THIN-LAYER DRYING CHARACTERISTICS FOR GREEN CHILLI

ABSTRACT

Thin-layer drying experiments under overflow–underflow and through flow conditions of green chilli were conducted with air temperature ranging from 40 to 65°C, relative humidity ranging from 10 to 60% and air velocity ranging from 0.10 to 1.0 m/s. The single exponential equation and the Page equation were used to determine the thin-layer drying characteristics for green chilli. Both the equations fitted well to the experimental data. The Page equation was found to describe the thin-layer drying of chilli better than the single exponential equation. The parameters of the Page equation and the single exponential equation were expressed as a function of drying air temperature, relative humidity and air velocity.     

Superheated Steam Regeneration of a Desiccant Wheel—Experimental Results and Comparison with Air Regeneration

A commercial zeolite desiccant wheel is tested with atmospheric pressure superheated steam regeneration over a range of air inlet conditions, steam inlet temperatures, and wheel rotation speeds. Results are compared with those from high-temperature air regeneration experiments on the same wheel obtained from the literature. For both cases the air stream to be dried was relatively hot and moist with inlet temperature and absolute humidity values of 50°C and 25 g · kg^?1 chosen to reduce heat carryover. Using steam at 160°C to regenerate the wheel leads to the same dehumidification as using hot air at approximately 90°C. The benefit of superheated steam drying is that a nearly closed-loop regeneration process can be used with potential energy savings on the order of 30%.     

Drying behavior of prina (crude olive cake) using different types of dryers

The purpose of this research is to investigate the drying kinetics and determine the suitable drying method of prina, which is obtained after pressing of olives in olive oil factories, and which cannot be used efficiently in certain sectors. Drying experiments were performed at drying temperatures of 60°C, 70°C, and 80°C at a fixed air velocity of 2 m/s using a hot air dryer and with microwave powers of 90 W, 360 W, and 600 W using a microwave dryer. The prina layer thicknesses were selected as 7, 9, and 11 mm for both drying methods. The minimum energy consumption values were measured as 42.0 Wh for 600 W power level and 7 mm layer thickness, and 10260 Wh for 7 mm layer thickness and 80°C temperatures. It was found that energy consumption during hot air drying was more than that of microwave drying. As a result, the suitable dryer and thickness of layer were selected as microwave dryer and 7 mm, respectively. The results of statistical analyses showed that the most suitable model to define the drying behavior of prina samples were found to be the Page model for the microwave dryer and Wang &; Singh model for the hot air dryer. Also, penetration depth, the loss tangent value (tanδ), dielectric constant of material (?^?), and dielectric loss factor (?^??) of dried prina were calculated as 34.51 cm, 0.1059, 75.65, and 8.01, at 2450 MHz, respectively.     

Oven-Drying Characteristics of Soils of Different Origins

Abstract

The accurate measurement of the moisture content of a soil is an important step in characterizing its engineering behavior. However, the oven-drying method can cause some chemical reaction (oxidation or loss of water of crystallization) to occur in certain soil types. The level of oxidation of the solid particles was studied over the drying temperature range of 60 to 140°C for different soils. The period of oven drying necessary for the specimen mass to equilibrate was also examined. The standard practice of oven drying the soil specimens at 110 ± 5°C or 105±5°C over a period of 24 h was confirmed as giving accurate moisture content values for inorganic soils. Oven drying of peat and other highly organic soils over a period of 24 h at 80°C produced similar levels of accuracy in the moisture content measurements as that for inorganic soils at the standard oven drying temperatures of 105 or 110°C. Some oxidation of the organic fraction commenced at between 80 and 90°C.     

SINGLE-LAYER DRYING BEHAVIOR OF MEXICAN TEA LEAVES (CHENOPODIUM AMBROSIOIDES) IN A CONVECTIVE SOLAR DRYER AND MATHEMATICAL MODELING

Single-layer solar drying experiments were conducted for Mexican tea leaves (Chenopodium ambrosioides) grown in Marrakech. An indirect forced convection solar dryer was used in drying the Mexican tea leaves at different conditions such as ambient air temperature (21° to 35°C), drying air temperature (45° to 60°C) with relative humidity (29 to 53%), airflow rate (0.0277 to 0.0556 m ³/s), and solar radiation (150–920 W/m²). The experimental drying curves showed only a falling rate period. In order to select the suitable form of drying curves, 14 mathematical models were applied to the experimental data and compared according to their statistical parameters. The main factor in controlling the drying rate was found to be the temperature. The drying rate equation was determined empirically from the characteristic drying curve. The diffusion coefficient of the Chenopodium ambrosioides leaves was estimated and varied between 1.0209 × 10^?9 and 1.0440 × 10^?8 m ²·s^?1.The activation energy was found to be 89.1486 kJ·mol^?1.     

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.     

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).     

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.     

Effect of Pulsed Electric Field and Osmotic Dehydration Pretreatment on the Convective Drying of Carrot Tissue

The influence of pulsed electric field (PEF) and subsequent centrifugal osmotic dehydration (OD) on the convective drying behavior of carrot is investigated. The PEF was carried out at an intensity of E = 0.60 kV/cm and a treatment duration of t _PEF  = 50 ms. The following centrifugal OD was performed in a sucrose solution of 65% (w/w) at 40°C for 0, 1, 2, or 4 h under 2400 × g. The drying was performed after the centrifugal OD for temperatures 40–60°C and at constant air rate (6 m³/h).

With the increase of OD duration the air drying time is reduced spectacularly. The dimensionless moisture ratio Xr = 0.1 is reached for PEF-untreated carrots after 370 min of air drying at 60°C in absence of centrifugal OD against 90 min of air drying after the 240 min of centrifugal OD. The PEF treatment reduces additionally the air drying time. The total time of dehydration operations can be shortened when OD time is optimized. For instance, the minimal time required to dehydrate untreated carrots until Xr = 0.1 is 260 min (120 min of OD at 40°C and 140 min of drying at 60°C). It is reduced to 230 min with PEF-treated carrots.

The moisture effective diffusivity D _eff is calculated for the convective air drying based on Fick's law. The centrifugal OD pretreatment increases drastically the value of D _eff . For instance, 4 h of centrifugal OD permitted increasing the value of D _eff from 0.93 · 10^?9 to 3.85 · 10^?9 m²/s for untreated carrots and from 1.17 · 10^?9 to 5.10 · 10^?9 m²/s for PEF-treated carrots.     

Evaluating a Solar Dryer for In-Shell Drying of Split Pistachio Nuts

Abstract

A thin-layer forced air solar dryer was designed to study the feasibility of drying pistachio nuts. The dryer was tested during the 2001 and 2002 drying seasons. The maximum temperature in the solar collector reached 56°C, which was 20°C above the ambient temperature. The required drying time was 36 h. During the first day of drying (0800 to 1700 h) the moisture content dropped to about 21% (wb). The final moisture content of the dried nuts was 6% wb, which was 1% below the recommended storage moisture. The drying constant of the pistachio nuts during solar drying was determined using two mathematical models, a one-term series solution of Fick's diffusion equation and an exponential decaying model. There was no significant difference between the two models (α = 0.05). In general, the quality of solar dried nuts was better than the conventional heated air due to slower drying rates.     

SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.     

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.     

Drying of Seabuckthorn (Hippophae rhamnoides L.) Berry: Impact of Dehydration Methods on Kinetics and Quality

Convective hot air drying and freeze drying were investigated as potential dehydration processes to obtain powders of seabuckthorn fruit pulp. Halved seabuckthorn fruits were placed in a hot air dryer and dried at 1 m/s and at 50 or 60°C or freeze dried at less than 30 mTorr and at 20 or 50°C shelf plate temperature. An initial characterization of the seabuckthorn pulp (moisture, pH, soluble solid content, vitamins C and E, total phenolics, and carotenoids) was performed. Water loss, total phenolic compounds, total carotenoids, and vitamin C were determined at different processing times. Vitamin E was determined before and at the end of drying.

Freeze-drying kinetics were faster than air drying, probably due to lower moisture diffusion in the compact, sugary, and oily structure of the air-dried tissue. The temperature had an important impact on hot air–drying and freeze-drying kinetics. Drying method and processing times affected the remaining phenolic, carotenoid, and vitamin contents of seabuckthorn berries. Freeze drying was revealed as a superior method to obtain seabuckthorn powders because of the lower residual moisture content, the ease of grinding, as well as the better nutritional retention.     

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.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Abstract

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

DRYING CHARACTERISTICS OF RED CHILLI

ABSTRACT

Red chilli is widely consumed as a food additive throughout the world. It is blanched/treated to minimize quality loss during processing. This paper reports on various pre-treatments applied before drying and their influence on drying kinetics as well as product quality. Inactivation of peroxidase enzyme was achieved by blanching chillies at 90°C for 3 min in hot water. The physical appearance of the dried product was found to be the best when the blanched samples were soaked in gum acacia solution (0.2% m/v) for 15 min at room temperature. The pre-treated chillies were dried in a tray dryer at selected temperatures (55, 60, 65 and 70°C). Results indicated that drying took place in the falling rate period; the drying kinetics were adequately described by the Page's model. The activation energy for drying was determined to be 41.95 and 41.06 kJ/mol respectively, for blanched and gum-treated chillies. Total pigment content decreased while non-enzymatic browning increased with increase in drying air temperature.     

Hot-Air Drying and Rehydration Characteristics of Red Kidney Bean Seeds

The effects of pretreatments such as citric acid and hot water blanching and air temperature on drying and rehydration characteristics of red kidney bean seeds were investigated. Drying experiments were carried out at four different drying air temperatures of 50°C, 60°C, 70°C, and 80°C. It was observed that drying and rehydration characteristics of bean seeds were greatly influenced by air temperatures and pretreatments. Four commonly used mathematical models were evaluated to predict the drying kinetics of bean seeds. The Weibull model described the drying behaviour of bean seeds at all temperatures better than the other models. The effective moisture diffusivities (D_eff) of bean seeds were determined using Fick's law of diffusion. The values of D_eff were between 1.25 × 10^?9 and 3.58 × 10^?9 m²/s. Activation energy was estimated by an Arrhenius-type equation and was determined as 24.62, 21.06, and 20.36 kJ/mol for citric acid, blanch, and control samples, respectively.     

Solar Drying of Peeled Longan Using a Side Loading Type Solar Tunnel Dryer: Experimental and Simulated Performance

This article presents experimental and simulated results of drying of peeled longan in a side-loading solar tunnel dryer. This new type of solar tunnel dryer consists of a flat-plate solar air heater and a drying unit with a provision for loading and unloading from windows at one side of the dryer. These are connected in series and covered with glass plates. A DC fan driven by a 15-W solar cell module supplies hot air in the drying system. To investigate the experimental performance, five full-scale experimental runs were conducted and 100 kg of peeled longan was dried in each experimental run. The drying air temperature varied from 32 to 76°C. The drying time in the solar tunnel dryer was 16 h to dry peeled longan from an initial moisture content of 84% (w.b.) to a final moisture content of 12% (w.b.), whereas it required 16 h of natural sun drying under similar conditions to reach a moisture content of 40% (w.b.). The quality of solar-dried product was also good in comparison to the high-quality product in markets in terms of color, taste, and flavor. A system of partial differential equations describing heat and moisture transfer during drying of peeled longan in this solar tunnel dryer was developed and this system of nonlinear partial differential equations was solved numerically by the finite difference method. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5. The simulated results agreed well with the experimental data for solar drying. This model can be used to provide the design data and it is essential for optimal design of the dryer.     

Drying Kinetics and Sorption Isotherms of the Nejayote

Abstract

The nixtamalization is an alkaline cooking process in corn tortillas industry that uses large quantities of water (5:1 corn weight) and producing a byproduct called nejayote. In this work, drying kinetic of insoluble fraction from nejayote was obtained using a hot air cabinet dryer at 60, 75, and 90°C. The air rate and water content were 1.5 m/s and 0.028 kg of water/kg of dry air, respectively. Sorption isotherms were measured by the COST method with sulfuric acid solutions at 30, 45 and 60°C. Results showed a difference in drying kinetics/drying time between drying temperatures of 60 and 75°C, and practically no difference between 75 and 90°C. This was probably due to the development of a superficial crust on the product. The shape of sorption curves was as expected; no temperature effect was identified but it is a common behavior for products with high sugar content. Finally the sorption isotherm was predicted based on primary chemical compounds (carbohydrates, proteins, crude fiber, and calcium hydroxide).     

High-Frequency Heating Controlled by Convective Hot Air: Toward a Solution for On-Line Drying of Softwoods

This article explores the potential of high-frequency heating combined with convective hot and moist air to dry softwood boards as fast as possible while maintaining a reasonable product quality. High-frequency drying experiments were performed in a multipurpose laboratory prototype. This device and the data logging equipment are briefly described in this article. The results obtained at different HF powers on Abies grandis board are presented and discussed. Our results prove that it is possible to dry a 50-mm-thick board from 150 to 5% in about 10 h while maintaining a good final quality by using air flow at 90°C for both dry-bulb and dew-point temperatures and an average HF power of 77 kW·m^?3.