain rye flour and 10% or 25% dried bilberry press cake powder. A consumer panel (n = 15) evaluated four extrudates on hedonic and Just-About-Right (JAR) scales, with a main focus on texture properties. The results indicate that different drying techniques implied a difference in processing time (40% reduction with MWHA drying). However, the retention of total phenolics and physical characteristics of extruded snacks containing bilberry powders were independent of drying techniques. In sum, powder of bilberry press cake can be incorporated in cereal based extruded snacks with enhanced phenolic content and potential for palatable sensory properties.

Effects of different drying methods on the physicochemical properties of pomegranate leather (pestil)

The present study examines the effects of various formulations (traditional and hydrocolloid) and drying techniques [hot air drying at 50, 60 or 70 °C, microwave-assisted hot air drying (90 or 180 W) at 50, 60 or 70 °C and refractance window drying 90, 95 and 98 °C), in order to develop new methods for pomegranate pestil production. Water activity, pH, color, textural properties, total phenolic content, total flavonoids, total proantocyanidins, total monomeric anthocyanin, ascorbic acid, HMF content, antioxidant activity, phenolic and anthocyanin profiles and volatile compounds of the pestils were determined. Hydrocolloid formulation resulted in superior physicochemical properties of the pestils compared to traditional formulation. While microwave-assisted drying (MWD) provided higher phenolic content, refractance window (RW) drying provided higher ascorbic acid and anthocyanin content, and lower content of 5-hydroxymethylfurfural. Volatile compounds in the pestils showed that MWD promoted non-enzymatic browning reactions. Overall, the hydrocolloid formulation and the RW drying technique are the most promising processing methods for high-quality pestil production with high content of bioactive compounds.

Hot Air Drying and Microwave-Assisted Hot Air Drying of Broccoli Stalk Slices (Brassica oleraceaL. Var.Italica)

Drying characteristics of broccoli stalk slices under hot air drying (HA) and microwave-assisted hot air drying (MWHA) at temperatures of 40, 50 and 60C is presented. In addition, the color and shape changes of the dried sample were compared. Midilli et al. model showed a good agreement with the experimental data obtained for both drying systems. The calculated effective moisture diffusivity are varied from 2.92 × 10−8 to 7.91 × 10−8 m2/s under HA and 6.64 × 10−8 to 13.31 × 10−8 m2/s under MWHA. Less changes in color was observed under MWHA at lower temperature. Meanwhile, higher circularity of the shape was observed under HA. According to these results, MWHA promotes faster drying time with higher moisture diffusion; however, pre-treatment prior to this drying process is suggested to enhance the final dried product. Practical Applications Habitually, just the edible floret of broccoli is consumed, leaving the stalk as crop remains. This part of the crop has the potential to be used as food since it has been identified as having many nutritional components. To develop this as a product, drying as a unit operation is essential. This work evaluates the drying behavior of different drying conditions of broccoli stalk. With these results, drying of broccoli stalk can be carried out by the food industry more effectively and efficiently.

Application and the Techno-economical Aspects of Integrated Microwave Drying Systems for Development of Dehydrated Food Products

The demand to increase food production to feed the growing number of the world population is becoming a primary concern for everyone. At the same time, one-third of the food produced is lost or wasted globally. The food wastes and the by-products of the food production and processing operations still have nutritional ingredients in them suitable for human consumption. To encounter this issue, drying methods can be carried out to add value to food waste, prolong the product’s shelf life and thus provide more food products. Integrated microwave drying systems have the potential to produce dehydrated products from the food waste. In this paper, integrated osmotic dehydration with microwave-assisted hot air drying method is described for the development of dehydrated broccoli stalk product. The techno-economical aspects in terms of raw material availability, process design, energy input, capital inputs, operation costs and environmental benefits are also discussed. This integrated microwave drying concept will be of use to process other biological materials that have the potential at the marketplace.

Effect of Different Drying Methods on the Microwave Extraction of Phenolic Components and Antioxidant Activity of Highbush Blueberry Leaves

In the last few decades, researchers have found blueberry leaves to be an interesting source of different phenolic compounds, and drying is an important part of their postharvest and sample preparation processes. In the current study, blueberry leaves were dried using microwave-assisted hot air drying or hot air drying alone at 45, 60, and 75°C to 10–15% wet basis. Corresponding drying characteristics were determined by fitting the moisture ratio obtained in each case with eight theoretical and semitheoretical models for hot air and microwave drying stages. It was observed that best fitting drying models for blueberry leaves depended not only on the nature of the biomaterial but also on the method of drying and the temperature of drying. Phenolic content, monomeric anthocyanin content, and the corresponding antioxidant activity in terms of 2,2-diphenyl-1-picrylhydrazyl (DPPH) inhibition activity and ferric reducing ability of plasma (FRAP) activity were evaluated for the leaf extract obtained following microwave extraction of the dried samples obtained using the above-mentioned drying methods and were compared to the corresponding bioactive compositional characteristics of freeze-dried blueberry leaves. It was observed that the freeze-dried sample had the highest content of total phenolics and total monomeric anthocyanins along with high antioxidant activity. Furthermore, extracts obtained from the microwave-dried leaf samples obtained with a drying temperature of 60°C had highest total phenolic content and highest total monomeric anthocyanin content among all other drying methods and the extract had an antioxidant activity similar to the freeze-dried blueberry leaf samples, implying that microwave drying at 60°C is a potential alternative to freeze drying for preservation of the phenolic components and antioxidant activity of dried blueberry leaves.

A Comparative Study of Four Drying Methods on Drying Time and Quality Characteristics of Stem Lettuce Slices (Lactuca sativa L.)

This article presents experimental results and analysis of four drying methods, viz. hot air drying (AD), hot air-assisted radio frequency drying (ARFD), infrared drying (IRD), and microwave-assisted hot air drying (MAD), on color, microstructure, density, rehydration capacity, and texture after rehydration of stem lettuce slices (Lactuca sativa L.). The drying time required for these drying protocols was also compared. These four drying tests were conducted at fixed air temperature (60°C) and velocity (1 m/s), as well as identical sample load (300 g), bed depth (20 mm), and the power level for ARFD, IRD, and MAD, which was fixed at 4 W/g. The results showed that the drying time required for stem lettuce slices using ARFD was the shortest (120 min), followed by MAD (140 min) and IRD (180 min); AD required the longest time (360 min). Notably, ARFD yielded uniform drying and the quality of the dried samples using ARFD was also the best among these four drying methods.

Thin Layer Modelling of Microwave-Convective Drying of Tomato Slices

Abstract The thin layer drying behaviour of tomato slices dried using microwave power densities of 1.13, 2.08 and 3.11 W/g combined with air ventilation (50°C) and hot air drying at 40, 50, 70 and 80°C was investigated. The tomato slice dried faster when subjected to microwave heating coupled with hot air ventilation. Drying time decreased considerably with increase in microwave power density and with increase in hot air temperature. Drying took place in a constant rate period followed by the falling rate period after a short heating period. The drying data were fitted to Newton (Lewis), Page, Henderson and Pabis, Logarithmic, Wang and Singh and Parabolic equations. The Parabolic model (R 2 = 0.9999; χ 2 = 0.0085; MBE = 0.0182 and RMSE = 0.0691) gave the best fit to predict the hot air ventilation drying of tomato slices while the Logarithmic model (R 2 = 0.9951; χ 2 = 0.0024; MBE = −0.0319 and RMSE = 0.0477) gave the best fit for microwave-assisted hot air drying of tomato slices. The values of the effective diffusivity coefficients of the tomato slices varied between 1.68 × 10–9 and 5.22 × 10–8 m2/s while the activation energy was 27.09 kJ/mol. The lower activation energy indicates that drying of tomato slices requires less energy and is hence a cost and energy-saving method. Microwave drying at 1.13 and 2.08 W/g maintained superior colour quality of the tomato slices.

Effects of Microwave-Assisted Hot Air Drying and Conventional Hot Air Drying on the Drying Kinetics, Color, Rehydration, and Volatiles of Moringa oleifera

In this study, the fresh Moringa oleifera pods (Drumsticks) were dehydrated by microwave-assisted hot air drying (MAHD) and conventional hot air drying methods. The samples were dried at three different temperatures, viz. 50, 60, and 70°C, with and without the application of microwaves. Microwave power density of 1 W/g was used for the MAHD. The final moisture content was targeted as 13% wb. The drying curves and drying rate curves were plotted and compared. The kinetics of drying obtained experimentally were correlated with the Page equation. The constants K and N of the Page equation were determined to predict the drying kinetics for varying conditions. The quality attributes, namely, color, rehydration ratio, and volatile compounds, were analyzed and compared with that of the fresh Moringa pods. The volatile compounds were analyzed using z-Nose (an electronic nose; Electronic Sensor Technology, Newbury Park, CA) and bioactive molecules were analyzed using gas chromatography–mass spectrometry. The results showed that the MAHD method had significantly (P < 0.05) reduced the loss of volatiles during drying. Also, MAHD preserved most of the bioactive molecules when compared to the conventional hot air drying method. The samples dried at 50°C using MAHD were the best in terms of all of the quality attributes tested in this study. Also, the results established that the z-Nose can be used as a quick and inexpensive means to assess the effect of different process parameters on the aromatic quality of the product and quantitatively classify quality based on aroma.

Optimization of microwave-assisted hot air drying conditions of okra using response surface methodology

Okra (Abelmoschus esculentus) was dried to a moisture level of 0.1g water/g dry matter using a microwave-assisted hot air dryer. Response surface methodology was used to optimize the drying conditions based on specific energy consumption and quality of dried okra. The drying experiments were performed using a central composite rotatable design for three variables: air temperature (40-70°C), air velocity (1-2m/s) and microwave power level (0.5-2.5W/g). The quality of dried okra was determined in terms of color change, rehydration ratio and hardness of texture. A second-order polynomial model was well fitted to all responses and high R(2) values (>0.8) were observed in all cases. The color change of dried okra was found higher at high microwave power and air temperatures. Rehydration properties were better for okra samples dried at higher microwave power levels. Specific energy consumption decreased with increase in microwave power due to decrease in drying time. The drying conditions of 1.51m/s air velocity, 52.09°C air temperature and 2.41W/g microwave power were found optimum for product quality and minimum energy consumption for microwave-convective drying of okra.

An Investigation of the Effects of Drying Methods and Conditions on Drying Characteristics and Quality Attributes of Agricultural Products during Hot Air and Hot Air/Microwave-Assisted Dehydration

Hot air and microwave-assisted hot air drying of apples (Golden delicious), strawberries (Fragaria vesca cv. Elsanta), tomatoes (Lycopersicon esculentom var. Roma), and mushrooms (A. bisporus) were performed in a commercial microwave oven that had been modified to incorporate a hot air dryer. The same microwave power intensity was used in all experiments. We investigated the effect of the above-stated methods on the structural, optical, mechanical, and rehydration characteristics of dried samples. Scanning electron microscopy (SEM) was used to evaluate the structural damage associated with two drying methods. The drying rate was significantly affected by microwave energy; therefore, drying time was reduced by 80, 70, 74, and 66%, respectively for apples, strawberries, tomatoes, and mushrooms, respectively. Specific volume, particle density, and porosity variation were estimated using models proposed in the literature. Some structural damage occurred, as observed in SEM micrographs. Optical, mechanical, and rehydration parameters were enhanced by microwave treatment for apples, strawberries, and tomatoes. However, negative effects were observed in the case of mushrooms.

Combined Drying Technologies for Development of High-Quality Shelf-Stable Mango Products

In this work, different combinations of osmotic dehydration followed by microwave-assisted hot air drying were studied for modeling and quality evaluation purposes. The results revealed that the two variables that most affected drying time of mango were the level of microwave power and osmotic dehydration time, rather than airflow velocity and temperature, which did not have an important influence until the final stage of drying. The results also revealed a nonuniform distribution of microwaves inside the dryer cavity that provoked the charring of pieces situated in a specific zone but an acceptable preservation of mango color after drying.