Spouted bed and microwave-assisted spouted bed drying of parboiled wheat

The effect of spouted bed and microwave-assisted spouted bed drying on drying rates of parboiled wheat was investigated. In addition, the effective moisture diffusivities of parboiled wheat were calculated. The drying experiments were performed using 200 g of parboiled wheat, at three different air temperatures (50, 70, 90 °C) and at two different microwave powers (3.5 W/g (db), 7.5 W/g (db)). Microwave-assisted spouted bed drying at microwave power of 3.5 W/g and 7.5 W/g reduced drying time by at least 60% and 85%, respectively compared to spouted bed drying. The effective diffusivity values were in the range of 1.44 × 10^?10–3.32 × 10^?10 in spouted bed drying while they were between 5.06 × 10^?10 and 11.3 × 10^?10 in microwave-assisted spouted bed drying at different experimental conditions.     

Quality evaluation of pineapple fruit during drying process

Pineapple (Anana comosus) slices were dried by hot-air convective drying technique at fixed temperature (45, 60 and 75 °C) and constant air velocity of 1.5 m/s. The effect of drying conditions (drying time and air temperature) on the pineapple quality was evaluated. The quality of dehydrated pineapple was analyzed by color and texture changes, l-ascorbic acid loss and the ability of water uptake during rehydration procedure. Water uptake during rehydration was described by Page model. Statistical analysis of data revealed not significant difference (p > 0.05) among color and mechanical characteristics of pineapple samples dried at different drying temperatures to preset moisture content. Pineapple samples dried at 45 °C had better rehydration ability and more l-ascorbic acid retention than those obtained by air drying 75 °C. Hence, 45 °C drying temperature was best condition for pineapple quality preservation.     

Thin layer drying kinetics of Gundelia tournefortii L.

The literature surveyed revealed that the drying kinetics of Gundelia tournefortii has not been investigated. In this study, mathematical modeling of the thin layer drying kinetics of G. tournefortii is investigated for both the microwave and open sun drying conditions. Five different microwave power levels ranging from 90 to 800 W were used for the microwave drying. Solar radiation for the open sun drying varied from 350 to 1100 W/m². Drying took place in the falling rate period. Increasing the microwave power caused a significant decrease in drying time. The experimental moisture loss data were fitted to the 14 thin layer drying models. Among the models proposed, the Midilli model precisely represented the microwave drying behavior of G. tournefortii with the coefficient of determination higher than 0.996 and mean square of deviation (χ²), root mean square error (RMSE) and mean bias error (MBE) lower than 1.82 × 10^?4, 12 × 10^?3 and 1.4 × 10^?4, respectively for all the microwave drying conditions studied. Values of drying constant (k) were in the range of 0.0098–0.2943 min^?1 and the effective moisture diffusivities (D_eff) of G. tournefortii ranged from 5.5 × 10^?8 to 3.5 × 10^?7 m²/s. The values of k and D_eff increased with the increase of microwave power level. The logarithmic model was found to best describe the open sun drying kinetics of G. tournefortii. The effective diffusivity of G. tournefortii under the sun drying condition was determined as 2.48 × 10^?10 m²/s.     

Effect of drying methods on volatiles of Chinese ginger (Zingiber officinale Roscoe)

Fresh ginger slices were dehydrated by air drying (AD), microwave drying (MD), vacuum drying (VD), and freezing drying (FD). Volatiles were extracted from fresh ginger pulp and dried ground ginger powder with solid-phase microextraction (SPME), and identified by gas chromatography–mass spectrometry (GC–MS). Results indicated that, 19, 28, 21, 20, 31 and 20 novel compounds (70 in total) appeared in dried gingers treated by AD at 50, 60 and 70 °C, MD at 60 W, VD in 13.3 kPa at 60 °C and FD in 0.203 kPa at chamber temperature of 22 °C, respectively. Principal component analysis for the main volatiles indicated that drying increased the relative contents of benzene,1-(1,5-dimethyl-4-hexenyl)-4-methyl-, 1,3-cyclohexadiene,5-(1,5-dimethyl-4-hexenyl)-2-methyl-,[S-(R*,S*)]-, α-farnesene and cyclohexene,3-(1,5-dimethyl-4-hexenyl) -6-methylene-,[S-(R*,S*)]- while decreased those of 2,6-octadienal,3,7-dimethyl-,(Z) and 2,6-octadienal,3,7-dimethyl-. Cluster analysis disclosed that MD was the most favorite drying way, followed by AD at 60 °C, VD, FD, and AD at 50 and 70 °C.     

Storage stability of vacuum-dried probiotic bacterium Lactobacillus paracasei F19

There is still lack of the insight into the storage stability of dry probiotics produced by vacuum drying. Therefore, in this study we assessed the stability of a vacuum-dried Lactobacillus paracasei F19 under varying storage conditions. L. paracasei F19 was vacuum-dried with and without sorbitol and trehalose. The dried cells were stored at 4, 20 and 37 °C, and at a_w = 0.07, 0.22 and 0.33. The survival was determined by viable counts on MRS agar plates. The inactivation rate constants were determined for each storage condition. The survival after drying of cells dried without and with trehalose and sorbitol was 29, 70 and 54%, respectively. All vacuum-dried cells were very stable at 4 °C. However, high stability at non-refrigerated temperatures was obtained only in the presence of sorbitol. In contrast to sorbitol, the supplementation of trehalose did not stabilize cells during storage. This is supposedly due to the rapid crystallization of trehalose during storage. While glass transition temperatures of dry cell-sorbitol increased from ?32 °C to 12 °C during storage at 37 °C and a_w = 0.07, T_g of dry cell-trehalose (?15 °C after drying) could not be determined after storage for only 24 h. In conclusion, we showed that high stability of probiotic cells at non-refrigerated temperatures could be obtained by vacuum drying process with appropriate protectant.     

Convective drying of onion: Kinetics and nutritional evaluation

The present work aimed, on one hand, to study of the drying of onions in terms of drying kinetics, which was evaluated at 30 °C, 50 °C and 60 °C. The experimental data was fitted to different empirical kinetic models from the literature, and this kinetic study was then complemented with the modelling if terms of Fick's diffusion equation, for estimation of the diffusion coefficients. On the other hand, the chemical characterization in fresh and dried onions at different temperatures (varying from 30 °C to 70 °C) was analysed, to evaluate the effect of drying and drying temperature on the chemical composition of the product. In this way, the analyses of moisture content, sugar content, crude protein, ash, fat, crude fibre, acidity and vitamin C were made and reported in this paper.From the results obtained it was verified that some chemical components of the onions are not affected by drying (ash, fat, protein and fibre) whereas some others are considerably influenced by drying (sugars, acidity and vitamin C). The present work allowed concluding that the three empirical models tested (Newton, Modified Page and Logarithmic) all describe relatively well the dehydration kinetics at the three temperatures analysed. Moreover, from the experimental data it was possible to estimate the diffusivities, which range between 3.33 × 10^?09 m²/s at 30 °C and 8.55 × 10^?09 m²/s at 60 °C.     

Changes in physico-chemical and functional properties during convective drying of aloe vera (Aloe barbadensis) leaves

Aloe vera leaves were dried at different temperatures in hot air oven and powdered. The percent powder yield was found 2.60%, 2.60%, 2.55% and 2.52% at 50, 60, 70 and 80 °C respectively. Powder samples had the pH (1% solution) 3.51, 3.53, 3.52 and 3.53 with the rise of drying temperature in the selected range. Statistically, yield and pH indicated no significant difference (p < 0.5) due to drying temperature variation. Wettability of powder at 70 °C was 32 s as compared to 35, 35 and 37 s in the samples obtained at 50, 60 and 80 °C respectively. Water absorption capacity of powder at 70 °C was 359% as compare to 351%, 354% and 356% of 50, 60, and 80 °C powder samples. The HPLC chromatogram obtained for the sample dried at 80 °C shows that as the temperature increased from 50 to 80 °C, aloin content decreased from 10.6 to 1.7 ppm. The “a” values were found 2.028, 2.226, ?0.282 and 2.531 for the samples obtained after drying at 50, 60, 70 and 80 °C respectively. Samples obtained at 70 °C showed negative “a” value indicated that the sample was more greenish in colour as compared to other samples.     

Effect of drying temperatures on starch-related functional and thermal properties of chestnut flours

The use of starchy flours in food systems greatly depends on the related functional properties of starch. The effect of drying temperatures on starch-related functional properties of flours obtained from fruits of the two most common Portuguese Castanea sativa varieties (Martainha and Longal) was evaluated. Flours were analysed for amylose and resistant starch contents, swelling ability, pasting properties and thermal characteristics. Drying temperature is positively correlated with amylose content, resistant starch and viscoamylographic properties, mainly the temperatures higher than 40 °C. Amylograms of fruits dried at 60 °C displayed higher peak viscosity (1370 B.U. and 2260 B.U. respectively for Longal and Martainha) when compared to the other temperatures tested (40 °C, 50 °C and 70 °C). Decreases in transition temperatures and in enthalpy evaluated by thermal analysis were observed with increasing drying temperatures, suggesting modifications in starch structure during the drying process. The effects of drying temperatures were more evident in Longal variety. The flours from the two chestnut varieties and from fruits dried at low temperatures and fruits dried at high temperature showed significant differences between the evaluated properties.     

Effect of temperature on structural properties of Aloe vera (Aloe barbadensis Miller) gel and Weibull distribution for modelling drying process

Aloe vera (Aloe barbadensis Miller) gel was dried at five inlet temperatures 50, 60, 70, 80 and 90 °C, in a convective dryer with a constant air flow of 2.0 ± 0.2 m/s. Rehydration ratio, water holding capacity, texture, microstructure and total polysaccharide content were evaluated. Drying kinetics was estimated using the Weibull distribution (r² > 0.97 and Chi-square < 0.0009). Values of scale and shape parameters ranged from 90.94 to 341.06 (min) and 1.43 to 1.49, respectively. Furthermore, the influence of temperature on the model parameters as well as on the quality attributes was analysed using a least significant difference test (p-value < 0.05). These effects were more evident for the long drying period (e.g. 810 min at 50 °C). However, minor alterations in the structural properties and total polysaccharide content were produced at drying temperatures of 60–70 °C, resulting in a high quality gel.     

Thin Layer Modelling of Recirculatory Convective Air Drying of Curd (Indian Yoghurt)

Dahi is one of the most popular fermented milk products consumed in India with excellent therapeutic properties. The initial moisture content of dahi is approximately 5.7 kg water kg dry solid⁻¹. The shelf life of dahi is limited and hence the long term storage is possible in the form of dahi powder which can further be used as a base for formulation of health drink mix. Dahi was dried in a laboratory scale recirculatory convective air dryer to a final moisture content of 0.04 kg water kg dry solid⁻¹. Drying characteristics of dahi were investigated under varying conditions of dahi thickness (0.003 m, 0.004 m and 0.005 m) and drying air temperatures (45°C, 50°C and 55°C) and velocities (1.5 m s⁻¹, 2.0 m s⁻¹ and 2.5 m s⁻¹). Different drying models were used to simulate the observed drying data. The mathematical models were compared based on R-square and reduced chi-square values. The drying characteristics were satisfactorily described by Page, modified page, logarithmic and Midilli et al. models. The Midilli et al. model followed by modified page model provided the best representation of data. Effective moisture diffusivity computed on the basis of Fick's law varied between 2.52 × 10⁻¹⁰ m² s⁻¹ and 1.3 × 10⁻⁹ m² s⁻¹ under experimental drying air temperatures and sample thicknesses at air velocity 2.5 m s⁻¹. The temperature and thickness dependence of effective moisture diffusivity was expressed by an Arrhenius type of equation. The equation showed best fit for diffusivity data at 2.5 m s⁻¹ and varying temperatures and sample thickness.     

Optical and mechanical properties of cocona chips as affected by the drying process

The effect of the application of a pre-osmotic treatment to obtain hot air dried cocona (Solanum sessiliofurum Dunal) chips was studied. The drying kinetics and the optical and mechanical properties of cocona chips obtained by the combined method of osmotic dehydration and hot air drying (OD + HAD) and by only hot air drying (HAD) were compared. Samples were dried by hot air at 60 °C. For the combined method, they were pre-dried to a moisture content of 75 g_water/100 g, immersed in a 55 °Brix sucrose solution at 25 °C for 48 min. The pre-osmodehydration applied did not influence the subsequent hot air drying kinetics, resulting in a final product with 0.055 ± 0.005 g_water/g_cocona. The optical properties of OD + HAD chips were more favorable, exhibiting a smaller color change with respect to the fresh fruit (±15 units) than the HAD samples (±23 units). On the other hand, the OD + HAD chips presented more fracture peaks than HAD ones, this related with a structure with a higher degree of crispness, a very desirable property for a chip product.     

Effect of different drying conditions on the vitamin C (ascorbic acid) content of Hayward kiwifruits (Actinidia deliciosa Planch)

In this study, drying kinetics of kiwifruit are investigated experimentally and theoretically under varying drying conditions. Experiments are conducted using air temperatures at 35, 45, 55 and 65 °C, mean velocities at 0.3, 0.6 and 0.9 m s^?1 and, relative humidity values at 40%, 55%, 70% and 85%. Initially, sorption isotherms of the dried kiwifruit slices are determined for different temperatures and equilibrium relative humidity values. The values of the moisture diffusivity, D_eff are obtained from the Fick's diffusion model. The effects of the governing drying parameters on the vitamin C content as well as on the total drying time are determined. The experimental moisture data were fitted to some models available in the literature, mainly the Henderson and Pabis model, the Lewis model and the two-term exponential model and, a good agreement was observed. In addition, it is disclosed that increasing drying air temperature causes more loss in vitamin C in the dried fruits while degradation of vitamin C is reduced with increasing relative humidity of drying air.     

Study of the Drying Kinetics of Green Bell Pepper and Chemical Characterization

The present work aimed, on one hand, the study of the drying of green peppers, in terms of drying kinetics evaluated at 30°C, 40°C, 50°C, 60°C and 70°C, having the experimental data been fitted to different empirical kinetic models from literature. This kinetic study was then complemented with the modelling in terms of Fick's diffusion equation.On the other hand, the chemical characterization in fresh and after drying at the lowest and highest temperatures was analysed, for evaluation of the effect of drying and drying temperature on the chemical composition of the product. In this way, the analyses made were: moisture content, sugar content, proteins, ash, fat, fibre and acidity.From the results obtained, it was concluded that the empirical models that best describe the dehydration kinetics were the Page and Newton models. From the experimental data was possible to estimate the diffusivities, which range between 9.0 × 10⁻¹⁰ at 30°C and 8.0 × 10⁻⁹ m² s⁻¹ at 70°C.Moreover, it was verified that drying influences the chemical composition of the peppers, but, conversely, the influence of the drying temperature was not very significant.     

Synthesis and characterization of balloons and porous blocks of β-SiC using silicone and urethane foam

A process to form different shapes of β-SiC was developed using precursors composed of silicone compounds and urethane foam. Two types of low molecular weight silicone, a Pt catalyst for curing, and a catalyst regulator were impregnated into urethane foam chips with different porosities and bulk densities, and cured at 200 °C for 1 h in air. Formed precursors were converted into β-SiC by pyrolysis at 1600 °C for 1–5 h in argon. Depending on the type of urethane foam, β-SiC balloons and β-SiC blocks with high porosities and low bulk densities were obtained. In addition, fine particles of β-SiC smaller than 100 nm were also obtained by crushing the blocks. For a disk formed by 5 h pyrolysis at 1500 °C, N₂ gas permeability, electric conductivity, and oxidation resistance at 900 °C in air were measured. The results indicated that the disk may be used as a gas permeable heater. The process is simple, and the materials for this process are not hazardous and commercially available at low cost.     

Microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics by different sintering processes

The effect of sintering processes, such as open sintering, sintering inside a closed crucible, and sintering within a powder bed, on the microstructure and V–I characteristics of ZnO–Bi₂O₃-based varistor ceramics was investigated at sintering temperatures in the range 1000–1200 °C. The results from the experiments showed that the microstructure and electrical properties of the samples varied according to the sintering method and temperature. Optimal values for the electrical characteristics of the varistor ceramics by different sintering processes were obtained when the sintering was conducted at 1100 °C. At the same sintering temperature, the different processes affected the properties differently. At 1000 °C, the samples sintered within a powdered bed showed better electrical properties than those subjected to the other two processes, while at 1100 or 1200 °C, the samples sintered in an open crucible exhibited the best electrical properties.     

The influence of drying air temperature on the physical properties of dried and rehydrated eggplant

Drying processes generally cause volume and surface change of foodstuffs. Information on the porous structure and the mechanical properties of dried food products is needed for determining food quality, process design and estimating properties such as density and moisture diffusivity.In this work we investigated the structural changes induced in eggplant by convective air drying at four different temperatures (40, 50, 60 and 70 °C) and their effect on the subsequent rehydration process. Drying and rehydration kinetic curves were also measured.The changes in physical properties, such as porosity, pore-size distribution and bulk density were determined by Hg porosimetry, scanning electron microscopy and optical microscopy while their effect on the textural characteristics by dynamometric measurements.As expected, the increase of the drying air temperature causes shorter drying times. The drying temperature influences strongly the microstructure of dried samples: the porosity increases with the air temperature, but the structure is better preserved at intermediate temperature (60 °C) as confirmed by the lower firmness values with respect to the other dehydrated samples (40, 50 and 70 °C). In these latter, the longer drying time and the higher temperature, respectively, causes the development of a wrinkled structure. In particular, at 70 °C the structure of dehydrated samples appears totally broken with a consequent faster water uptake during rehydration.     

Characterisation of industrial tomato by-products from infrared drying process

The thin-layer infrared drying behaviour of industrial tomato residues, peels and seeds, was experimentally investigated in the temperature range from 100 °C to 160 °C. The drying rate was found to increase with temperature, hence reducing the total drying time. In particular, as drying temperature was raised from 100 °C up to 160 °C, the time period needed to reduce the moisture content of the sample from 236.70 wt% down to 5.26 wt% (dry basis) was observed to decrease from 99.5 min to 35 min.Using a non-linear regression (Marquart's method) together with a multiple regression analysis, a mathematical model for the thin-layer infrared drying process of industrial tomato residues was proposed. The effective moisture diffusivity is dependent on moisture content; the average values for the diffusivity coefficients at each temperature were obtained using Fick's second law of diffusion, and varied from 5.179 × 10^?9 m²/s to 1.429 × 10^?8 m²/s over the temperature range. The temperature dependence of the effective diffusivity coefficient was described following an Arrhenius-type relationship. Activation energy for the moisture diffusion was determined as 22.23 kJ/mol.     

Modelling of dehydration–rehydration of instant rice in combined microwave-hot air drying

The drying operation is one of the critical steps in the preparation of instant rice. Drying methods and conditions play important roles in achieving the desired quality. In this study, instant rice was subjected to convective hot air, microwave and combined microwave-hot air dehydration. Three air temperature (70 °C, 80 °C, 90 °C) and three microwave power (210 W, 300 W, 560 W) settings were investigated to find the drying kinetics, rehydration kinetics and colour change. The results showed that combined microwave-hot air drying decreased the drying time required when compared to drying with either hot air or microwave energy alone. Predictive models were developed to describe dehydration and rehydration kinetics. Dehydration rate, rehydration rate and total colour change of rehydrated product generally increased with microwave level and air temperature. Combination drying with MW = 300 W and T = 80 °C was optimal in terms of drying time, rehydration time and colour.     

Convective drying characteristics of sludge from treatment plants in tomato processing industries

The present work is mainly focused on the study of the thin layer drying behaviour of sludge from water treatment plants in tomato processing industries, using a convective dryer. The drying experiments were conducted at inlet temperatures of drying air of 30 °C, 40 °C and 50 °C and at an airflow rate of 0.9 m/s and 1.3 m/s. The drying rate was found to increase with temperature and velocity, hence reducing the total drying time. In particular, as drying temperature was raised from 30 °C up to 50 °C, the time period needed to reduce the moisture content of the sample from 173 wt% down to 7 wt% (dry basis) was observed to decrease from more than 760 min to 470 min (0.9 m/s) and from 715 min to 295 min (1.3 m/s).Using a non-linear regression (Marquart's method) together with a multiple regression analysis, a mathematical model for the thin-layer convective drying process of sludge from treatment plants in tomato processing industries was proposed. The values of the diffusivity coefficients at each temperature were obtained using Fick's second law of diffusion, and varied from 6.11 × 10^?10 m²/s to 2.54 × 10^?9 m²/s over the temperature and velocity range. The temperature dependence of the effective diffusivity coefficient was described following an Arrhenius-type relationship. The activation energy for the moisture diffusion was determined as 30.15 kJ/mol and 36.70 kJ/mol, for airflow rates of 0.9 m/s and 1.3 m/s respectively. Air temperature 40 °C and drying airflow rate 1.3 m/s were found adequate to reduce drying energy consumption as well as to optimise the dryer loading/unloading periods.     

Experimental modelling of infrared drying of industrial grape by-products

The thin-layer infrared drying behaviour of industrial grape by-products was experimentally investigated in the temperature range from 100 to 160 °C. The drying rate was found to increase with temperature, thus reducing the total drying time. In particular, as drying temperature was raised from 100 °C up to 160 °C, the time period needed to reduce the moisture content of the sample from 204.32% down to 38.89% by weight (dry basis) decreased from 60.5 to 21 min.Using a non-linear regression (Marquart's method) together with a multiple regression analysis, a mathematical model for the thin-layer infrared drying process of wet grape residues was proposed. The values for the diffusivity coefficients at each temperature were obtained using Fick's second law of diffusion. They varied from 11.013 × 10^?9 to 26.050 × 10^?9 m²/s along the temperature range. The temperature dependence of the effective diffusivity coefficient was expressed by an Arrhenius type relationship. Activation energy for the moisture diffusion was determined as 19.27 kJ/mol.