Determination of Moisture Diffusivity of Lentil Seed During Drying

The mechanism of drying and the diffusion of water in green (fresh), red and whole lentils (4.20-4.32 mm diameter and 2.20-2.30 mm thickness) were successfully interpreted and modelled by using Fick's law. The moisture content of fresh green lentil was 55 % and moisture content of the red and whole lentils was increased to 39-40 % and drying temperature (45-60°C) was varied, but the velocity and the humidity of the drying air was kept constant.

The effective diffusuvity was estimated from drying rate curves and expressed by an Arrhenius relation.     

Standard and Superheated Steam Schedules for Radiata Pine Single-Board Drying: Model Prediction and Actual Measurements

An assessment was made on the predicted drying behavior with its actual measurement for a radiata pine single-board drying model. The predicted temperature profiles within the board showed reasonable agreement compared with experimental drying tests using both fibre optic and thermocouple sensors. The agreement between predicted drying rates and its measurements was acceptable. This model was used to assess predicted drying behavior applying schedules (dry-bulb/wet-bulb: 140/90°C, 140/99°C, 160/90°C, 160/99°C). The difference in drying rate was small for wet-bulb at 90°C compared with that at 99°C.     

DETERMINATION OF PARAMETERS AND PRETREATMENT SOLUTION FOR GRAPE DRYING

The drying of Iranian seedless white grapes (sultana) was investigated in a batch operation in a laboratory dryer. The effect of pretreatment and temperature on the drying rate of grapes at constant air velocity were studied. Pretreatment solutions contained different alkaline materials in different concentrations and temperatures. Dipping grapes in an alkaline solution increased the drying rate substantially. Increasing the air temperature in the dryer from 60 to 70°C accelerated the drying rate (for a decrease in drying time) of the pretreated grape. Grapes dried in 450-900 min depending on pretreatment and air temperature. The shorter drying time and best quality dried product were obtained with grapes dipped in a solution of potassium carbonate of 5% at 42°C. The mean apparent diffusivity of moisture for K₂CO₃ pretreatment was calculated at 50, 60, and 70°C, and the values were fitted to an Arrhenius type model. Moisture sorption isotherms of currants were determined at 20, 30, and 40°C using static gravimetric methods. A modified Halsay equation was used to predict experimental data for water activity in a range of 0.1-0.85.     

HIGH TEMPERATURE DRYING FROM THE THREE PRIMARY ORTHOGONAL SURFACES OF LOBLOLLY PINE

While several permeability studies have been performed, little information is available on the flow of water in wood along its three orthogonal axes during high temperature drying. Much southern yellow pine construction lumber is dried at temperatures above 100°C, thus how water moves in wood is of concern. In this experiment individual Loblolly pine (Pinus taeda) sapwood cubes were coated on two of their three orthogonal faces (longitudinal, radial, and tangential) and the effect of this sealing on the drying rate was determined. From these cubes, drying rates from each of the three primary planes were determined. The free water movement/evaporation between the radial and tangential faces was not statistically different, while the longitudinal rate was approximately three times faster. In commercial practice, the faster longitudinal rate is negated by the very small portion of transverse surface area relative to total lumber surface area. This partially explains why end checking and moisture variation in high temperature dried southern yellow pine lumber are not significant causes of degrade. There was no statistically significant difference in permeability across the seasons tested.     

HIGH ELECTRIC FIELD DRYING OF JAPANESE RADISH

Japanese radish (Raphanus sativus L.) slices were dried under an alternating current (AC) high electric field (HEF) of 430 kV/m using a multiple point electrode and a grounded copper mesh. HEF with multipoint to a mesh electrode system could evaporate 87.5% of the total moisture from fresh radish slices on 7 h exposure. Oven-drying at 60°C and ambient air at 25°C for 7 h could dry 86.9% and 26.5% of the total moisture from the fresh radish slices respectively. The average rate of HEF drying from second hour to sixth hour was 0.025 g/min which was 0.0012 g/min higher than the oven-drying. Quality factors for HEF-dried radish exhibited less shrinkage, high absorption of water, better rehydration, less solids loss and better color than the oven-drying.     

ADSORPTION ISOTHERMS OF RAW AND ROASTED HAZELNUTS

Adsorption isotherms of raw and roasted (at 100°C, 120°C, 140°C and 160°C) hazelnuts of Tombul variety were determined at 25°C, and over relative humidities ranging from 11 to 90% using the gravimetric static method. Eleven sorption models (GAB, BET, Modified BET, Halsey, Iglesias-Chirife, Oswin, Chung-Pfost, Smith, Kuhn, Harkins-Jura and Caurie) were evaluated by using the linear regression method. GAB (Guggenheim-Anderson-de Boer) model satisfactorily described adsorption characteristics of the samples over the experimental range of study. Monolayer values at 25°C were estimated from the GAB model as 0.0155 kg water/kg dry solid for raw and 0.0100-0.0163 kg water/kg dry solid for roasted hazelnuts. Caurie security moisture content for raw hazelnuts was also determined from Caurie model as 0.048 kg water/kg dry solid.     

DRYING CHARACTERISTICS OF THE HAZELNUT

Equilibrium moisture content isotherms for Spanish hazelnut (Corylus avellana L.) at different temperatures (30°C-80°C) were determined using static gravimetric method. Thin layer drying experiments were done with forced air circulation and were conducted with different operating conditions to determine the drying characteristics of hazelnuts. The effect of air temperature (30°C-70°C), air velocity (0.5 m/s - 2 m/s) and drying bed loading density (50 kg/m² - 150 kg/m²) on drying of unshelled and shelled hazelnuts was studied. Six mathematical models were used to fit the experimental equilibrium moisture content data, from which the G.A.B. model was found to give the best fit. Diffusion coefficients were determined by fitting experimental thin-layer drying curves to the Fick's diffusion model. Variation of the effective diffusion coefficient with temperature was of the Arrhenius type. The Page equation was found to describe adequately the thin layer drying of hazelnut. Page equation drying parameters k and n were correlated with air temperature and relative humidity.     

DEHYDRATION OF PEARS WITHOUT SULPHUR DIOXIDE

The experimental dehydration behaviour of Bon Chretien pears dehydrated without sulphites was investigated during isothermal (67°C) and simulated counter-flow dehydration (48°→60°C and 58°→67°C). Dehydration of peeled halves and quarters treated with sodium chloride retarded browning during isothermal dehydration relative to untreated halves. This retarding effect was enhanced by using dehydration conditions favouring high drying rates, low superficial product water activity and low superficial product temperatures Ascorbic acid-treated samples exhibited considerable browning, practically to the same degree as untreated samples-Objective colour measurements of dried products subjected to different dehydration conditions are presented. Constant rate drying periods were observed in counter-flow simulations (48°→60°C) at higher humidity (30% RH), but not at relatively low humidity (20% RH). It was found that enzymatic browning reactions could be controlled during dehydration by selecting dehydration conditions favouring low superficial product temperature and water activity (low humidity) in conjunction with a suitable inhibitor of polyphenol oxidase. Thus, it was found that simulated counter-flow dehydration (48°→60°C and 58°→67°C) of pear quarters gave significantly lighter coloured products when dehydrated at 20% RH in comparison to dehydration at 30% RH     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

DRYING CHARACTERISTICS OF RED CHILLI

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.     

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.     

EFFECT OF WATER SPRAY LOG STORAGE ON SOUTHERN PINE VENEER DRYING

A reduced temperature drying schedule was investigated as a means of improving grade recovery of structural veneer cut from water stored southern pine logs. Fresh cut, one year old, and three year old logs were evaluated. In total, close to one thousand sheets of veneer were evaluated. Reducing the temperature in the last zone of the veneer dryer from 193 to 177°C substantially decreased incidence of grade-reducing splits. Moisture content average and distribution were not effected. Water sorption tests showed favorable results from the one year old material that was dried at the reduced temperature schedule. Relatively high thickness variation of veneer cut from water stored logs had no apparent affect on final moisture content average or distribution. Reducing the zone three temperature by 16°C increased the total veneer drying times by 13 and 19%, respectively, for the three year old and one year old material.     

EFFECT OF TEMPERATURE AND RELATIVE HUMIDITY ON THE DRYING RATES AND DRYING TIMES OF GREEN BELL PEPPERS (CAPSICUM ANNUUM L)

Green bell pepper dices were dehydrated at different dry bulb air temperatures (55°, 60°, 65°, 70° and 75°C) and relative humidities (15, 20, 25, 30, 35 and 40%). The effects of temperature and relative humidity (RH) on the drying rates and drying period of diced green bell peppers were determined. Drying rate curves were characterized by a short induction (heating) phase followed by a falling rate period. Near constant rate drying was observed only at 55°C at 15% RH and at 65°C (15% RH). Drying rates generally increased with increasing temperatures and decreasing RH. The effect of temperature on the drying rates became less pronounced with increasing RH Drying rate maxima at 70°C and RH of 15, 20 25 and 40% exceeded those at 75°C, possibly due to case-hardening.     

DRYING KINETICS AND CHARACTERISTIC DRYING CURVE OF LIGHTLY SALTED SARDINE (SARDINELLA AURITA)

Drying of lightly salted sardine (Sardinella aurita) was accomplished using three air temperatures (35°C, 40°C, 50°C) and three air velocities (0.5 m/s, 1.5 m/s, 2 m/s); the effects of drying conditions on drying kinetics were studied. As for all biological products, air temperature is the main factor influencing the drying kinetics. However, over a given temperature which seems to correspond to protein modification (50°C), and at a high air flow rate (2 m/s and 2.5 m/s) a crust formation on the surface of the fish, due to the combined effect of heat and salt was observed. This phenomenon inhibited the drying rate. From the drying curves, two falling rate periods were observed. The dimensionless drying rate versus a dimensionless moisture content data were regressed by the Marquardt Levenberg non-linear optimization method to obtain an empirical equation describing the salted sardine characteristic drying curve.     

ACOUSTIC DRYING OF GREEN RICE

The effects of acoustic drying of rice in a fluidized bed were evaluated at frequencies of 12 kHz and 19 kHz and air temperatures of 20.5° C and 40° C. The intensity levels were 132 and 128 dB respectively, intensities that are considerably lower than the 145 dB and higher usually applied in drying experiments of this type. The results confirmed that the effectiveness of acoustic drying is more pronounced at the lower temperatures, a significant fact especially in drying heat sensitive materials. The rate of water removal increased when acoustical energy at either frequency was applied with room temperature air. Increase in drying rate was not obvious when acoustic energy was applied at 40° C at the intensity levels of these experiments.     

Drying Kinetics and Sorption Isotherms of the Nejayote

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).     

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.     

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

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.     

THE DEHYDRATION OF GARLIC 2. THE EFFECTS OF PRETREATMENTS ON DRYING KINETICS

The kinetics of drying of pretreated garlic slices of thickness 1.5 - 5.0 mm were determined at between 40 and 60°C using an air flow rate of 2.5 m/s. Pretreatmenl consisted in immersion of the slices in water or solutions of K₂CO₃, K₂CO₃ with olive oil, Na₂S₂O₅ or NaOH, at 25°C for 60 or ISO s. The effects of each pretreatmenl on the drying kinetics and the quality of the dried product were evaluated, in the latter case by determining its rehydration ratio. Pretreatment with water or the Na₂S₂O₅ solution gave good results regardless of the thickness of the garlic slices, whereas pretreatment with a dilute NaOH solution gave good resuhs for thin slices (≤2.5 mm). The experimental kinetic data were satisfactorily fitted by a difiusional kinetic model developed in Part 1 of this work.     

PROTOTYPE HIGH TEMPERATURE/HIGH PRESSURE KILN FOR THE EVALUATION OF WOOD DRYING SCHEDULES

A new laboratory kiln was developed and built to perform over a very wide range of drying conditions. For example, the dry bulb temperature can vary from 30°C to 150°C and the dew point can be adjusted between 20°C and 130°C. Obviously, with such a high level of dew point, pressures over atmospheric pressure may be induced inside the chamber. For this reason, the kiln has been designed to withstand pressure of up to 3 bars. This kiln can also perform vacuum drying.

A programmable controller allows the temperature levels to be maintained within ± 0.2°C. Because the whole kiln can be heated only through the agitated water present at the bottom of the kiln, the load temperature can be increased up to 130°C in saturated conditions, without any change of moisture content.

The kiln has various sensors attached and is capable of withstanding severe conditions (high temperature, saturated vapour and elevated pressures). At present, air and water temperatures as well as temperature at different locations within the board can be collected during the drying process. A load cell and pressure gauges are also available. The first tests performed using this equipment are presented at the end of the paper.