Drying Kinetics and Antioxidant Phytochemicals Retention of Salak Fruit under Different Drying and Pretreatment Conditions

Kinetics of hot air drying and heat pump drying were studied by performing various drying trials on salak slices. Isothermal drying trials were conducted in hot air drying and heat pump drying at a temperature range of 40–90°C and 26–37°C, respectively. Intermittent drying trials were carried out in heat pump drying with two different modes: periodic heat air flow supply and step-up air temperature. It was observed that the effects of relative humidity and air velocity on drying rate were significant when moisture content in salak slices was high, whereas the effects of temperature prevailed when the moisture content was low. As such, it was proposed that drying conditions should be manipulated according to the moisture transport mechanisms at different stages of drying in order to optimize the intermittent drying and improve the product quality. Generally, loss of ascorbic acid during drying was attributed to thermal degradation and enzymatic oxidation, whereas the loss of phenolic compounds was mainly due to thermal degradation. Experimental results showed that heat pump drying with low-temperature dehumidified air not only enhanced the drying kinetics but produced a stable final product. Heat pump–dried samples retained a high concentration of ascorbic acid and total phenolic compounds when an appropriate drying mode was selected.     

Effect of Osmotic Dehydration with Pulsed Vacuum on Hot-Air Drying Kinetics and Quality Attributes of Cherry Tomatoes

The effects of osmotic dehydration (OD) with or without pulsed vacuum (PV) on hot-air drying kinetics and quality attributes of cherry tomatoes were investigated. Both OD and PVOD pre-treatments were performed for 3 h at 50°C in 50 and 70^o Brix sucrose solutions with a solution-to-fruit mass ratio of 4:1, and PVOD was applied for 15 min before OD at atmospheric pressure. Samples were further dried at air temperature of 70°C. Effective moisture diffusivity (D _eff) of osmotically dehydrated samples increased gradually while the D_eff curve of fresh samples had a plateau stage during hot air drying. Lower glass transition temperature, T_g, values of osmotically dehydrated samples indicated that they needed a lower storage temperature. Both OD and PVOD pre-treatments had advantages in shortening drying cycles and improving quality of products. Compared with air drying, osmo-air drying decreased the total drying time, color change, and hardness of dried samples by 32.26%, 18.11%, and 88.21%, respectively, and increased volume ratio and vitamin C retention rate by 72.31% and 125.82%. As compared with OD, PVOD decreased color change and hardness by 28.48% and 45.17%, increased volume ratio and vitamin C retention rate by 27.41% and 17.77%, but there was no significant difference shown in drying time. Therefore, osmotic pre-treatment can shorten the total dehydration time, and improve the general quality of dried cherry tomatoes.     

Microwave,Vacuum, and Air Drying Characteristics of Collard Leaves

Collard leaves (Brassica oleracea L. var. acephala) with an initial moisture content of 6.65 on percentage dry basis (%db) were dried by three different drying methods: microwave, air, and vacuum. Samples of fresh leaves, 25 g each, were dried until their moisture was down to 0.1 on a dry basis. The following drying levels were used in each of the drying processes: 350, 500, 650, 750, 850, and 1000 W for microwave drying; 50, 75, 100, 125, 150, and 175°C for air drying; and 0.4, 50, and 100 mmHg at 50 and 75°C for vacuum drying, respectively. Drying times ranged between 2.5 to 7.5 min, 8 to 210 min, and 35 to 195 min for microwave, air, and vacuum drying, respectively. The data obtained compared well with a thin-layer drying model. Microwave drying at 750 W provided optimal results with respect to drying time, color, and ascorbic acid content (vitamin C).     

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.     

The Effects of Predrying Treatments and Different Drying Methods on Phytochemical Compound Retention and Drying Characteristics of Moringa Leaves (Moringa oleifera Lam.)

The most appropriate maturity stage of Moringa oleifera leaves was selected for drying based on phytochemical content, including quercetin and kaempferol. Desorption isotherms were developed and were best fit by the modified Henderson model. Prior to drying, samples were left untreated, blanched in boiling water, and blanched in NaHCO₃/MgO. The leaves were dried by hot air tray drying (TD) and heat pump–dehumidified drying air (HPD) at air temperatures of 40, 50, and 60°C. Alternatively, leaves were subject to microwave drying (MWD) at 150, 450, and 900 W and to freeze drying (FD). The moisture versus time data were fitted to five drying models. In general, a three-parameter model gave the best fit. The drying constant was related to the drying temperature or microwave power using an Arrhenius model. Effective moisture diffusivity (D _eff) increased with higher drying temperature, higher microwave power, or blanching treatments. Structural changes in the leaves after drying and upon rehydration were observed by scanning electron microscopy (SEM). Leaves blanched and dried using HPD at 50°C and fresh and dried using FD showed a partial breakdown of the tissue structure upon rehydration. HPD and blanching reduced the drying time by 8.3% and increased quercetin and kaempferol levels by 42.1 and 51.4%, respectively, compared to TD at 50°C. MWD provided the quickest drying followed by HPD and TD, respectively. HPD drying of M. oleifera after blanching resulted in relatively greater quality compared to TD and MWD.     

Study on quality attributes and drying kinetics of instant parboiled rice fortified with turmeric using hot air and microwave-assisted hot air drying

Abstract

This study investigated the quality and drying kinetics of instant parboiled rice fortified with turmeric (IPRFT) by using hot air (HA) and microwave-assisted hot air (MWHA) drying. The cooked long grain parboiled rice (LGPR) fortified with turmeric was dried with HA at temperatures of 65, 80, 95, and 110?°C. The microwave power density of 0.588 Wg^?1 was incorporated for drying with MWHA. Drying was performed until the dried IPRFT reached 16% (d.b.) of moisture content. The quality of the dried IPRFT was evaluated in terms of color, total phenolics content (TPC), total antioxidant capacity (TAC), rehydration ratio, volume expansion ratio, texture and microstructure. The results showed that the incorporation of microwave power with HA drying helped to reduce the drying time by 50% compared to conventional HA drying. A prediction of the moisture ratio by using the Page model provided the best R² and RMSE in drying kinetics. The drying conditions had small effects on the color, TPC, TAC, and microstructure of the dried IPFRT. The rehydration ratio, volume expansion ratio and texture of the rehydrated IPFRT showed minimal variations from changes in the drying conditions. The TPC and TAC of the dried IPRFT clearly increased compared to the TPC and TAC of the initial LGPR.     

Effects of Different Drying Methods on the Quality Changes of Granular Edamame

In this article, the quality changes of the granular fruits and vegetables dried by vacuum microwave drying, freeze drying, hot air drying, and combined hot air–vacuum microwave drying are investigated, and the quality parameters compared on the basis of vitamin C and chlorophyll contents, shrinkage and rehydration capacity, color, texture, and microstructure changes. The quality parameters of products dried by vacuum microwave drying are slightly lower than those obtained by freeze drying, but much better than those obtained using conventional hot air drying. The quality characteristics of product dried by combined hot air–vacuum microwave are significantly improved compared to those simply hot air–dried.     

Combined Microwave-Vacuum and Freeze Drying of Carrot and Apple Chips

A combination of microwave-vacuum (MWV) drying and freeze drying was investigated as potential means for drying carrot and apple chips. The sample was first dried by microwave-vacuum to dehydrate some amount of internal free water and then by freeze drying to a final moisture content of less than 7% (wet basis). Chemical properties (carotene and vitamin C retention) and physical properties (shrinkage, color, texture, and rehydration ratio) of carrot and apple slices dried by this method were evaluated and compared with those dried by freeze drying alone, MWV drying alone, and conventional hot air drying, respectively. The comparison showed that the carotene retention of carrot slices and the vitamin C retention of apple slices dried by the current method were close to those of freeze-dried carrot and apple slices and much better than those of conventional hot air–dried ones. The samples prepared by the current method exhibited very close rehydration capacity, color retention, and texture with those of the freeze-dried ones but with a little higher shrinkage. However, the samples still showed the attractive external appearance without marked warp.     

Dehydration of Concentrated Ganoderma lucidum Extraction by Combined Microwave-Vacuum and Conventional Vacuum Drying

Combination of microwave-vacuum drying and conventional vacuum drying was investigated as a potential method for drying concentrated Ganoderma lucidum extraction. The Ganoderma lucidum was extracted by hot water (60–65°C) and then concentrated to moisture of about 70% (wet basis) in a rising-film evaporator. The concentrated sample was dried by microwave-vacuum until the moisture content reached 10% (wet basis), and then by conventional vacuum drying at the temperature of 55–60°C to final moisture content about 6% (wet basis). The retention of polysaccharide and triterpenes of Ganoderma lucidum dried by this method were evaluated and compared with those dried by freeze drying and conventional vacuum drying alone. The comparison showed that the quality of extraction dried by the current method was close to that of freeze-dried extraction and much better than that of conventional vacuum-dried ones.     

Design and Performance Evaluation of a Solar-Assisted Biomass Drying System with Thermal Storage

A solar biomass hybrid air heating system that does not require a conventional auxiliary heater but can still provide a daily load fraction exceeding 90% and supply hot air at a steady temperature and flow rate continuously for 24 h a day has been developed. The system, which combines an unglazed transpired solar collector, rock bed, and a biomass gasifier stove with heat exchanger, was evaluated by drying chilli using air at 60°C and 90 m³/h. The chilli was dried from 76.7% moisture (w.b.) to 8.4% over 32.5 h of continuous drying. The dryer reduced the drying time by 66% compared to open sun drying and provided 91.6% load fraction during the 24-h operation. The temperature of hot air supplied was stable at 60±3°C for about 21 h during the entire drying duration.     

Effects of Different Drying Methods on the Quality Changes of Granular Edamame

In this article, the quality changes of the granular fruits and vegetables dried by vacuum microwave drying, freeze drying, hot air drying, and combined hot air-vacuum microwave drying are investigated, and the quality parameters compared on the basis of vitamin C and chlorophyll contents, shrinkage and rehydration capacity, color, texture, and microstructure changes. The quality parameters of products dried by vacuum microwave drying are slightly lower than those obtained by freeze drying, but much better than those obtained using conventional hot air drying. The quality characteristics of product dried by combined hot air-vacuum microwave are significantly improved compared to those simply hot air-dried.     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

Dehydration of Garlic Slices by Combined Microwave-Vacuum and Air Drying

Abstract

Combination of microwave-vacuum drying and air drying was investigated as a potential mean for drying garlic slices. The sample was dried by microwave-vacuum until the moisture content reached 10% (wet basis), and then by conventional hot-air drying at the temperature of 45°C to final moisture content less than 5% (wet basis). Pungency, color, texture, and rehydration ratio of garlic slices dried by this method were evaluated and compared with those dried by freeze drying and conventional hot-air drying. The comparison showed that the quality of garlic slices dried by the current method was close to that of freeze dried garlic slices and much better than that of conventional hot-air dried ones. The lab microwave-vacuum dryer which the materials to be dried could be rotated in the cavity was developed by the authors.     

Optimization of the Drying Process of Carrot (Daucus carota v. Nantes) on the Basis of Quality Criteria

The drying curves and the degradation kinetics of three different quality attributes (total carotenoids (TC) and total polyphenols (TP) contents and antioxidant activity (AA)) of carrots during drying at different temperatures (from 40 to 90°C) have been experimentally evaluated and modeled. A diffusional model taking into account the solid shrinkage and both the external and internal water transfer resistances was used to accurately represent the water transfer in carrot during drying (average mean relative error (MRE) of 3.3 ± 0.6%). The effective moisture diffusivity was found to follow the Arrhenius relationship (E_a = 76.0 kJ/mol) and the mass transfer coefficient a linear dependence with air temperature. The Weibull model was used to satisfactorily simulate the degradation kinetics of the three quality attributes considered (average MRE of 2.8 ± 1.2% for TC content, 5.7 ± 1.0% for TP content, and 3.6 ± 1.8% for AA); these were the E_a of 52.7 kJ/mol for TC; 22.1 kJ/mol for TP; and 27.5 kJ/mol for AA kinetics. By using the proposed models, the estimated optimum drying temperature to best retain the total carotenoids content ranged between ca. 42–46°C; meanwhile, in order to maintain the TP content and the antioxidant activity at the highest levels, the drying needed to be carried out at temperatures of ca. 60–75°C. These results indicate that the TC retention is more influenced by the drying temperature, while the TP and AA retentions are more sensitive to drying time exposure. However, it was possible to establish a global optimum air temperature which ranged between 52.6 and 57.7°C, decreasing the TC, TP, and AA retentions by less than 2.2% from their respective optimal values.     

Effect of Fluidized Bed Drying Temperature on Various Quality Attributes of Paddy

Abstract

As reported by many researchers, it was found that fluidized bed paddy drying using high drying air temperatures of over 100°C affected the head rice yield and whiteness of dried rice. However, only a few studies on fluidized bed paddy drying with drying air temperatures below 100°C were so far reported. The main objective of this work was therefore to study the effect of fluidized bed drying air temperature on various quality parameters of Suphanburi 1 and Pathumthani 1 Indica rice. Paddy was dried from the initial moisture contents of 25.0, 28.8, and 32.5% dry basis to 22.5 ± 1.2% dry basis using inlet drying air temperatures between 40 and 150°C at 10°C/step. After fluidized bed drying, paddy was tempered and followed by ambient air aeration until its final moisture content was reduced to 16.3 ± 0.5% dry basis. The results showed that the head rice yield of Suphanburi 1 was significantly related to the inlet drying temperature and initial moisture content whilst there was no significant relationship between the head rice yield, drying temperature and initial moisture content for Pathumthani 1. The whiteness of the two rice varieties was slightly decreased with increase in drying air temperature and initial moisture content. It was also found that the hardness of both cooked rice varieties exhibited insignificant difference (p < 0.05) comparing to rewetted rice, which was gently dried by ambient air aeration in thin layer. The thermal analysis by DSC also showed that partial gelatinization occurred during drying at higher temperatures. Using inlet drying air temperatures in the range of 40–150°C therefore did not affected the quality of cooked rice and paddy. The milling quality of paddy was also well maintained.     

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.     

Study and comparison of different drying processes for dehydration of raspberries

The aim of the present study was to evaluate and compare different drying methods (microwave, hot air?+?microwave, and osmotic dehydration?+?microwave) in raspberries (cv. Heritage). A portion of raspberries was pretreated with osmotic dehydration (60°Brix sucrose solution at 20°C for 360?min) and another with hot air drying (HAD) (1.5?m/s air speed at 60°C for 300?min). Pretreated raspberries were then dried by microwave and at three different intensities (3.5, 7.5, and 11?W/g). Physicochemical properties (moisture content, water activity, and drying rate) and quality parameters (optical properties, mechanical properties, antioxidant capacity, and rehydration capacity) of dried raspberries were evaluated. Results showed that the microwave drying (MWD) at 7.5?W/g (50?min and final temperature of 79?±?5.1°C) allowed a high yield of dried raspberries. The combined processes were not efficient to accelerate the decrease of moisture content, due to the low drying rate of the pretreatments. In terms of quality, none of the drying processes allowed a high retention of the antioxidant capacity. However, they allowed an appropriate rehydration performance. The combination of HAD with MWD allowed obtaining a good appearance and desirable texture on the dried product. Thus, this last option seems to be the best among the drying methods tested, but additional studies are required to improve the efficiency of the process and the effect on the antioxidant capacity during drying.     

Mathematical Model of Fixed-Bed Drying and Strategies for Crumb Rubber Producing STR20

The objectives of this research were to investigate empirical and diffusion models for thin-layer crumb rubber drying for producing STR20 rubber using hot air temperatures of 110–130°C and to study the effect of drying parameters such as inlet drying temperature, volumetric flow rate, and initial moisture content on the quality of dried rubber. Finally, a mathematical drying model for predicting the drying kinetics of crumb rubber was developed using inlet air flow rates of 300–600 m³/min-m³ of crumb rubber (equivalent to 1.8–5.0 m/s) with the crumb rubber thickness fixed at 0.25 m. The average initial moisture content of samples was in the ranges of 40 and 50% dry basis while the desired final moisture content was below 5% dry basis. The results showed that the drying equation of crumb rubber was highly related to the inlet air temperature, while the drying constant value was not proportional to the initial moisture content. Consequently, the experimental data were formulated using nine empirical models and the analytical solution of moisture ratio equation was developed by Fick's law of diffusion. The result showed that the simulated data best fitted the logarithmic model and was in reasonable agreement to the experimental data. The effective diffusion coefficient of crumb rubber was in the range of 1.0 × 10^?9 to 2.15 × 10^?5 m²/s corresponding to drying temperatures between 40 and 150°C, respectively. The effects of air recirculation, inlet drying temperature, initial moisture contents, air flow rate, and drying strategies on specific energy consumption and quality of samples were reported. The experiments were conducted using two different drying strategies as follows: one-stage and two-stage drying conditions. The results showed that initial moisture content and air flow rates significantly affected the specific energy consumption and quality of rubber, while the volumetric air flow rate acted as dominant effect to the specific energy consumption. The simulated results concluded that the percentage of recycled air between 90 and 95% provided the lowest specific energy consumption as compared to the others.     

Rehydration Behavior of Vacuum-Microwave-Dried Potato Cubes

Rehydration properties and microstructure of vacuum-microwave and hot air–dried potato cubes were examined. Two kinds of models were considered to describe the hydration kinetics: a diffusion model for a cube and two empirical equations, Peleg and Weibull. The values of the effective moisture diffusivity of soaked potatoes were in the range 1.17 × 10^?9 to 4.73 × 10^?9 m²/s. The vacuum-microwave drying technique resulted in puffed potato particles characterized by porous microstructure with a network of open cavities and the hot air drying gave the potato particles containing compacted cells with the low amount of open micro-caves. Higher rehydration ability was observed for the samples dried with microwaves under low pressure. Vacuum-microwave drying at 6 kPa seems to be the optimal drying condition for potato cubes, ensuring porous microstructure of dried material and low shrinkage of dried potato particles as well as their high recovery properties and rehydration.     

Fluidized Bed Drying of Aonla (Emblica officinalis)

In the present work an attempt has been made to study the dehydration of aonla (Indian gooseberry) fruits. Aonla fruits, being highly perishable, cannot be kept for long periods. Aonla contains a very high amount of vitamin C, which is highly volatile and susceptible to heat. Sun drying required the longest period of drying (660 min), while the shortest time of drying is with fluidized bed drying at 80°C with 115 m/min air velocity (120 min). The results indicate that there is great loss of most of the ascorbic acid in the aonla slices. This suggests that the drying exposure caused the loss of volatile biochemical compounds. The retention of ascorbic acid in the samples dried in fluidized bed drying is greater compared to those dried under sun and hot air tray.