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.     

Thin-Layer Drying of Green Peas and Selection of a Suitable Thin-Layer Drying Model

The thin-layer drying of three varieties of green peas was carried out in hot air-drying chamber using an automatic weighing system at five temperatures (55–75°C) and air velocity of 100 m/min. The green peas were blanched and sulphited before drying. The variety Pb-87 dried at 60°C was judged to be best for quality on the basis of sensory evaluation and rehydration ratio. The Thomson model was found to represent thin-layer drying kinetics within 99.9% accuracy. The effective diffusivity was determined to be 3.95 × 10^?10 to 6.23 × 10^?10 m²/s in the temperature range of 55 to 75°C. The activation energy for diffusion was calculated to be 22.48 kJ/mol. The variation in shrinkage exhibited a linear relationship with moisture content of the product during drying. The Dincer number at drying air temperature 60°C and drying air velocity 100 m/min was determined to be 2,838,087. The difference between temperatures of drying air and that of green pea kernels was found to decrease with drying time for all the drying temperatures taken for investigation.     

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).     

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.     

Rapid Drying of Parboiled Paddy Using Hot Air Impinging Stream Dryer

A method for rapid drying of parboiled paddy via the use of an impinging stream dryer was proposed and assessed. The effects of the drying air temperature, number of drying cycles, as well as time of tempering between each drying cycle on the moisture reduction, head rice yield, and whiteness index of the dried parboiled paddy were studied. The drying experiments were carried out at drying air temperatures of 130, 150, and 170°C; inlet air velocity of 20 m/s; impinging distance of 5 cm; and paddy feed rate of 40 kg_{dry_paddy}/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle the parboiled paddy was tempered for a period of either 0 (no tempering), 15, 30, 60, or 120 min. After impinging stream drying, paddy was ventilated by ambient air flow until its moisture content reached 16% (db). Moisture reduction of the paddy was noted to depend on both the impinging stream drying temperature and tempering time. Drying at a high temperature along with tempering for a suitable period of time could maintain the head rice yield of the paddy at a level similar to that of the reference parboiled paddy. To avoid discoloration and low head rice yield, parboiled paddy should not be dried at a temperature higher than 150°C and should be tempered for at least 30 min.     

Drying of Burdock Root Cubes Using a Microwave-Assisted Pulsed Spouted Bed Dryer and Quality Evaluation of the Dried Cubes

Burdock cube samples were dried using hot air and microwave pulsed spouted bed drying (MPSBD). Hot air drying was carried out at three temperatures (70, 80, and 90°C). MPSBD was carried out at three microwave power levels (1.0, 2.0, and 3.0 W/g). The results showed that MPSBD samples dried at 2.0 W/g for 30 min and at 1.0 W/g for 40 min had desirable color, flavor, and textural attributes. Gas chromatography–mass spectrometry results showed that the samples dried using MPSBD were richer in flavor compounds, especially in esters, compared to the hot air–dried samples.     

The Influences of Drying Process on Crude Protein Content of Naked Oat Cut Herbage (Avena nuda L.)

The effects of thin-layer drying naked oat cut herbage process (hot air temperature, air velocity, and drying time) on crude protein content were investigated. It was observed that an increase in drying temperature could lead to a significant drop in the crude protein content of naked oat cut herbage. The crude protein content also decreased with an increase in drying time and air velocity, and air velocity had a minimal impact during processing. A thin-layer drying mathematical model of crude protein content for naked oat cut herbage was established. The optimal drying parameters were determined as hot air temperature of 88.6–94.2 ^° C, air velocity of 0.28–0.32 m/s, and drying time of 16.4–20.6 min.     

Effect of air velocity in dehumidification drying environment on one-component waterborne wood top coating drying process

In this study, the effect of air velocity in dehumidification drying environment on one-component waterborne wood top coating drying process is analyzed by drying time and moisture content and surface temperature of coating, in which air temperature is 35°C and relative humidity is 50%, and the air velocity is the only change parameter, varying from 0.2 to 1.2?m/s. It is found that drying time of top coating shortens and moisture content of top coating decreases with increasing air velocity. Surface drying time is about 15?min, hard drying time 21?min, and sanded drying time 37?min. To accelerate the drying speed, the air velocity is increased to more than 0.4?m/s. Moisture content of top coating is 58.2% during surface drying, 31.4% during hard drying, and 21.9% during sanded drying time. An infrared thermometer is used to measure the surface temperature of coating. Surface temperature of top coating is 30.0°C when it is dried to the surface drying degree, 33.5°C when the top coating is dried to the hard drying degree, and 34.6°C when the top coating is dried to the sanded drying degree. The drying degree of coating can be judged from the drying time and surface temperature and moisture content of coating. The drying degree of top coating is better when surface temperature is higher and the moisture content is lower.     

Drying Kinetics and Color Retention of Dehydrated Rosehips

Abstract

Freshly harvested rosehips (Rosa canina L.) were dehydrated in a parallel flow type air dryer at six air temperatures (30, 40, 50, 60, and 70°C) at air velocities of 0.5, 1.0, and 1.5 m/s. Drying air temperature and velocity significantly influenced drying time and energy requirement. Minimum and maximum energy requirement for drying of rosehips were determined as 6.69 kWh/kg for 70°C at 0.5 m/s, and 42.46 kWh/kg for 50°C, 1.5 m/s. In order to reduce drying energy consumption, it is recommended that the drying air velocity must not be more than 0.5 m/s and drying air temperature should be 70°C. In addition, the influence of drying air temperature and air velocity on the color of dried rosehip has been studied. Hunter L, a, b values were used to evaluate changes in the total color difference (ΔE) on dried rosehips. 70°C drying air temperature and 1 m/s air velocity were found to yield better quality product.     

Superheated Steam Vacuum Drying of Rubberwood

The effect of superheated steam vacuum drying (SSVD) on the drying time and mechanical properties of rubberwood was studied. Rubberwood boards with dimensions of 1000 mm × 76.2 mm × 25.4 mm were dried at 86.7–89.3 kPa vacuum pressure (14.6–12.0 kPa absolute) and temperatures of 60, 70, and 80°C. Superheated steam at 110°C was injected intermittently to relieve stress buildup in wood and eliminate cracking. The prong test was used to evaluate the initial acceptability of the dried wood and the mechanical properties of wood were measured. From this study, the total drying time was reduced from 168 h to less than 20 h (MC reduction from 0.80 to 0.06 db). In addition, compared to the reference values shown in the parentheses, the shear parallel-to-grain, the compression parallel-to-grain, the compression perpendicular-to-grain, the modulus of rupture (MOR), the modulus of elasticity (MOE), and the hardness for the optimum drying temperature of 70°C were 28.87 (11.0) MPa, 59.09 (32.0) MPa, 21.09 (5.0) MPa, 101.97 (66.0) MPa, 9838.5 (9240.0) MPa, and 6475 (4350) N, respectively. Thus, the vacuum-dried wood showed a 32% increase in hardness, a 12% increase in compression parallel-to-grain, and an 88% increase in shear parallel-to-grain.     

Temperature and Quality Characteristics of Infrared Radiation–Dried Kelp at Different Peak Wavelengths

Drying experiments on kelp (seaweed species) were conducted using air drying (AD) and infrared radiation drying (IRD) at different emission peak wavelengths of 2.4, 3.0, 5.0, and 6.0 µm. Temperature characteristics of the dried kelp were determined in terms of temperature distribution and surface–interior temperature variation. Rehydrated ratio, color, and texture before and after rehydration were measured to evaluate the quality of dried kelp products. Dielectric properties were also studied to observe the characteristics of rehydrated dried products. The results indicated that the total drying time required for IRD products was approximately 120 min, reduced by 56% compared to AD (275 min). Infrared-dried products at 2.4 μm wavelength and AD products were found to be more uniform from the thermal images and had higher rehydration ratios compared to others. IR-2.4 rehydrated products were the closest to blanched samples in hardness, springiness, cohesiveness, and chewiness. This research work concluded that infrared radiation drying has potential to be used for drying of kelp.     

Drying Kinetics of Oil Palm Frond Particles in an Agitated Fluidized Bed Dryer

The drying kinetics of oil palm frond particles in a laboratory-scale agitated fluidized bed dryer were investigated under various operating conditions: inlet air temperature (50–80°C), superficial air velocity (0.6–1.0 m/s), bed load (200–300 g), and agitation speed (300–500 rpm). To study the effects of these variables on the drying time and drying rate, an experimental design using Taguchi orthogonal array was employed. Based on analysis of variance (ANOVA), the results indicated that inlet air temperature greatly affected the drying rate, followed by superficial air velocity and bed load. The effect of agitation speed on the drying rate was found to be small. The experimental drying kinetics data were compared with the values obtained from three different models, namely, the Page model, modified quasi-stationary method (MQSM), and a new composite model. It was found that the proposed new model could satisfactorily predict the complete drying rate curve for the drying of oil palm fronds.     

Comparison of two alternatives of combined drying to process blueberries (O'Neal): Evaluation of the final quality

In this work, we examined and compared two combined alternatives for the drying of blueberries (O’Neal). Pretreatments of osmotic dehydration (60°Brix sucrose solution at 40°C for 6 h) and hot air drying (HAD) (60°C, 2.5 m/s for 90 min) were performed to reach the same water content. Pretreated blueberries were then dried by microwave at different microwave output power values: 562.5, 622.5, and 750 W. The combined drying processes were also compared with HAD alone (control). The effects of the processes over blueberries were studied in terms of decrease in water content, drying rate (DR), mechanical properties (firmness and stiffness), optical properties (L*, a*, and hue angle (h)), antioxidant capacity, and rehydration capacity. The hot air–microwave drying decreased the process time and presented a high drying rate compared with the osmotic dehydration–microwave processes and the control drying. In terms of quality, the antioxidant and rehydration capacities were the most affected. The results showed that the best drying method to obtain the desired final product was the hot air–microwave drying (750 W).     

Drying and Quality Characteristics of Shredded Squid in an Infrared-Assisted Convective Dryer

Drying is one of the most common methods for processing and preserving squids. A novel forced convective dryer based on infrared heating was developed with an online temperature control. By setting the drying medium temperature of 50°C, we studied the effects of infrared wavelength and air velocity on drying characteristics of the shredded squid and qualities of dried squid products. We also compared it with the conventional hot-air drying (HAD) and advanced microwave vacuum drying (MVD). The infrared heating rate increase was faster than that of HAD. The heating and drying at the wavelength of 2.5–3.0 µm were more effective than those at the infrared wavelength of 5.0–6.0 µm. Specific energy consumption linearly increased with the air velocity. Microstructure observation showed that the infrared-dried rehydrated sample displayed a muscle fiber structure similar to the fresh sample. The infrared-dried squids had less drying shrinkage, brighter color, and better rehydration capacity than HAD products. Their sensory qualities were better than HAD and MVD products. Above all, infrared drying with wavelength of 2.5–3.0 µm and air velocity of 0.5 m/s was suggested as the best drying condition for squids in this study.     

Drying Characteristics of Barley Grain Dried in a Spouted-Bed and Combined IR-Convection Dryers

A study was performed to determine the drying characteristics and quality of barley grain dried in a laboratory scale spouted-bed dryer at 30, 35, 40, and 45°C and an inlet air velocity of 23 m/s^?1, and in an IR-convection dryer under an infrared radiation intensity of 0.048, 0.061, 0.073, and 0.107 W cm^?2 at an air velocity of 0.5 m/s^?1. The results show that the first, relatively short, phase of a sharp decrease in the drying rate was followed by the phase of a slow decrease. The time of barley drying depended on temperature of inlet air in a spouted-bed dryer and on radiation intensities in an IR-convection dryer. Barley drying at 45°C in a spouted-bed dryer was accompanied by the lowest total energy consumption. The average specific energy consumption was lower and the average efficiency of drying was higher for drying in a spouted-bed dryer. The effective diffusivities were in the range 2.20–4.52 × 10^?11 m² s^?1 and 3.04–4.79 × 10^?11 m²/s^?1 for barley dried in a spouted-bed and in an IR-convection dryer, respectively. There were no significant differences in kernel germination energy and capacity between the two drying methods tested.     

Forced Convective Drying of Wastewater Sludge with the Presentation of Exergy Analysis of the Dryer

The main objective is studying the fundamental aspect, by means of drying kinetics and the application of forced convective drying of wastewater sludge with the determination of the optimum drying conditions. The drying system is composed of two units; small samples of 2.5 g are dried in the first unit and a bed of sludge weighing 250 g is dried in the second unit. The experiments are performed under air temperatures varying between 80°C and 200°C. The range of the air velocity and its humidity is 1–2 m/s and 0.005–0.05 kg_water/kg_{dry air}, respectively. The experiments are performed for two different sludges: activated sludge (AS) and thermalized and digested sludge (TDS). Usually, three main drying phases are observed during drying of bed of sludge. These phases are reduced to only two for small samples. Determination of the influent parameters shows that the temperature of the drying air and sludge origin can profoundly influence the drying kinetic of the sludge. The exergy analysis of the two units of the drying system allows selecting 140°C, 2 m/s, and 0.05 kg_water/kg_{dry air} as optimum drying conditions with an exergy efficiency reaching 90%.     

Influence of Temperature and Air Velocity on Drying Time and Quality Parameters of Pistachio (Pistacia vera L.)

Abstract

Drying is one of the important steps in pistachio processing. In this step kernel moisture content is decreased from 50 to less than 5% (d.b.) which will result in suitable condition for storage. Study of effective parameters in pistachio drying is important since these parameters influence drying time and kernel quality. In this research, a mono layer of pistachios was dried at three different temperatures (60, 75, and 90°C), and three levels of drying air velocity (1.5, 2, and 2.5 m/s). Changes of drying time, protein, fat and peroxide value were investigated for two common Iranian pistachio varieties Kalehghouchi and Fandoghi. Sensory tests were also used to check flavor of pistachios dried at the three temperature levels (60, 75, and 90°C). Statistical analysis of the data indicated that increasing the temperature to 90°C reduced drying time down by about 37% and caused a change in pistachio flavour. Taste tests indicated a consumer preference for pistachios dried at 75°C. If the air velocity is increased from 1.5 to 2.5 m/s, drying time reduces about 10 percent. Changes in temperature and air velocity have no significant effects on protein and fat content of pistachios, but if temperature reaches 90°C, peroxide value will increase to 0.55 meq/kg, which is still within the permissible limit for processed pistachios.     

Infrared assisted dry-blanching and hybrid drying of carrot

Infrared (IR) blanching and IR assisted hot air (hybrid) drying of carrot slices were attempted and their performance (processing time, retention of vitamin C and rehydration characteristics) was compared with conventional blanching and drying techniques. Intermittent heating of carrot slices using IR radiation (chamber maintained at 180–240 °C) for 8–15 min resulted in desired level of enzyme inactivation. The time required for blanching of carrot slices (10 mm thick) using hot water, steam and IR radiation was 5, 3 and 15 min, respectively. Retention of water soluble vitamin C was higher (62%) in IR blanched carrot as compared to water (43%) and steam (49%) blanching. IR blanching reduced the moisture content by 13–23% (absolute). IR blanched samples dried by hybrid mode took ～45% lesser time compared to water blanched–hot air dried samples. Higher rehydration moisture of dried samples indicated the retention of cell structure during IR blanching. Vitamin C retention was ～39% higher in IR blanched–hybrid dried slices compared to water blanched–hot air dried. The study shows the potential application of dry-blanching and IR assisted hybrid drying in food processing for improving product quality.     

Drying Kinetics of Tomato Slices in Vacuum Assisted Solar and Open Sun Drying Methods

A lab model vacuum-assisted solar dryer was developed to study the drying kinetics of tomato slices (4, 6, and 8 mm thicknesses) compared with open sun drying under the weather conditions of Montreal, Canada. The drying study showed that the time taken for drying of tomato slices of 4, 6, and 8 mm thicknesses from the initial moisture content of 94.0% to the final moisture content of around 11.5 ± 0.5% (w.b.) was 360, 480, and 600 min in vacuum-assisted solar dryer and 450, 600, and 750 min in open sun drying, respectively. During drying, it was observed that the temperature inside the vacuum chamber was increased to 48°C when the maximum ambient temperature was only 30°C. The quality of tomato slices dried under vacuum-assisted solar dryer was of superior quality in terms of color retention and rehydration ratio. The drying kinetics using thin-layer drying models and the influence of weather parameters such as ambient air temperature, relative humidity, solar insolation, and wind velocity on drying of tomato slices were evaluated.     

The Loss of Aroma Components of the Fruit of Vangueria infausta L. (African Medlar) After Convective Drying

The loss of aroma components after drying the fruit of Vangueria infausta L. was studied by means of convective air drying and gas chromatography techniques. The samples were dried at 80°C, with an air flow of 3 m/s at different drying times in the range of 60 up to 420 min. Aroma composition, dry matter, and water activity were measured for each sample throughout the drying time. The study shows that the targeted aroma components present in the fresh fruit sample identified in our previous work (hexanoic acid, methyl hexanoate, methyl octanoate, octanoic acid, ethyl hexanoate, and ethyl octanoate) are retained in the matrix for at least 240 min of drying. Samples dried for 300 min showed decreased amounts of volatiles and practically no aromas were found after 420 min. It is hypothesized that the results are explained as a consequence of the crystallization of sugars during the drying process. The suggested explanation is supported by a prediction of the water activity that is critical for obtaining crystallization of a mixture of sugars corresponding to the fruit.