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Diffusivity of moisture in air-drying of raisins
... 2 h. Uniform berries (4.0H.lg) were selected and the stems were hand removed prior to pretreatment. The sample size was about 100 g. The pretreatment consisted of dipping the grapes in an alkaline emulsion of 2% ethyl oleate in 0.5% sodium hydroxide at 80°C for a period of 30 s. These procedures are in accordance with the earlier studies reported (Saravacos and Raouzeos. 1986;Tulasidas et al.. 1993). ...
... Inlet air temperature was found to strongly influence MW drying characteristics as indicated by drying time requiremenrx (Tables 2 and 3) 30%. In the case of convective drying of grapes, the increase of drying rare with the increase of air temperature is attributed to the increased moisture diflusivity at higher temperatures (Saravacos and Raouzeos, 1986). ...
... Low diffusivities in the low moisture region explain the' possibility of internal control of mass transfer which could k the reason for the rapid drop in DR. Such a khaviour has been observed in the drying of many food materials (laros el al.. 1992;Karthanos et al.. 1990;Saravacos and Raouzeos, 1986;Tulasidas. 1994). A similar behaviour of T as discussed above was noticed at other combinations of P and u. ...
Grapes were dried into raisins using combined microwave and convective drying in a single mode cavity specially designed for the studies. Effect of processing parameters, viz.microwave power density (P) air temperature (T) and velocity (u) on drying kinetics was studied in an effort to optimize the operating conditions. The individual and interactive effects of these variables were srudied using three levels of P, four levels of T and two levels of u. Higher drying rates observed with the higher P and T were atuibuted to the increased product temperature and higher partial vapour pressure difference between the surface of the fruit and the air stream. Increase of air velocity increased the drying times. Low drying rate at higher air velocity was attributed to decreased fruit temperature due to rapid convective heat mnsfer at the surface.
... Varios trabajos sobre encogimiento de productos alimenticios tales como uvas [17], champiñones [2], papas [4,10,16,18,20], yuca [1], ajo [9], frutas y legumbres diversos [7,8,19,21] durante el secado, poden ser encontrados en la literatura. Pero, pocos son los trabajos sobre el encogimiento del lecho de estos productos durante el secado [15]. ...
... Estos resultados están de acuerdo con varios resultados encontrados en la literatura [4,5,7,8,11,12,16,17,18,20,21]. ...
Neste trabalho, o porosidade de leitos de cubos de batata, beterraba e cenoura com arestas iniciais de 8mm, foi determinada experimentalmente em função da umidade. A porosidade do leito, com diferentes umidades, obtidas por secagem a 70ºC por tempos diferentes, foi calculada utilizando-se as determinações experimentais de volume do leito e volume real das partículas. Esses dois volumes foram bem representados por equações lineares baseadas na lei da adtividade dos volumes. A porosidade foi representada satisfatoriamente por uma equação parabólica. A relação entre o volume do leito admensional e a umidade foram representadas por polinômios de terceiro grau. A queda de pressão através de um leito fixo de cubos de batata, beterraba e cenoura de 100mm de espessura foi determinada experimentalmente utilizando-se um equipamento com sistema de controle. Os resultados foram representados de maneira satisfatória por uma equação desenvolvida empiricamente a partir do modelo de Ergun. Como era de se esperar, a queda de pressão aumentou com a diminuição da porosidade do leito, conseqüência do encolhimento dos produtos.
... Eq. (5) can be simplified for the first term of the series (Saravacos and Raouzeos, 1986). The new equation is written as given below, Eq (6): ...
Drying kinetics, water-soluble vitamins, total phenolic content (TPC), antioxidant capacity (AC) of the jujube fruits dried at 50, 60, and 70°C, and degradation kinetics of the quality parameters were investigated. The models fitted to drying were determined as Page at 50 and 70°C, Parabolic at 60°C. Increment in the drying temperature increased the drying rate and decreased the drying time. Water-soluble vitamins, TPC, and AC were significantly reduced by the drying process. Degradation of water-soluble vitamins increased with the drying temperature, although TPC and AC were not significantly affected by temperature. Thermal degradations of quality parameters were fitted to first-order kinetic.
... Equation 6 can be reduced (Saravacos and Raouzeos, 1986) and a new equation is provided below: ...
Drying kinetics, color properties, water-soluble vitamins, antioxidant capacity, total phenolic content, and thermal degradation kinetics of bioactive compounds in goji berries were investigated. Drying experiments were conducted at 50°C, 60°C, and 70°C. Page model was determined as the best model to predict experimental moisture ratio for all temperatures. Increment in drying temperature increased effective moisture diffusivity and drying rate values. Vitamins C and B 6 , antioxidant activity and total phenolic content were significantly reduced by drying. Thermal degradation of vitamins C and B 6 , antioxidant capacity and total phenolic content were found to fit the first-order kinetic model.
... where D eff is the effective diffusivity coefficient (m 2 /s), r is the half thickness of the samples (m), n is the positive integer and t is the drying time (s). For long drying times (setting n = 1), Saravacos and Raouzeos (1986) demonstrated that Eq. (12) could be further simplified to a straight-line equation and can be expressed in a logarithmic form by taking the natural logarithm of both sides. ...
This research paper describes the drying kinetics of ber (Zizyphus mauritiana) fruit and economic evaluation of hybrid photovoltaic/thermal (PV/T) solar dryer fabricated using locally available materials, e.g. galvanized iron sheet, glass, PV panel and fan. Ber fruits were dried to the safe level moisture content (20%) in a period of 240 h with the drying load of 18 kg. Four mathematical models were evaluated to predict the behaviour of ber (Zizyphus mauritiana) drying in a combined hybrid PV/T solar dryer. Experimental data of moisture ratio (MR) and temperature obtained from dryer were fitted to the four empirical drying models. The performance of the drying model was compared based on their statistical parameters: coefficient of determination (R²), root mean square error (RMSE) and reduced chi-square (χ²) between the observed and predicted MR. The logarithmic model was found to be the most suitable for describing the thin layer drying behaviour of ber (Zizyphus mauritiana). The effective moisture diffusivity was 3.34 × 10⁻⁷ m²/s and the efficiency of this dryer was found to be 16.7%. The developed hybrid PV/T drying system produces better quality products in shorter time by the efficient use of solar energy. The economic evaluation of the hybrid photovoltaic thermal (PV/T) solar dryer revealed that high value of IRR (54.5%) and low value of payback period (2.26 Years) make the dryer unit cost-efficient ensuring its economic viability.
... Grain drying generally occurs under falling rate period and diffusion is considered as the most likely physical mechanism governing the moisture movement (Saravacos and Raouzeos, 1986). Fick's second law of diffusion can be simplified for describing mass transfer during drying since food dehydration is assumed to be taken place by diffusion process. ...
p>The study was carried out for the purpose of determining the drying kinetics as well as moisture sorption isotherm of hybrid-81 corn. Corn at about 31% moisture content (wb) was dried in a forced convective hot air cabinet dryer at different drying conditions, such as variable air dry bulb temperature (40°C, 50°C and 60°C) and loading density (3.56 kg/m2, 7.12 kg/m2, and 10.68 kg/m2) as well as in shining sun at different layers (3.56 kg/m2, 7.12 kg/m2, and 10.68 kg/m2). The water sorption isotherm of the dried corn was developed using vacuum desiccators, which contained saturated salt solutions in the range of 11-93% RHs. The mono-layer moisture content calculated by the Brunauer–Emmett–Teller (BET) model (6.76 g/100 g solid) was lesser than that calculated by the Guggenheim–Anderson–De Boer (GAB) model (10.53 g/100 g solid). The energy constants were 10.45 and 4.64 as per BET and GAB equation, respectively. Both models gave suitable fits for corn. The activation energy (Ea) for diffusion of water was found to be 11.09 kcal/gm-mole for corn. Furthermore, it was noticed that, with the increase of corn layer, the drying rate decreased in case of both sun and mechanical drying. However, higher loading density resulted in efficient drying, at least up to 10.7 kg/m2. It was shown that the drying time to obtain stability was the lowest for moisture content (12.08 %) corresponding to aw of 0.65 in case of BET or GAB monolayer moisture content. This finding could be helpful in predicting the storage life of corn.
J. Bangladesh Agril. Univ. 15(2): 309-317, December 2017</p
... Materials and methods for shrinkage studies The volume of grape berries/raisins was determined by the liquid displacement technique using toluene at room temperature (Mohsenin 1986;Saravacos and Raouzeos 1986). A sample of ten grapes/raisins was used to determine the average volume. ...
Thompson seedless grapes were dried into raisins under combined convective and microwave drying in a single mode cavity type of applicator at 2450 MHz with varying operating parameters viz., air temperature, microwave power density, and superficial air velocity. A number of experiments designed to determine the relationships between grape physical and electrical properties and grape moisture content were executed. These relationships were then used in the development of a model of microwave drying. The model was based on the continuum approach and included the terms describing internal heat generation, convective heat transfer, and evaporation. The internal resistance to moisture transfer was described by a Fick's type diffusion equation. The effective diffusivity parameter included the combined effect of microwave power and liquid diffusion on moisture transport. A moving co-ordinate system based on non-variate dry solids was adopted to handle the problem of shrinkage. The model was solved by the "Method of Lines". The simulated results were tested with the experimental data and good agreement was found proving the validity of the procedures.
... It is well known that diffusivity of starch varies with change in its moisture content (Saravacos and Raouzeos, 1983). Few researchers have investigated such moisture dependence of diffusivity in rice. ...
... obtenues pour le raisin, ont été introduites respectivement dans l'équation (8). Ensuite, cette dernière a été résolue numériquement par la méthode de Runge-Kutta à l'ordre 4. Afin de vérifier le modèle à C.C., les courbes théoriques sont établies pour les mêmes conditions des essais effectués au laboratoire. ...
Résumé -Afin d'étudier le séchage des produits agroalimentaires sous serre, en plein air, et dans un séchoir, nous avons effectué des essais de séchage combiné convectif -radiatif à conditions constantes sur le raisin de variété 'sultanine'. Des séries d'expériences ont été réalisées en soufflerie de laboratoire, équipée d'un projecteur de puissance 1 kW simulant le rayonnement solaire, dans le but d'analyser l'effet simultané de l'irradiation, de la température, et de la vitesse de l'air séchant sur la cinétique de séchage. Un simple modèle du séchage pour le raisin en relation avec le processus de l'évaporation de l'eau, a été développé et vérifié. Des essais de séchage en plein air, dans un séchoir, et sous serre pour valider le modèle ainsi établi en soufflerie où les paramètres de séchage sont constants. Nous constatons que ce modèle surestime le processus de séchage sous conditions constantes. Pour ajuster les prédictions du modèle sous conditions variables, un facteur de correction a été introduit dans le modèle. En conséquence le modèle adapté aux conditions variables décrit correctement les cinétiques de séchage d'une couche mince de raisin en plein air, dans le séchoir, et sous serre. Abstract -In order to study the drying of crops in tunnel greenhouse, open sun and natural convection solar drier conditions drying experiments at constant laboratory conditions and at varying outdoor conditions were carried out. Laboratory drying experiments were undertaken, inside a wind tunnel where solar radiation was simulated by a 1 kW lamp, for different external parameters (incident radiation, ambient temperature and air velocity). Effect of drying parameters on moisture content and drying time were determined. A simple drying model of Sultanine grape related water evaporation process was developed and verified. Outdoor drying experiments were carried out in tunnel greenhouse, open sun and natural convection solar drier conditions respectively to validate the drying model obtained at laboratory under constant conditions. It was found that the laboratory model overestimates the drying process under time varying conditions. A correction factor was then introduced in the formulation of the model to adjust these predictions. Mots clé: Séchage -Plein air -Serre -Séchoir -Rayonnement solaire -Raisin -Modélisation.
... The initial moisture content of grapes is about 78%. ln sorne cases, a chemical treatment of the product was performed before desiccation, in order to preserve its nutritive properties and to prevent browning (Saravacos and Raouzeos, 1986). The prepared samples were spread on perforated trays, dried by hot air and weighted at regular time intervals. ...
Ta increase the temperature gradient along the collector and optirruse the dryer efficiency, we have constructed and tested a solar drying installation. Based on simplified heat and mass balance equations, a simple numerical model was developed taking into account the dryer operating conditions. A comparison with experimental results was made. It allows to think that this general modelling procedure is reasonably accurate for sizing such solar dryers. f"
... A presença de soluções adicionadas para melhorar as propriedades reológicas, ou textura do produto (ex: salga úmida), podem também influenciar na posição do cruzamento [4]. Resultados semelhantes foram obtidos por LOPES FILHO et al. [10] no estudo de isotermas de sorção da carne de jacaré e por SARAVACOS et al. [18] para isotermas de adsorção de uva passa. ...
As isotermas de sorção da carne de tambaqui desidratada osmoticamente foram determinadas a três temperaturas (5, 17 e 29ºC) e com duas concentrações de solução osmótica (10 e 30% de NaCl), pelo método gravimétrico. Quatro modelos de sorção foram testados para verificar o melhor ajuste; GAB, Oswin, BET e PELEG. Os dados experimentais se ajustaram satisfatoriamente aos modelos. O modelo escolhido para este trabalho foi o de PELEG. Para o ajuste nas curvas de sorção, foi feita a análise de regressão não-linear, usada o programa estatístico ORIGIN 4.0 para estimar as constantes dos modelos. A avaliação do melhor ajuste foi feita pela análise do coeficiente de determinação do ajuste (r2) e teste de Qui-quadrado (x2). Foram analisadas as influências da variação da temperatura e concentração de NaCl na atividade de água da carne do tambaqui. À medida que decresce a temperatura há um decréscimo na atividade de água. Um aumento na concentração do NaCl diminui a atividade de água. A propriedade termodinâmica estudada foi o calor isostérico de sorção. A medida que aumenta a concentração da solução osmótica aumenta o valor do calor isostérico de sorção.
... Most of the studies have been used a diffusion model for describing the diffusion that controls the drying behavior (Saravacos and Raouzeos, 1986;Raghavan, 1994). For the materials undergoing shrinkage, like grapes, moisture diffusion coefficient varies with moisture content. ...
... where, D eff is the effective diffusivity (m 2 /s) and L is the half thickness of the slab (m). For long drying times, only the first term (n=0) in the series expansion of the above equation can give good estimate of the solution (Saravacos and Raouzeos 1986), which is expressed in logarithmic forms as follows (Madamba 2003): ...
The thin-layer vacuum drying behavior of Zizyphus jujuba Miller slices was experimentally investigated at the temperature of 50, 60, and 70 °C and the mathematical models were used
to fit the thin-layer vacuum drying of Z. jujuba slices. The increase in drying air temperature resulted in a decrease in drying time. The drying rate was found to increase
with temperature, thereby reducing the total drying time. It was found that Z. jujuba slices with thickness of 4 mm would be dried up to 0.08 kg water/kg dry matter in the range of 180–600 min in the vacuum
dryer at the studied temperature range from 70 to 50 °C. The Midilli et al. model was selected as the most appropriate model
to describe the thin-layer drying of Z. jujuba slices. The diffusivity coefficient increased linearly over the temperature range from 1.47 × 10−10 to 3.27 × 10−10 m2/s, as obtained using Fick’s second law. The temperature dependence of the effective diffusivity coefficient followed an Arrhenius-type
relationship. The activation energy for the moisture diffusion was determined to be 36.76 kJ/mol.
... It can be seen that D eff values for the pretreated samples are greater than those obtained for the control samples under the same drying conditions. The increase of D eff in pre-treated with ethyl oleate samples is attributed to the partial chemical breakdown of the sample skins resulting in higher permeability of water (Saravacos and Raouzeos, 1986). The values of D eff are reported to vary between 2.4 and 12.1 × 10 −9 m 2 /s for the drying of strawberry leathers in the temperature range of 50–80 @BULLET C (Lee and Hsieh, 2008), (0.65–6.92) × 10 −10 m 2 /s for quince 35–55 @BULLET C (Kaya et al., 2007 ), (6.76–12.6) ...
The effects of alkali emulsion of ethyl oleate and air temperature (60, 70 and 75°C) on the drying characteristics of sweet cherry were studied using a hot air dryer at a constant air velocity of 2.0m/s. It was observed that both the alkali emulsion of ethyl oleate and air temperature affected the drying time. The drying times of pre-treated samples were 19.5–22.6% shorter than those of control samples. Five semi-theoretical thin-layer models, namely, Lewis, Henderson and Pabis, Logarithmic, Page, Wang and Singh models were used for the modeling of the drying kinetics. The fit quality obtained with each model was evaluated using statistical tests. After comparing experimentally obtained values with the calculated values from the models, it was concluded that Page model represents the drying characteristics better than the other models. The effective moisture diffusivity was determined by using Fick's second law and was observed to lie between 5.683×10−10 and 1.544×10−9m2/s for the pre-treated and control samples. Rehydration ratio was significantly affected by pre-treatment and air temperature. It was found to increase proportionally with the increase in air drying temperature.
... The diffusivity depends on moisture concentration and temperature. Experimentally determined diffusivity, referred to as effective diffusivity, is used to estimate the moisture transport pmpenies of the material (Saravacos and Raozeos, 1986). ...
This study was conducted to evaluate the potential of a natural zeolite (chabazite) as the particulate medium for grain drying. The granular zeolite was heated in an electric oven and mixed with grain corn. Drying experiments were conducted in a rotary batch dryer equipped with a computer and a data acquisition and control unit simulating an adiabatic process. Five initial medium temperatures (140, 160, 180, 200, and 220 °C) were used. Drying curves for grain corn and the values of effective diffusivity are reported. The amount of moisture removed and
... The shrinkage ratio was calculated using the volume at any moisture content with the initial volume. Assuming that the ellipsoid shape of the berries can be approximated with a sphere of equivalent volume V, an equivalent diameter "De" was calculated using the relation De= (6Vln)'" (Saravacos and Raouzeos, 1986). The mean equivalent diameter of the berries was determined at different moisture contents, and correlated with the moisture ratio. ...
Strawberries were pretreated with ethyl oleate and sodium hydroxide solution to make the skin transparent to moisture diffusion. The treated berries were dried with convective air at 45°C and microwaves at two power levels of 0.1 and 0.2 W/g. During the drying process change in shrinkage ratio with respect to moisture ratio was studied. Also, the change in equivalent diameter of strawberries in correlation with moisture ratio was investigated. During drying the variation in surface and centre temperatures with moisture content and time was studied. It was observed that there is a direct relation between the shrinkage and moisture ratio. The equivalent diameter of the berry has a logarithmic function with respect to the moisture ratio.
... Shrinkage ratio (V/Vo ¼ Sb) was calculated using the volume at any moisture content with the initial volume. Assuming that the ellipsoid shape of the berries can be approximated with sphere of equivalent volume (V), an equivalent diameter (De) was calculated using the relation De ¼ (6V/p) 1=3 (Saravacos & Raouzeos, 1986). ...
The subject of this paper is to study the shrinkage of strawberries as a function of the moisture ratio during microwave-convective drying. Strawberries were pre-treated and osmotically dehydrated and dried at power levels 0.1 and 0.2W/g, based on initial mass. The main results showed that the shrinkage has a linear relation with moisture ratio; the equivalent diameter of the strawberry has a reciprocal logarithmic function with moisture ratio; the change in volume was bigger for the fruits osmotically dehydrated than for the not osmotically dehydrated ones, power level exerts influence on shrinkage and equivalent diameter for pre-treated and osmotically dehydrated samples.
... The effects of chemical pre-treatments solutions on seedless grapes and black grapes during drying are reported in literature (Bolin et al.,1975;Rıva & Perı, 1986;Saravacos & Raouzeos, 1986;Aguilera et al., 1987;Dincer, 1996;Doymaz & Pala, 2002;Doymaz, 2006). ...
In this work, heat and moisture transport occurred during drying of black grapes in a laboratory dryer were investigated. In the experiments, the air was passed through the chamber at a variety of flow rates (0.5, 0.75, 1.0 and 1.25 m s−1) and temperatures (40, 50, 60 and 70 °C). Thermal and moisture diffusivities were determined. The possibility of expressing the moisture removal from the grapes was searched with the fourteen one-layer drying models selected from the literature. Among all the models, the Page model was found the best for explaining the experimental results. The effects of the drying temperature and air velocity on the constants and coefficients of Page model and diffusivities were also investigated by multiple combinations of the different equations as the linear, power, logarithmic, exponential, polynomial, inverse polynomial and Arrhenius type by non- linear regression analyses. Models obtained were also analysed statistically using t-test, RMSE, MBE and χ2.
... Grain drying generally occurs under falling rate period and during which diffusion is considered as the most likely physical mechanism governing the moisture movement (Saravacos and Raouzeos, 1986). It has already been established that drying of porous materials in the falling rate period is approximated by Fick's second law of diffusion ( Zogzas et al., 1996). ...
Both heating and drying of high moisture paddy using vibrating platform coupled with infrared heating source dryer give uniform but quick drying by exposing each and every grain uniformly to the IR source. A laboratory apparatus capable of modulating both frequency and amplitude of vibration had been devel-oped to study vibration aided drying characteristics of high moisture paddy. The effective moisture dif-fusivity of the grain during drying under this mode has been evaluated. The drying characteristics of high moisture paddy was studied under different vibrating conditions for five levels of radiation intensity (1509, 2520, 3510, 4520 and 5514 W/m 2) and four levels of grain bed depths (single kernel thickness of 3, 6, 12 and 25 mm). The optimum frequency and amplitude of the vibrations were found to be in the range of 20–22 Hz and 8–9 mm for all the three grain bed depths of paddy (25, 12 and 6 mm) for obtaining nearly complete and uniform mixing (mixing index ffi 1.0) [Das, I., Das, S.K., Bal, S., 2003a. Determination of mixing index of paddy grains under vibrating conditions. Journal of Food Process Engineering 26 (2), 121–133]. Drying occurred almost under falling rate period. The value of the average effective moisture diffusivity (D eff) avg ranged between the 0.778 Â 10 À10 and 3.884 Â 10 À10 m 2 /s for different drying treatments. Both the radiation intensity and grain bed depth found to have significant effect on the (D eff) avg value at 1% level of significance.
Two identical units of a hybrid photovoltaic-thermal (PV/T) solar dryer designed and constructed at ICAR-Central Arid Zone Research Institute, Jodhpur, were used to dry Indian jujube (Zizyphus mauritiana) fruit, one operated under natural and the other under forced convection modes. The fruits were dried to safe moisture content (24 %) in a period of 192 h in forced convection mode, and in 240 h in natural convection mode with drying load of 18 kg. There was a significant difference in performance of the dryer under forced and natural convection mode. The average thermal efficiency of solar energy utilization under forced convection mode was higher (16.7 %) than that of natural convection solar dryer (15.6 %). Logarithmic drying model was suitable for describing the thin layer drying behaviour of the fruit. Effective moisture diffusivity of forced convection dryer was 3.34 × 10-7 m2 .s-1 . Economic evaluation of the solar dryer indicated high value of IRR (54.5 %), and low value of payback period (2.26 years), suggesting the dryer to be cost efficient.
Indian jujube (Ziziphus mauritiana) and Date palm (Phoenix dactylifera L.) the most important fruit crops that can grow in India’s hot arid regions. The perishable nature of Indian jujube and date fruits leads to post-harvest loss. Both of these fruits are highly perishable, containing more than 80% moisture. Moisture needs to be reduced to less than 20% for storing it for a longer period of time. A phase change material (PCM) based hybrid solar dryer was developed, which consisted of the collector of area 1.06 m2 and
two drying trays made of SS (0.50 m2) and that two half trays (0.24 m2). The performance of the developed PCM dryer was evaluated by drying fresh Indian jujube and date palm fruits having an initial moisture content of 65–80% (wet basis) and compared with the developed dryer without PCM. The drying system works so that phase-change material stores the thermal energy during sunshine hours and releases it at night. Therefore, the dryer is effectively operative for the next 5–6 h. The temperature of
the drying chamber was observed as 6 �C higher than the ambient temperature in July 2020 during date palm drying after sunshine hours till midnight and it falls gradually, and temperature in the drying chamber was observed as 5�Celsius higher than the ambient temperature in January 2021 during Indian jujube drying. The variations in moisture content (wet basis) of the Indian jujube fruit are reduced from 80% to 22% within seven days by using the dryer with PCM and in 9 days dryer without PCM. In date palm fruit,
MC reduced from 65 to 20% in 6 days in the dryer with PCM and eight days without PCM. The thermal efficiency (g) of PCM based hybrid photovoltaic thermal (PV/T) solar dryer (g = 18.0%) was found better than that of natural convection mode without PCM based hybrid solar dryer (g = 15.8%). The developed solar dryer provides a promising alternative for drying other arid food produced after sunshine hours with a real-time data acquisition system. It is also found to be superior in retaining quality.
Two identical units of a hybrid photovoltaic-thermal (PV/T) solar dryer designed and constructed at ICAR-Central Arid Zone Research Institute, Jodhpur, were used to dry Indian jujube (Zizyphus mauritiana) fruit, one operated under natural and the other under forced convection modes. The fruits were dried to safe moisture content (24 %) in a period of 192 h in forced convection mode, and in 240 h in natural convection mode with drying load of 18 kg. There was a significant difference in performance of the dryer under forced and natural convection mode. The average thermal efficiency of solar energy utilization under forced convection mode was higher (16.7 %) than that of natural convection solar dryer (15.6 %). Logarithmic drying model was suitable for describing the thin layer drying behaviour of the fruit. Effective moisture diffusivity of forced convection dryer was 3.34 × 10-7 m 2 .s-1. Economic evaluation of the solar dryer indicated high value of IRR (54.5 %), and low value of payback period (2.26 years), suggesting the dryer to be cost efficient.
The diffusivity of water is an important transport property in the processing and storage of food products. Diffusivity data are essential in modelling and calculations of water transport in fbur/raisin mixtures (bakery products). The water diffusivity at 20–60% moisture and 15–70°C was determined from moisture distribution data in two contacted cylinders of wheat flour/raisin pulp, applying the diffusion equation. Higher water diffusivities were found in the flour than in raisin. A sharp discontinuity of moisture distribution was observed at the flour/raisin interface, with higher concentration in the raisin side, apparently due to differences in water activity.
Sewage sludge is generated in wastewater treatment processes. It has a solids content of about 1-2% typically. The key step to treating sludge is dewatering.. Dewatering of sludge by belt presses, filters, and centrifuges can lead to dry solids contents in the range of 15-25%. This step can substantially reduce the volume of the sludge. Characteristics of sludge include high water content, bulk mass, and containment of pathogenic microorganisms. Landfilling of sludge has the disadvantages of occupying land and causing secondary pollutions, especially to groundwater. Thermal drying of dewatered sludge is another step to reduce the volume of dewatered sludge. The drying process consists of complex mechanisms such as molecular diffusion, capillary flow, Knudsen flow, water uptake kinetics flow, and surface diffusion. In order to study the effective diffusion coefficient and the activation energy characteristics of the sludge layer in the process of superheated steam drying and hot air drying, an internal-circulation drying test-bed under normal pressure was built to carry out superheated steam drying and hot air drying tests on sludge layers with thicknesses of 4 mm and 10 mm respectively at the temperature range of 160-280°C. The linear relationship between effective diffusion coefficient and drying time was established through the Fick diffusion model. It was found that the effective diffusion coefficients for the 4 mm sludge layer ranged 7.1515×10-9-2.4852×10-8 m2/s and 1.2414×10-8-2.2769×10-8 m2/s for superheated steam drying and hot air drying respectively. The effective diffusion coefficients for the 10 mm sludge layer ranged 1.9659×10-8-5.8811×10-8 m2/s and 2.8042×10-8-5.6095×10-8 m2/s for superheated steam drying and hot air drying respectively. The linear relationship between effective diffusion coefficient and temperature was established based on the Arrhenius empirical formula. Thus, the average activation energies of 4 and 10 mm sludge layers can be obtained respectively as 21.173 and 18.085 kJ/mol by superheated steam drying and 9.485, 11.191 kJ/mol by hot air drying. These values are mostly in conformity with the effective diffusion coefficient and activation energy obtained by the Midilli thin layer drying model This test showed that when temperature exceeds 260°C, the effective diffusion coefficient of a sludge layer by superheated steam drying is greater than that created by hot air drying. Values obtained showed a linear increase in diffusion coefficients to temperature by superheated steam drying but a curve in hot air drying, suggesting the possibility of oxidation and combustion of the sludge layer by hot air drying.
In this study, changing of drying kinetics and color parameters of halved and whole strawberry tree fruits were evaluated by using hot air drying at 50 °C, 60 °C and 70 °C and vacuum drying at 60 °C, 70 °C and 80 °C. Fruits were pretreated before drying with dipping ethyl oleate (EO) and water blanched using 80 °C hot water (WB) for one minute and compared non pretreated (NPT) fruits. The drying behaviors of strawberry tree fruits were measured by using tri-stimulus color parameters such as Hunter L*, a* and b* values. HMF occurring was also observed to find color changing. EO pretreatment and dividing to fruits was found highly effective on decreasing of drying time both drying methods. WB pretreatment found to be effective to keep color properties of fruits. Color properties were found to be better in half samples compared to whole dried fruits. HMF values verified this result.
Cowpea, also known as beans, black-eyed peas, is an important leguminous vegetable, which is extensively grown in China. Fresh cowpeas having relatively high moisture content must be distributed to customers or processed as quickly as possible in order to prevent microbial fermentation and thermal degradation. Cowpeas resources have great losses as which are putrescible at normal temperatures and pressures, and this severely limits the transportation, storage and process of cowpea, therefore the development of cowpea deep processing is necessary. As Asian people have the habit of eating dried vegetables such as dried cowpea, drying can not only solve the serious problem of huge losses, but also create more economic benefits for farmers; and because most of deep processing technology needs dried cowpea, drying is of great importance as a preprocessing method. The tunnel type of hot air drying technology has been widely utilized in the drying of carrots, preserved fruits, and plums etc. In this paper, hot air drying characteristics and a drying model of tunnel type of hot air drying technology was studied in drying Cowpea, in order to establish the best model to simulate and predict the water ratio and to provide a technical basis for the industrial application of the control and prediction of the drying process and the tunnel type of hot air drying for cowpea. The tunnel dryer was made by the Institute of Agricultural Product Processing, Chinese Academy of Agricultural Engineering and College of Mechanical Engineering, Tianjin University of Science and Technology. According to the pre-test results, the drying characteristics of Cowpea was investigated under different air temperatures (70°C, 80°C and 90°C), air velocities (0.3, 0.4 and 0.5 m/s) and material thickness (6 mm, 18 mm and 30 mm). The empirical relationships between Cowpea moisture then natural logarithmic lnMR and drying time, water effective diffusion coefficient Deff and drying air temperature was established. And then using a MATLAB software system to process a large number of experimental data, afterwards the results were analyzed through nonlinear regression in order to get the function between seven mathematical model equations and the experimental data. The results show that the drying time can be shortened by increasing air temperature and air velocity and reducing material thickness, and the effect of air temperature and material thickness is more effective than that of air velocity. At different dying conditions, the drying rate always changed from constant-rate drying or quasi-constant-rate drying to falling-rate drying stage at 2.0 g/g dry basis moisture content of cowpea. The moisture effective diffusivity was increased with the addition of the air temperature and air velocity, and decreased with the addition of the material thickness, and the activation energy was 33.9 kJ/mol, which was determined from the Arrhenius equation. According to the statistical parameters, as the correlation coefficient(R2), root mean square error (RMSE) and sum of squared error (SSE), the Page model was the best suitable to descript the relationship of Cowpea moisture and drying time by the tunnel type of hot air drying, which could be used in production to predict the moisture content on a certain air temperature, air velocities and layer-thickness using tunnel dryer for drying cowpea.
A mathematical model for simulation of simultaneous heat and mass transport in air-drying of prunes is presented. The differential equations of heat and mass transfer for a spherical body were numerically solved using a variable-grid finite difference method applying simultaneous heat and mass balances in each node. This provides predictions of moisture content, temperature and dimensional change in a space-time domain during the drying process. A computer-based dehydration system was designed and built to obtain quality kinetics data for prune drying and were used to test the results of the proposed model. This paper presents the development and validation of the model and its relevance to interpreting the mechanisms involved in the drying process of prunes.
Solar dryers are developed for drying various agricultural products such as fruits, vegetables, spices, medicinal plants, or marine products. The direct solar dryer (dryer I) with a simple design can be assembled by farmers themselves, using locally available materials. The indirect solar dryer (dryer II) is predestined for manufacture by small scale industries. Current and projected food preservation and solar crop dryer availability figures are given along with potential renewable energy sources. Solar dryers are classified according to their heating modes or the manner in which the heat derived from the solar radiation is utilized. These solar dryers may be one of three known types: direct, indirect, and mixed mode solar dryers. Various applications of solar dryers are also given.
The shrinkage of potato cubes during the drying process has been studied. Our main purpose was to characterize the behavior of the volume, density and dry solid density of the cubes under different drying conditions: air temperature between 40 and 100 degreesC; air velocity: 0.7 and 2.0 m/s; humidity: ambient and 0.114 kg water/kg dry air. We evaluated the volume of potato cubes by toluene displacement in pycnometers; the observed decrease was linearly associated with the moisture content during drying for moisture losses of less than 80%. Consequently, the dry solid density decreased as a hyperbolic curve. Equations based on the additivity of volume agreed with our experimental results. At the end of drying, for X/X-o : 0.2, potato cubes density decreased sharply. This behaviour corresponds to the formation of porosity and could be depicted by an empirical equation only. Drying conditions effects are important in this domain only.
Research on air impingement drying of chestnut was carried out using an air impingement drying dryer. The drying characteristic of chestnut was investigated under different air temperatures (70, 75, 80 and 85°C) and air velocities (10, 12, 14 and 16 m/s). The results show that the drying time can be shortened by increasing air temperature and air velocity, and the effect of air temperature is more effective than that of air velocity. The moisture effective diffusivity was increased with the addition of the air temperature and air velocity, and the activation energy was 53.63 kJ/mol, which was determined from Arrhenius equation. According to the statistical parameters as the correlation coefficient(R2), root mean square error(RMSE) and X2, the Page model was the best suitable to descript the relationship of Chestnut moisture and drying time by air impingement drying. The expression and values of model parameters were also obtained subsequently. It showed that the Page model could sufficiently predict and control the air impingement drying process of chestnut, according to the comparison values and calculated values.
Traditional drying process of minced fish-sheet snack uses either solar drying or hot-air drying that requires long drying time. Products also have a short shelf-life due to poor packaging. This research explored improving process efficiency by optimizing drying time and extending product's shelf-life using active packaging technology. Drying conditions were hot-air drying, microwave heating, and a combination of both. Physical properties of samples were compared with control from hot-air drying at 45°C for 4 hr. Then, samples were fried and packed in various packages including polypropylene bag (PP) as control, PP with oxygen-absorber sachet and PP with moisture-absorber sachet then being kept at 30°C during storage. For hot-air oven, increasing temperature and time raised the drying rate of treatments, resulting in samples with lower moisture content, water activity and slightly darker color than control (p≤0.05). For microwave heating, treatment at power of 50 Watts for 6 min and 30 Watt for 6-8 min achieved moisture content and water activity that were closer to control (p>0.05). However, samples treated with lower power resulted in a more desirable color (yellowish-brown). The combination of microwave heating and hot-air oven significantly reduced drying time (p≤0.05), and the optimum condition was using microwave at 30 Watts for 8 min followed by hot-air oven of 45°C for 1 hr. During storage, moisture content, water activity and TBA increased whereas crispness decreased with increasing storage time. Using oxygen absorber resulted in samples with the least oxidation judging by TBA changes whereas moisture absorber help prevented moisture-content increasing, consequently resulting in better texture. According to standard for total plate count and yeast & mold count, samples could be stored up to 60 days. Thus, active packaging could be an alternative way to extend shelf-life of crispy minced-fish snacks.
The thin-layer drying characteristics of pomegranate arils were investigated at the temperature of 55, 65 and 75°C, and the thin-layer drying models were used to fit the drying data. The increase in drying air temperature resulted in a decrease in drying time. Five different thin-layer drying models were used to predict the drying characteristics. The Midilli et al. model showed a better fit to experimental drying data as compared to other models. Effective moisture diffusivities were calculated based on the diffusion equation for a spherical shape using Fick’s second law, and varied from 9.373 × 10−11 to 3.429 × 10−10 m2/s over the temperature range. Moisture diffusivity values increased as air temperature was increased. The dependence of moisture diffusivity on temperature was described by an Arrhenius-type equation. The activation energies of control and pre-treated samples were determined to be 49.7 and 40.1 kJ/mol, respectively.
The kinetics of drying fruit and vegetable foodstuffs are studied using a three-dimensional finite element model. The heat transfer effect is taken into account using an overall heat balance. The shrinkage of the sample during drying is also included in the system of equations. Two types of elements are used for the representation of the three-dimensional domain: the right hexahedral prism and the triangular prism. The model predicts accurately the experimental moisture content of fruits and vegetables. Literature values for the diffusion coefficient and the activation energy of diffusion agree well with those obtained by the finite element model.
The transport and equilibrium properties of water in food mixtures are important in various processing operations and in storage stability of food products. The dough/raisin mixture is a typical mixture of different food materials, used in bakery products. The water diffusivity at 15–70 °C in dough of 20–60% moisture and in raisins of 20–40% moisture was determined from moisture distribution measurements in two contacted cylinders of dough/raisin pulp, applying the diffusion equation. The sorption isotherms of the two materials were determined using a water activity apparatus and the standard gravimetric method. A sharp discontinuity in moisture distribution was observed at the dough/raisin interface with higher concentration in the raisin side, evidently due to differences in water activity of the two materials. The water diffusivity in raisins, estimated from moisture distribution was similar to the diffusivity obtained from drying data (1–5 × 10−10 m2/s). The water diffusivity in the dough was in the same range. The diffusion and equilibrium properties of moisture may control the physicochemical and microbial changes occurring in dough/raisin mixtures.
An electric heat blast dryer was constructed to carry out thin layer drying studies of Spratelloides gracilis at the temperatures of 60, 70, and 80ºC and the air velocity of 1.00±0.07m/s. Strong quadratic dependence between drying rate and drying time (R²>0.967) was found. The effective water diffusivity and dry basis moisture content showed a good linear relationship (R²>0.998). Furthermore, out of the nine models considered, the Wang and Singh model was found to be the most suitable for describing the drying behavior of Spratelloides gracilis with the highest value for coefficient of determination (R), the lowest value for chi-square (χ), root-mean-square error (RMSE), and mean relative percent error (PE). The activation energy of the sample was 20.9796 kJ/mol.
The thin-layer drying behaviour of black grapes in a laboratory dryer was examined. Various pre-treatments were applied to black grapes, which were dried at 60°C with an air velocity of 1.1m/s. The shortest drying time (25h) was obtained with black grapes dipped in ethyl oleate plus potassium carbonate solution. Different thin-layer drying models such as Lewis, Page, Henderson and Pabis, and Two-term exponential models analysed the moisture variation of black grapes, which untreated/pre-treated with various solutions. All the models were compared according to two statistical parameter, i.e. coefficient of determination (R2) and reduced chi-square (χ2). The Page model was found to be the most suitable for describing drying curves of black grapes. The effective moisture diffusivity varied from 3.82×10−10 to 1.28×10−9m2/s.
A critical presentation of the moisture diffusivity estimation methods in the field of solids drying is attempted. Methods are classified, discussed, and tabulated with corresponding references. Each particular method is briefly described and discussed in terms of its simplicity, accuracy of moisture diffusivity prediction, and applicability to various materials
Effects of ohmic pretreatment on the drying rates of seedless red grapes and adsorption isotherms of the raisins produced were investigated. The treated grape samples were heated in a medium containing 2% citric acid to a final medium temperature of 60C using an alternating current at 14 V/cm field strength and 30 Hz, 60 Hz, and 7.5 kHz frequencies.Results of this study reveal that the drying rate of the grapes was significantly increased by the ohmic pretreatment, especially at low electrical frequencies. The effect of the ohmic pretreatment on equilibrium moisture content of the raisins produced was evident at 0.75 or higher water activities but there was no or limited effect at low to moderate water activities.
Influences of temperature and moisture content on diffusional transport property and kinetics of both urease inactivation and color change during soybean treatment with superheated steam in the temperatures of 120–150°C were investigated. The experimental results have shown that the effective diffusivity and apparent rates of such reactions are related to temperature in a way that the rates of water transport, urease inactivation, and brown pigment formation are higher with higher temperature. In addition, these rates are also enhanced by increasing moisture content, except for the browning reaction that is inhibited by this factor due to the dilution effect. The reduction in urease activity is reasonably described by modified first-order reaction and the color change of soybeans is characterized by three Hunter parameters in which the changes of L-, a- and b-values are followed according to zero-order, modified Monod and first-order reactions, respectively. The kinetics of inactivation and color change, along with the experimental data of protein solubility and lysine content, have been suggested that the soybean should be treated with superheated steam at the temperature ranging from 120 to 135°C. The superheated-steam fluidized-bed technique can simultaneously be applied for both drying and inactivating such components in soybeans by a single operation when the initial moisture content falls within a suitable range, approximately lower than 20%d.b. At elevated initial moisture contents, two-stage drying technique is recommended in order to avoid excessive cooked soybeans.
The masa obtained from nixtamalized corn (Zea mays L.) to produce tortillas is very susceptible to the loss of moisture, because it contains a high percentage of water. The dehydration process of masa and tortillas can be evaluated by calculating the moisture diffusion coefficient. In our experiment the effective moisture diffusion coefficient (EMDC), the degree of starch crystallinity and the thermal diffusivity were determined in raw corn masa and cooked corn masa prepared by extrusion at six processing temperatures (crude, 60, 70, 80, 90 and 100 o C) and two resting periods (0 and 24 h, at 5 o C). Values for EMDC were determined using a simple and quick method based on the room temperature dehydration curves. The EMDC was obtained in samples where the residual moisture concentration was in the range of 5 to 60 % (w/w). All corn masa samples presented a maximum value of the EMDC for a residual moisture concentration of about 10 % (w/w). The coefficients obtained for cooked masa without resting time were larger than those obtained in masa stored for 24 h at 5 o C. Masa processed at 80 o C, with and without rest, has the minimum value for the diffusion coefficient, the maximum degree of starch crystallinity and the highest thermal diffusivity. These masas showed lower dehydration rates, and they were more adequate during machinability. Tortillas with the best quality were produced from masas processed at 80 o C with 24 h rest.
The drying of grapes is a more complex process compared to the dehydration of other agricultural materials due to the necessity both of a pretreatment operation prior to drying and further processing operations after drying. To date, most researchers have concentrated on pretreatment and drying where the primary effects of pretreatment on drying have been evaluated. It seems appropriate that a review should now be presented to critically examine all the corresponding published data in the literature and to compare the conditions used in these studies in order to highlight the grape drying process by considering the important aspects of both the pretreatment and post-drying operations. The result of this study will be useful in identifying the future research directions needed for grape drying and also to facilitate the development of an analytical framework for the design of novel drying equipment.
A theoretical model for the convective drying of food materials was developed. These materials shrink considerably, as moisture content decreases; for this reason, the proposed model takes shrinkage into account as a fundamental part of the drying operation. This is accomplished by modeling the movement of dry solids and the consequent movement of bound substance during drying. The resulting system of three non-linear partial differential equations is solved numerically and the predicted values of moisture content and temperature are compared with experimental data for the drying of Sultana grapes. The effective moisture diffusivity in the mass of grapes was described by an Arrhenius-type equation. A sensitivity analysis of the predicted values and the examination of the confidence regions of the estimated parameters showed that the activation energy for mass transfer was estimated with a high degree of precision. The agreement between the calculated results and the experimental measurements, using various criteria, was very satisfactory.
The kinetics of apricot dehydration was examined using a factorial experimental design in order to evalute the effect of air temperature, air velocity and pretreatment of the sample on the drying time and the transport coefficients of apricots. A finite element method was used to solve the differential equations describing the transfer of moisture in the irregular shape of the apricot. The two transport coefficients of the system were estimated using non-linear regression analysis. It was found that the moisture transfer is entirely controlled by the external resistance to mass transfer for the air velocities examined. A new method is proposed for the evaluation of the relative significance of the external and the internal mass transfer during, which should repiace the well known Biot number criterion.
Chinese jujube fruits were dried at 50, 60 and 70 °C in an electric heat blast dryer. Verma model was superior to the others for explaining change of moisture ratio in Chinese jujube by comparing thirteen commonly used mathematical models, suggested by the lowest percent error (1.211%). A strong linear dependence between drying rate and drying time was suggested by high values of determination coefficient (R2 > 0.97). Linear model (effective diffusivity model) was found to be excellent adequacy in describing relationship between effective diffusivity, Deff, and dry-basis moisture content, M. The effects of drying air temperature on the coefficients of the model were determined by multiple regression method. Lower percent error (3.44%) proved that the effective diffusivity model has higher precision. Our study indicated that it is possible to describe moisture content, drying rate, effective diffusivity, with the models including the effect of drying air temperature on the model constants and coefficients.
The drying effects of sultana grapes in two different air-drying shelters were tested and analyzed. Ten different thin-layer drying mathematical models were compared according to their coefficients of determination andχ2test. The Wang and Singh model was found to be the most suitable for describing the air-drying curves of sultana grapes. As compared with the local traditional shelter, the reformed shelter with a greenhouse can reduce about 12–18% of air-drying time, increase about 23% in green-grade rate of raisins and increase 33% in the effective moisture diffusivity of sultana grapes.
Grapes are one of the most popular and palatable fruits in the world. The preservation of grapes by drying is a major industry in many parts of the world where grapes are grown. Drying grapes, either by open sun drying, shade drying or mechanical drying, produces raisins. Air-drying of solar energy has been demonstrated to be cost-effective and could be an effective alternative to traditional and mechanical drying systems, especially in locations with good sunshine during the harvest season. The traditional air-drying shelter of grapes has been used for thousands of years in Asia and other places around the world. However, less investigation was done about the drying characteristics and the optimization of the shelter. The work of this manuscript provides interesting information that is useful for design of the drying shelter of raisins and for the improvement of raisin quality, especially using air-drying of solar energy.
In this present study, samples of plums were pretreated in alkali solution containing ethyl oleate and dried in a laboratory dryer at 65 °C and 1.2 m/s air velocity. The effect of ethyl oleate solution on drying time of plum samples was investigated. Pretreatment of plums with ethyl oleate increased the rates of convective drying. Drying time of pretreated samples was 29.4% shorter than that of untreated samples. Diffusion coefficients of treated and untreated samples were estimated to lie between 2.171 × 10−10 and 2.401 × 10−10 m2/s. Six semi-theoretical thin layer models were compared according to their coefficient of determination (R2), sum square error (SSE) and root mean square error (RMSE) to estimate air-drying curves. Comparing the R2, SSE and RMSE values of all models, it was concluded that the two-term exponential model for untreated, Wang and Singh model for pretreated plums represents drying characteristics better than the other models.
The effect of ethyl oleate on the drying rates for starches and seedless grapes was investigated in a pilot plant air-dryer. Incorporation of ethyl oleate in unheated pastes of high-amylose starch significantly increased the drying rate during the initial stage of drying but had little effect on the drying rates for granular high-amylopectin starch pastes and gelatinized starches. Dipping grapes in alkali emulsions of ethyl oleate increased significantly the drying rate throughout the drying period.Ethyl oleate may act as a wetting agent, increasing the evaporation rate of water from porous surfaces in the early stages of drying. The initial interphase mass transfer coefficients during the drying of the starches were close to the predicted values for wet surfaces. The acceleration of drying in grapes may be caused by solvent action of ethyl oleate on the skin wax and the grape cell walls.
The aim of this work is to evaluate experimentally the drying kinetics of agricultural products as a function of drying conditions and to evaluate the diffusion coefficient. The study is performed on two kinds of grapes grown on both sides of the Mediterranean sea. Two models of diffusion have then been performed to evaluate the effective diffusivity. A simplified one based on Fick’s law and a second one that takes account of the shrinkage as a fundamental parameter by modeling the movement of dry solids and the consequent movement of the bound substance. A comparison is then made between the results obtained for the two considered varieties as well as between our results and those found in literature. The drying kinetics are linked to the experimental conditions by means of two parameters K and N. A regression analysis based on Fick’s law determined the dependence of these parameters on drying conditions for each variety of grape.
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