Mechanisms of Drying of Skin-Forming Materials. II.Droplets of Heat Sensitive Materials

ABSTRACT

Drying studies were carried out on single droplets of gelatin solution, typifying a skin forming, heat-sensitive material. Each droplet was suspended from a novel rotating glass nozzle which enabled direct measurement of its weight and temperature during drying. Observations using this appartatus successfully detected the decrease in core temperature and increase in drying rate resulting from granule dissolution at 30 °C.

The study covered air temperatures in the range 19 °C–175 °C. Scanning Electron Microscopy was used to examine the dried crusts. The resistance of the skin to vapour diffusion was significantly increased with an increase in the air temperature. At air temperature > 60 °C the dried particle lost the structure and properties of the original powder. The optimum drying conditions to avoid this were predicted for this type of skin forming-material.     

Temperature stability of vitamin D2 and color changes during drying of UVB-treated mushrooms

The study investigates the effect of drying temperature on vitamin D₂ content and color changes of UVB-treated shiitake (Lentinula edodes), oyster (Pleurotus ostreatus), and white and brown button mushrooms (Agaricus bisporus). Fresh samples were UVB treated up to 1.5?J/cm² for 20?min at 25°C and either dried in a high precision dryer (temperatures: 40, 60, 80°C, specific humidity: 10?g/kg, air velocity: 0.6?m/s) or frozen at ?25°C, and then freeze-dried (pressure: 0.28?mbar). Vitamin D₂ content was not negatively affected by the increased temperatures of the drying air. The highest content of vitamin D₂ was detected in freeze-dried (171.84?µg/g) and hot-air dried shiitake at 60°C (169?µg/g), followed by oyster (121.96?µg/g), whereas the lowest amount was observed in brown button mushrooms at 40°C (34.65?µg/g). Although vitamin D₂ indicated a remarkable stability even at 80°C, the dried samples were characterized by intensive tissue darkening.     

Kinetics of ultrasound pretreated apple cubes dried in fluidized bed dryer

Abstract

This work evaluated the effect of ultrasonic pretreatment on the production of dehydrated apples (Malus domestica L. var Granny Smith) in a fluidized bed dryer. Cube-shaped apple samples were subjected to ultrasound in an ultrasonic bath and dried in a fluidized bed drier. The experimental design evaluated the effect of ultrasound pretreatment time (0 to 30?min) on the soluble solids loss during pretreatment and on the drying time. The ultrasonic pretreatment was carried out in a bath ultrasound operating at 25?kHz and outputting 55?W/m³ of power density. Distilled water was applied in the pretreatment to produce low-calorie apple cubes. Fluidized bed drying was carried out at 30, 40, and 50?°C. Fick’s law was used to model the drying process and to determine the apparent water diffusivity. The soluble solid loss ranged between 8.7 and 21.2% during the pretreatment, and the apparent water diffusivity during air drying ranged from 1.09?×?10^?6 to 2.81?×?10^?6 m²/min. Ultrasound pretreatment increased the apparent water diffusivity up to 58%. Apple cubes subjected to 20?min of ultrasound pretreatment and dried at 50?°C presented the highest apparent water diffusivity and dried to achieve a water activity of 0.4 in 100?min.     

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

ABSTRACT

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.     

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.     

Artificial neural network approach for protection of the color of dried golden and pink oyster mushrooms with pretreatments

In this study, the possibilities of protecting the color of dried golden and pink mushrooms were investigated, and color parameters of dried mushrooms were modeled by artificial neural network (ANN). For this purpose, first, the golden oyster mushroom (Pleurotus citrinopileatus) and pink oyster mushroom (Pleurotus djamor) were cultivated. Then, pretreatments were applied using citric acid (CA) and potassium metabisulfite (KMS) with different rates (0.5%, 1.0%, and 1.5%) separately, excluding control group mushrooms. All mushrooms were dried for 330 minutes in a laboratory type oven at two different temperatures (40°C and 50°C) until completely dehydrated. Colorimetric values (L*, a*, and b*) were determined using Konica Minolta CM‐2600d spectrophotometer for 30 minute intervals during the drying process. The obtained data were modeled using the ANN technique. The results show that darkening of mushrooms increased as the drying temperature increased. CA and KMS showed better results for dried golden and pink mushrooms, respectively. Thanks to the pretreatment, the mushroom's original color was protected compared with control samples. All mean absolute percentage error values of models were determined, which were lower than 4.0%. It was concluded that ANN can be a good way to predict the color of dried golden and pink mushrooms (pretreated or not) with a high degree of accuracy.     

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.     

Comparative Evaluation of the Effects of Electrohydrodynamic,Oven, and Ambient Air on Carrot Cylindrical Slices during Drying Process

Electrohydrodynamic (EHD) drying is a novel method of nonthermal drying. A corona discharge using multiple electrodes and a high-voltage electric field of 5.2 kV · cm^?1 was produced to investigate the drying enhancement of carrot slices and its effect on color and shrinkage. The EHD setup consisted of 13 stainless steel needle points connected to a DC power supply and a stainless steel plate. EHD⁺ drying, EHD^? drying, oven drying at 55°C, and ambient air drying control at 25°C for 5 h resulted in 79.5, 77.7, 77, and 22.5% total moisture removal from the fresh carrot slices, respectively. The final shrinkage of the EHD^± drying was less than that of oven drying but was higher than that of ambient air drying. It was estimated that the energy consumption of oven drying was several times greater than those of EHD^± drying. The conventional drying processes changed all color parameters, whereas the color for EHD^± dried samples remained almost the same. The carrot slices’ temperature during drying by EHD^± was significantly less than that of those dried by oven and ambient air drying.     

CONCURRENT FLCW RICE DRYING

ABSTRACT

Long- and medium-grain rice were dried in a commercial multi-stage concurrent-flow dryer. Drying air temperatures varied fran 82°C to 177°C. Over six points of moisture were removed in one dryer pass without affecting the rice head-yield. Energy consumption of the dryers was half that of conventional rice dryers. Simulation played a major role in the design of the mUlti-stage concurrent-flow rice dryers.     

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.     

An Experimental Study on Drying of Green Apples

Drying behavior of green apples in a laboratory dryer was examined. Prior to drying, the apples were cut in 8 mm thick slices, which were then treated with citric acid solution and blanched hot water at 80°C. Next, they were dried at 65°C with an air velocity of 2.0 m/s. The shortest drying time (270 min) was obtained with apples pretreated with citric acid solution. The drying data were fitted with 11 mathematical models available in the literature. Selection of the best model was investigated by comparing the determination of coefficient (R ²), reduced chi-square (χ²), root means square error ( RMSE ), and mean relative percentage error (P) between the experimental and predicted values. The results showed that the Wang and Singh, logarithmic, and Verma et al. models gave the best results in describing thin-layer drying of apple slices. The effective moisture diffusivity of pretreated samples with citric acid solution was higher than the other samples.     

Ultrasound-assisted heat pump drying of pea seed

ABSTRACT

The work is for the purpose of studying the influence of drying temperature and ultrasound on drying kinetics, antioxidant enzymes, and germination performance of pea seeds. The drying experiments were performed at air temperature of 30, 35, and 40°C without ultrasound and with three ultrasonic levels of 28?kHz?+?60?W, 28?kHz?+?100?W, and 40?kHz?+?60?W. The antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and the toxic substances such as malondialdehyde (MDA) of the dried seeds were determined, and germination percentage (GP), germination index (GI), and mean germination time (MGT) were measured. The result showed that high temperature and ultrasound application had significant (P?<?0.05) enhancing of pea seed drying kinetics, which shortened the drying time and improved the diffusion coefficient from 3.528?×?10^?11 to 5.668?×?10^?11?m²/s. Page model can well describe the drying curves under different experiment conditions. Ultrasound application significantly (P?<?0.05) improved the activities of SOD, POD, and CAT and reduced the MDA content. In addition, high ultrasonic power contributed to the increase in GP and GI and the reduction of MGT of seeds.     

DRYING OF SUGAR BEET PULP USING A LABORATORY AIR DRIER

ABSTRACT

Drying curves for sugar beet pulp were obtained in a laboratory-scale air drier. Dehydration conditions were: temperatures in the range of 35 to 90 ^°C, air flow velocities of 0.5 to 2.0 m/s, and air moisture content of 0.005 to 0.06 kg water/kg air. Using data from thin layer experiments, a mathematical model was derived to describe the dehydration behaviour under the described conditions. The model was able to predict data obtained from deep bed experiments. Results from this work could be used to develop a design methodology for dryers that operate at relatively low temperatures (<90^°C).     

Effect of Pretreatments on Drying and Rehydration Kinetics and Color of Sweet Potato Slices

The effect of air temperature and pretreatments (KMS: citric acid) on drying kinetics of sweet potato slices was investigated. Drying experiments were performed in a tray dryer. In falling rate period, moisture transfer from sweet potato slices was described by applying the unsteady-state Fickian diffusion model, and the rate constant (k) were calculated. The effect of temperature on k could be interpreted according to Arrhenius law. Drying rate and therefore k values were found to be affected by pretreatments. Rehydration rates of dried sweet potato slices at 25, 40, 80°C were also determined and found to be independent of drying conditions and rehydration temperature. The ΔE value was found to be the highest for slices treated at 50°C with 0.5:1.0% KMS and citric acid.     

Diametral strength testing of hydroxyapatites doped with yttrium and fluoride

Abstract

Abstract

This study aimed to investigate the diametral strength testing of hydroxyapatite (HA) doped with Y and fluoride with different compositions. Hydroxyapatites were synthesised by precipitation method and sintered at 900, 1100 and 1300°C for 1?h. High amounts of doping caused a decrease in relative densities of HAs. Higher sintering temperatures helped in increasing the relative densities. No second phases were observed by X-ray diffraction spectra of 2·5?mol.-%Y and 2·5?mol.-%F doped HA after the sintering at all temperatures. Trace amounts of β-tricalcium phosphate was found in 7·5?mol.-%Y and 2·5?mol.-%F doped HA sintered at 1100 and 1300°C. Diametral strength of doped HAs mostly enhanced with the addition of Y³⁺ and F^?. 2·5YFHA sintered at 1300°C had the highest diametral strength of 11·6?MPa with a relative density of 94·3% of theoretical density.     

Convective Drying of Wastewater Sludge: Introduction of Shrinkage Effect in Mathematical Modeling

Drying of two kinds of wastewater sludge was studied. The first part was an experimental work done in a discontinuous cross-flow convective dryer using 1 kg of wet material extruded in 12-mm-diameter cylinders. The results show the influence of drying air temperature for both sludges. The second part consisted of developing a drying model in order to identify the internal diffusion coefficient and the convective mass transfer coefficient from the experimental data. A comparison between fitted drying curves, well represented by Newton's model, and the analytical solutions of the equation of diffusion, applied to a finite cylinder, was made. Variations in the physical parameters, such as the mass, density, and volume of the dried product, were calculated. This allowed us to confirm that shrinkage, which is an important parameter during wastewater sludge drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by the air temperature and the origin of the sludge. The values of the diffusion coefficient changed from 42.35 × 10^?9 m² · s^?1 at 160°C to 32.49 × 10^?9 m² · s^?1 at 122°C for sludge A and from 33.40 × 10^?9 m² · s^?1 at 140°C to 28.45 × 10^?9 m² · s^?1 at 120°C for sludge B. The convective mass transfer coefficient changed from 4.52 × 10^?7 m · s^?1 at 158°C to 3.33 × 10^?7 m · s^?1 at 122°C for sludge A and from 3.44 × 10^?7 m · s^?1 at 140°C to 2.84 × 10^?7 m² · s^?1 at 120°C for sludge B. The temperature dependency of the two coefficients was expressed using an Arrhenius-type equation and related parameters were deduced. Finally, the study showed that neglecting shrinkage phenomena resulted in an overestimation that can attain and exceed 30% for the two coefficients.     

EVALUATION OF DIFFERENT METHODS OF DRYING ON THE QUALITY OF OYSTER MUSHROOM (Pleurotus sp)

ABSTRACT

Mushrooms are extremely perishable and have a short shelf life period. To increase the post harvest lives, different methods of drying viz., sun diving, fluidized bed drying and thin layer drying with potassium metabisulphite (KMS) and blanching was done. The quality of mushroom dried in fluidized bed condition at 50°C for 80-120 minutes with 0.5 KMS was found to be superior to other drying methods. Rehydration ratio was also maximum in KMS treated mushroom and no significant difference at higher concentration. The treatment with KMS and blanching reduces the nutritive quality but improves the colour of the mushrooms when compared with sun dried samples. Storage of mushrooms after treatment with KMS at higher concentration (1.5%) reduces the microbial spoilage.     

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.     

DRYING BEHAVIOR OF MUSHROOM SLICES

Drying of mushroom slices is an unsteady-state process under the control of diffusion race of water through the mushroom hyphae. Therefore, temperature, thickness of the mushroom slices and the ratio of the air film to mushroom resistance toward the diffusion of water control the drying rate. In this work, the initial thickness of mushroom slices was kept constant at 2 mm and the effects of structural parameters, temperature and humidity were investigated. Through the measurements of weight, center and surface temperatures of the mushrooms, and variation of the surface area, actual drying behavior of mushrooms under different dryer conditions were determined. Color of the dried mushroom slices were used as criteria for the evaluation and determination of the optimum conditions.     

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

ABSTRACT

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.