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Diffusivity of Moisture in Air Drying of Starch Gels
... Since the full-length grain is the preferred form of rice, such broken rice has typically lower market value. [2,3] Moisture diffusivity in food materials depends on its temperature45678910 and instantaneous moisture content (MC).11121314151617 In rice, temperature dependence of the diffusivity is well understood45678 ; however, few researchers have investigated its moisture dependence. ...
... In the mass transfer model, rice kernel was represented by a spherical shape and it consisted of a single homogeneous material. Moisture diffusivity of other starchy materials, such as starch gels,1112 sponge cake, [14] dry biscuits, [15] and starch films, [17] was also found to depend upon the instantaneous MC. In this study, our goal was to determine moisture diffusivity of different rice components at different absorption and desorption conditions. ...
... Endosperm is primarily composed of starch. Such increase in moisture diffusivity of starches with increase in moisture content is extensively reported in the literature.11121317] Elbert et al. [13] have explained such an increase in moisture diffusivity with MC during parboiling by relating it to the energy requirement involved in moisture transport, which was higher at lower moisture contents because water molecules are more firmly bound inside the starch matrix. ...
... Since the full-length grain is the preferred form of rice, such broken rice has typically lower market value. [2,3] Moisture diffusivity in food materials depends on its temperature45678910 and instantaneous moisture content (MC).11121314151617 In rice, temperature dependence of the diffusivity is well understood45678 ; however, few researchers have investigated its moisture dependence. ...
... In the mass transfer model, rice kernel was represented by a spherical shape and it consisted of a single homogeneous material. Moisture diffusivity of other starchy materials, such as starch gels,1112 sponge cake, [14] dry biscuits, [15] and starch films, [17] was also found to depend upon the instantaneous MC. In this study, our goal was to determine moisture diffusivity of different rice components at different absorption and desorption conditions. ...
... Endosperm is primarily composed of starch. Such increase in moisture diffusivity of starches with increase in moisture content is extensively reported in the literature.11121317] Elbert et al. [13] have explained such an increase in moisture diffusivity with MC during parboiling by relating it to the energy requirement involved in moisture transport, which was higher at lower moisture contents because water molecules are more firmly bound inside the starch matrix. ...
Moisture diffusivity values of different rice kernel components, namely endosperm, bran and husk, are required to solve mathematical models describing absorption and desorption processes. In addition to the rice variety and temperature, the moisture diffusivity also depends on its instantaneous moisture content or water activity (aw) and whether rice is absorbing or desorbing moisture. This research was undertaken to determine moisture diffusivity values of rough rice components in different aw ranges during absorption and desorption. Experiments were performed to measure sorption rates of different rice forms, including white rice, brown rice, and rough rice kernels. Mathematical models were developed to predict their moisture distribution during moisture sorption processes. These models were solved by finite element method using Comsol Multiphysics® simulation program. Moisture diffusivity values of different rice components were calculated and found to be different during absorption and desorption. Diffusivity of rice endosperm was higher during desorption than absorption at aw higher than 0.20 and increased with an increase in aw in 0.20–0.80 aw range. Diffusivity of bran remained almost the same with aw while diffusivity of husk decreased with an increase in aw. Results obtained in this research demonstrated that the moisture diffusivity of different rice components varies significantly with the change in water activity or moisture and should be accounted in the mathematical models.
... Crank (Crank, 1979). The method of slopes (Perry, Green, & Maloney, 1984) has been used to estimate effective moisture diffusivity by analysis of water sorption data of systems stored under saturated salt solutions (Lomauro, Bakshi, & Labuza, 1985;Saravacos & Raouzeos, 1984). The subsequent development of this method used dynamic vapour sorption (DVS) devices (Guillard et al., 2003;Yu et al., 2007). ...
... The effective moisture (vapor phase) diffusivity was calculated using the method of slopes (Lomauro et al., 1985;Saravacos & Raouzeos, 1984;Lomauro et al., 1985). Data was obtained for each dairy powder assuming the product was represented as an infinite slab. ...
A new method for the visualisation and determination of local diffusion coefficients in dairy powders is described
based on real-time visualisation of penetration of fluorescent dyes into individual particles of spray-dried
dairy powders including skim milk powder, milk protein concentrate and whey protein isolate. The rehydration
process was controlled by adding polyethylene glycol (PEG) as a viscosity modifier to the aqueous phase in ratios
of 1:0, 1:1, 1:3 and 1:4 aqueous rhodamine to PEG, respectively. Real-time effective diffusivity values were
obtained from analysis of confocal laser scanning microscope images. Particle size was measured optically.
Results indicated that for all dairy powders, rehydration rates were highly dependent on particle size. Effective
diffusivity increased linearly with increasing particle size and average effective diffusivity of the liquid phase
was calculated for all particle size distributions using this dependence. The Guggenheim-Anderson-de Boer
(GAB) water sorption relationship was used to model water sorption isotherms over a broad range of water
activities. Vapour phase systems had significantly higher effective diffusivity than liquid phase systems. The
results obtained by this new method is broadly in agreement with previously published works, suggesting this
new method may be used to measure the hydration of individual powder particles.
... 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. ...
... Such increase in moisture diffusivity of starches with increase in moisture content is extensively reported in literature. Saravacos and Raouzeos (1983) Desorption starch diffusivity to increase with moisture content at these moisture content ranges during its drying at different temperatures. The same trend was reported by Vagenas and Karathanos (1993) during dehydration of two native starches consisting of amylose and amylopectin content in 0 -40% moisture content range. ...
... It has been verified that D eff varies with internal material conditions such as moisture content, temperature, and structure [21,22]. Due to the unfeasible theoretical prediction of D eff in complex materials, the value of D eff is usually obtained from experimental results through computer optimization and the slopes method [23]. The computer optimization method can give D eff at different grid point and time step through the iteration and the discretization [9]. ...
Using plant-based polysaccharide gels to produce hard capsules is a novel application of this technology in the medicinal field, which has garnered significant attention. However, the current manufacturing technology, particularly the drying process, limits its industrialization. The work herein employed an advanced measuring technique and a modified mathematical model to get more insight into the drying process of the capsule. Low field magnetic resonance imaging (LF-MRI) technique is adopted to reveal the distribution of moisture content in the capsule during drying. Furthermore, a modified mathematical model is developed by considering the dynamic variation of the effective moisture diffusivity (Deff) according to Fick’s second law, which enables accurate prediction of the moisture content of the capsule with a prediction accuracy of ±15%. The predicted Deff ranges from 3 × 10⁻¹⁰ to 7 × 10⁻¹⁰ m²·s⁻¹, which has an irregular variation with a time extension. Moreover, as temperature increases or relative humidity decreases, there is an increased acceleration of moisture diffusion. The work provides a fundamental understanding of the drying process of the plant-based polysaccharide gel, which is crucial for enhancing the industrial preparation of the HPMC-based hard capsules.
... The moisture transport can be described by the Fick's second law equation of unsteady state diffusion, which for the case of drying relates the spatial and temporal changes of the moisture content X (kg liquid/ kg dry solid) via moisture-dependent diffusivity D eff (m 2 /s) [34]. The diffusivity can be estimated by analysis of the drying data and applying the method of slope [35][36][37][38][39]. This method is described in more details in chapter three and we utilize it in this thesis to calculate the effective moisture diffusivity (D eff ) of the drying Pickering foams. ...
Pickering foams, aqueous foams stabilized by solid particles, can be used as a precursor to fabricate solid porous polymers, ceramics, and composite materials. The fabrication process usually comprises of two subsequent steps of drying and solidification (e.g. sintering) after foam is prepared. Drying (or aging) in aqueous Pickering foams is a complex transport process which involves evaporation and drainage (induced by gravity) of the excess liquid along with deformation of the foam and possible formation of cracks caused by capillary-induced stresses. Crack formation is therefore depended on the drying conditions and the mechanical properties of sample. A comprehensive understanding of the drying process can provide us with predictive tools to select efficient process parameters (e.g. required drying time prior to sintering) as well as providing input prameters for developing numerical models.
In this thesis, we first provide experimental data (end of drying time, average moisture content, and effective moisture diffusivity) on drying of Pickering foams stabilized by polymer particles under controlled conditions (i.e. relative humidity and temperature). Drying curves are presented for samples of various initial thicknesses and shapes on substrates of different hydrophobicity and temperatures. Moisture transport is represented via calculating the effective moisture diffusivity coefficients using method of slope. Also, we investigate drying for a bi-component Pickering foam prepared using Multi-walled Carbon Nanotubes (MWCNTs) and polymer particles. We show that the effective moisture diffusivity increases as the average moisture content decreases for all trials and that all data can be collapsed on a master curve. Also, effective moisture diffusivity increased as initial sample thickness as well as substrate temperature increase. On the other hand, effective moisture diffusivity does not depend strongly on the sample shape and MWCNTs concentration.
In next part, we explore the effect of initial sample thickness and shape, substrate temperature and wettability, as well as MWCNTs concentration on the crack formations in the samples. We demonstrate that substrate wettability, initial sample thickness, and MWCNTs concentration have a strong influence on the formation and propagation of cracks. We found that decreasing the wettability of the substrate reduces crack formation. Also, increasing the initial sample thickness reduces crack formation. On the contrary, increasing the MWCNTs concentration increases crack formation for all types of substrates. We demonstrate that substrate temperature and sample shape do not seem to influence crack formation but these parameters influence the crack patterns. Such information can be beneficial in using these foams as a precursor to fabricate porous composite porous materials.
... The diffusion coefficient D eff increased with increase of the damage degree P and was a non-linear function of the conductivity disintegration index Z (Fig. 22). In approximation of the parallel model of diffusion (Saravacos and Raouzeos 1984), the diffusion coefficient can be presented as ) (10)) relating P and Z was used. ...
Recently, the electrotechnologies based on the effects of pulsed electric fields (PEF), such as ohmic heating (OH) and DC electric field, have gained real interest in the field of food processing. These techniques efficiently enhance methods of extraction from food plants and dehydration of biosolids. The PEF and pulsed OH techniques preserve the nutritional, functional, structural and sensory properties of products better than conventional extraction technologies. The electrofiltration and electro-osmotic dewatering can be very effective for the separation of bioproducts and dehydration of food wastes.
The book gives an overview the fundamental principles of electrical techniques, electrophysical properties of foods and agricultural products, application of various emerging electrotechnologies for enhancing the solid-liquid separation and drying processes, extraction techniques of pigments, processing methods of different in-plant tissues and biosolids, electro-osmotic dewatering and electrofiltration of biomaterials, recent industrial- scale gains, and other aspects. Each chapter is complementary to other chapters and addresses the latest efforts in the field.
... The slope method (Saravacos & Raouzeos, 1984) was also used to estimate the moisture diffusivity. The experimental drying curve (MR versus heating time) was compared to the theoretical diffusivity curve (MR versus F 0 ). ...
To design efficient infrared (IR) dryers for rough rice, it is important to understand the drying behavior of rough rice under IR heating. The objective of this study was to determine the moisture diffusivity of rough rice under IR heating followed by cooling. The effects of initial moisture content, rice temperature, drying bed thickness, tempering, and cooling methods on moisture diffusivity and moisture diffusivity coefficient were investigated. Samples of freshly harvested medium grain rice (M202 variety) with initial moisture content (MC) of 25.8, 31.2 and 33.8g moisture/100g dry solid were used. They were dried with IR radiation intensity of 5348W/m2, for six exposure times, 15, 30, 40, 60, 90 and 120 s. The tested drying bed thicknesses were single-layer, 5mm and 10mm. The unsteady diffusion equation based on Fick’s law and slope methods were used to describe moisture diffusivity. The results indicated that rough rice moisture diffusivities under IR heating and cooling were significantly affected by rice temperature and tempering treatment, respectively. High heating rate and moisture diffusivity were achieved with IR heating. It took only 60, 90 and 120 s to achieve about 60°C rice temperature with corresponding moisture diffusivities of 4.8 × 10−9, 3.6 × 10 −9 and 3.4 × 10−9 m2/s during heating for drying bed thicknesses of a single layer, 5mm and 10mm, respectively. The moisture diffusivity coefficients during heating and cooling of IR dried rice with tempering were much higher than those of convective drying, which reflected the high drying rate of the IR drying method.
... In all cases, the value of EMDC has maximum values for the residual moisture around 10 % (w/w) (Figures 3a,b). This result is similar to that observed by King (1968), Saravacos and Raouzeos (1990) and Vagenas and Karathanos (1991), who have found that in different granular materials which include native starch and their gels, the water diffusivity has a maximum value at a moisture concentration similar to the reported in the present work. Such behavior of the EMDC is typical for the granular materials, especially food products, where the water diffusion to the atmosphere, at low moisture concentrations, is restricted by the diffusion of water molecules absorbed at the grain surfaces exposed to atmosphere. ...
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.
... Even though it is generally recognized that the diffusion coefficient in starchy foods depends on moisture concentration (5aravacos and Raouzeos, 1984;Zhang et al., 1984;Leslie et al., 1991 ), until now few theoretical expressions of such dependence has been formulated. A rather simple expression can be obtained starting from the Arrhenius equation and using the relationship postulated by de Boer (1968) between the activation energy for the diffusive process, E D , and the heat of sorption, Es, as, ...
A diffusion equation to describe the isothermal absorption of liquid water in a spherical solid that undergoes uniform swelling was derived. The resulting partial differential equation was solved using a finite difference method, taking into consideration water content dependence of the diffusion coefficient. The developed model was applied to simulate the water uptake of brown rice for the soaking temperatures of 25, 45, 55 and 65 _C. The estimated ‘‘differential” diffusion coefficients were a strongly increasing function of moisture content for all temperatures tested, approaching to the self-diffusion coefficients of water for brown rice moisture contents near to the saturation values. The ‘‘integral” diffusion coefficient corresponding to range of moisture content resulting from soaking conditions were calculated and correlated according to Arrhenius equation with an activation energy of 32.5 kJ/mol.
... kg water/kg of dry solid (shown by closed circles in Fig. 5). This moisture level approximately corresponds to the maximum in bound water diffusivities, that widely discussed in the literature (Aguerre & Suarez, 2004;Fish, 1958;Saravacos & Raouzeos, 1984). The second stage of the temperature rise is observed at the point of maximum water diffusivity x % x b , that practically do not depend upon the character of material pretreatment (Fig. 5). ...
The influence of pulsed electric field (PEF) treatment on convective drying of potato tissue was investigated. The drying experiments for potato disks were done in the interval of drying temperatures 30-70 °C. The effects of pre-treatment before drying and PEF treatment in the course of the drying are discussed. The temperature dependencies of moisture effective diffusion coefficient Deff for intact, PEF-treated and freeze-thawed potato tissues are compared. It is shown that Deff is a function of the conductivity disintegration index Zσ. The moisture effective diffusion coefficient Deff is sensitive to the pre-treatment procedure, and the highest values of Deff are always observed for freeze-thawed pretreated samples. The data show that thermal pre-treatment of samples at high temperatures (T = 70 °C) has practically no beneficial effect on the drying rate, though the same thermal pre-treatment at mild temperatures (T = 50 °C) increases the moisture effective diffusion coefficient Deff and gives an effect that is comparable with that for the PEF pre-treated samples. © 2005 Elsevier Ltd. All rights reserved.
... Thus, Fish (1958) reported moisture diffusivities in potato starch gels increasing with the increase of moisture, reaching the maximum at moisture levels about 0.3 kg water/kg of dry solid. Saravacos and Raouzeos (1984) found that for corn starch gels, water diffusivity reached a maximum value at moisture of about 15%, dry basis, decreasing significantly at lower and higher moistures. ...
A model for isothermal bound water diffusion in grains and other starchy materials was derived. Assuming that spreading pressure gradient is the driving force for diffusion, an analytical expression for the variation of diffusion coefficient with moisture content was derived. When applied to predict the variation of the diffusion coefficient with moisture for some cereal grains and starches, the curves show a maximum for moisture contents about 0.10–0.15 kg water per kg dry solid. The expression for moisture variation of the diffusion coefficient was used to simulate the drying behavior of cereal grains for known values of initial and equilibrium moisture contents with satisfactory results.
... This is not surprising given the complexity of the drying mechanisms and the differences in magnitude in experimentally determined diffusivities by several investigators. At this respect, Fish (1958) found that water diffusivity in potato starch gels increases with moisture content, reaching a maximum at moisture level of 0.3 kg water/kg dry solid; conversely, Saravacos and Raouzeos (1984) found that water diffusivity in corn starch reaches a maximum at moisture levels of 12-15%, dry basis, decreasing at lower and higher moisture. By assuming that: (a) the grains are spherical; (b) diffusion takes place only in radial direction and (c) the effect of volume change due to moisture loss is negligible, one can use the diffusion equation in spherical coordinates (Crank, 1975): ...
Thin-layer drying of amaranth grain was conducted at 40, 50, 60 and 70 �C and a moisture range from about 32.5% to 5.0%, dry basis. A mathematical model based on Fick’ s second law of diffusion with variable diffusion coefficient for spherically shaped grain was used to predict the drying behavior of the grain. An analytical expression describing the variation of the diffusion coefficient with moisture was derived, based on the fact that the ratio of activation energy for diffusion and the heat of desorption is a constant value and equal to 0.5. A correlation describing the activation energy of the drying process with moisture content and temperature is established.
... This is not surprising given the complexity of the drying mechanisms and the differences in magnitude in experimentally determined diffusivities by several investigators. At this respect, Fish (1958) found that water diffusivity in potato starch gels increases with moisture content, reaching a maximum at moisture level of 0.3 kg water/kg dry solid; conversely, Saravacos and Raouzeos (1984) found that water diffusivity in corn starch reaches a maximum at moisture levels of 12-15%, dry basis, decreasing at lower and higher moisture. By assuming that: (a) the grains are spherical; (b) diffusion takes place only in radial direction and (c) the effect of volume change due to moisture loss is negligible, one can use the diffusion equation in spherical coordinates (Crank, 1975): ...
Thin-layer drying of amaranth grain was conducted at 40, 50, 60 and 70 °C and a moisture range from about 32.5% to 5.0%, dry basis. A mathematical model based on Fick' s second law of diffusion with variable diffusion coefficient for spherically shaped grain was used to predict the drying behavior of the grain. An analytical expression describing the variation of the diffusion coefficient with moisture was derived, based on the fact that the ratio of activation energy for diffusion and the heat of desorption is a constant value and equal to 0.5. A correlation describing the activation energy of the drying process with moisture content and temperature is established.
... Fish (1958) reported increasing moisture diffusivities in potato starch gels with an increase in moisture, reaching a maximum at moisture levels of about 0.3 kg water/kg of dry solid. Similarly, Saravacos and Raouzeos (1984) found that for corn starch gels, water diffusivity reached a maximum value at a moisture of about 15%, dry basis, decreasing significantly at lower and higher moistures. Becker and Sallans (1957) postulated two mechanisms of moisture migration during the dehydration of wheat kernels. ...
A model for isothermal diffusion of bound water was used to simulate the thin-layer drying kinetics of amaranth grain. The model assumes that the driving force for the transport of bound water is the gradient of spreading pressure. The gradient of spreading pressure was related to the moisture gradient using the GAB isotherm. This variation shows a relative maximum moisture content about 8% (d.b), after which the diffusion coefficient falls sharply as the moisture content is further reduced. To verify the model, drying tests of amaranth grain were conducted at 40 to 70ºC in a laboratory drier from 32.5 to 6% moisture (d.b.). Equilibrium moisture contents were also determined using an electronic hygrometer at temperatures and relative humidities corresponding to drying conditions. The applicability of the model to simulation of drying curves was satisfactory in the full range of moisture.
... The diffusion coefficient D eff increased with increase of the damage degree P and was a non-linear function of the conductivity disintegration index Z (Fig. 22). In approximation of the parallel model of diffusion (Saravacos and Raouzeos 1984), the diffusion coefficient can be presented as ) (10)) relating P and Z was used. ...
The purpose of this contribution is to review the existing approaches to pulsed electric field (PEF) application as a tool
for enhancing the processing of plant tissues. The PEF-treatment as a nonthermal method, which allows to preserve the natural
quality, color, and vitamin constituents of food products. The numerous laboratory attempts to modernize the optimal PEF application
protocols still lack universality. The problem is inherently multidisciplinary and integrates different biological, electrophysical,
and chemical processes. The fundamental aspects of electroporation in application to plant tissues, electrically induced damage,
optimal power consumption, synergetic effect of combined PEF-thermal treatment, and influence of pulse protocol parameters
are presented and critically discussed. The experimental data on PEF-induced acceleration in expression, diffusion, and drying
processes are also analyzed.
... Such a model could contribute to the development of an efficient evaluation phase in the research and development of air-dried alginate products or to the optimization of processing times in industrial applications. However, a review of the literature on the drying of gels [19][20][21][22][23][24] suggests that no simple model can be used to describe the entire drying process. In this paper, we point out that for the specific case of calcium alginate beads, nearly the entire process can be described with the simple model described herein. ...
The dehydration of calcium alginate beads is observed to have a simple power-law time dependence. The ratio of the water content, M(t), to the initial water content, M0, can be expressed as M(1)/Mo=(1 - t/t) 3/2. The parameter tc is insensitive to the guluronic content of the alginate or the degree of cross-linking. Using this simple model, tc can be estimated from the properties of water vapor, the initial water content, and the relative humidity, in addition to an empirically derived correction factor, K, which has been found to be 2.8 ± 0.3.
This research investigated the application of combined hot air and radio frequency drying technology on the drying efficiency and the quality of black carrot pomace (BCP). Electrode gap (8, 9 and 10 cm), thickness (2, 3 and 4 cm), weight (500, 750 and 1000 g) and compaction density (0.44, 0.66 and 0.88 g/cm³) were the parameters studied to determine the drying characteristics of the pomace by hot air-assisted radio frequency (HARF) drying system. Electrode gap of 8 cm and thickness of 2 cm, weight of 500 g and compaction density of 0.44 g/cm³ were identified as the ideal HARF drying conditions. HARF drying shortened the drying time by 39% compared to conventional hot air (HA) drying. Six different mathematical models were used for describing the drying characteristic of BCP. The effective moisture diffusivities were estimated by two approaches: the diffusion equation method and the method of slope for HARF drying and HA drying methods. HARF drying provided the dried products with better color, phenolic content, anthocyanin content and antioxidant capacity.
Industrial relevance
Black carrot pomace is an industrial waste left after the juice extraction from black carrot. The most common utilization way of this valuable by-product is to use it as animal feed. The reason behind this fact is the difficulties in drying of a high moisture bulk material in huge quantities produced in winter season. If it is not dried on time, it gets fermented and become useless for the separation of valuable components such as anthocyanin, pectin and fiber. So, a fast and efficient drying method and its characterization is essential for the utilization of this valuable by-product.
An empirical equation is available for predicting apparent water diffusivity in rice starch/water system in arbitrary moisture content and at arbitrary temperature. In the present paper, water diffusivity in wheat starch (wheat flour)/water system was measured by PFG-NMR (pulsed field gradient-nuclear magnetic resonance). The result of experiment suggested that the predicting equation for water diffusivity in rice starch/water system could be applied for that in wheat starch (wheat flour)/water system. © 2001, Japan Society for Food Engineering. All rights reserved.
Moisture removal from solids is an integral part of food processing. Moreover, convective drying is one of the most important techniques for preservation of biological products. An essential prerequisite for interpretation of the mass transport phenomena during drying is a knowledge of the equilibrium characteristics of the foodstuff. A further necessity is a knowledge of the drying kinetics and a detailed understanding of the moisture transport mechanism. During the drying process, a multiphase system undergoes simultaneous physical and structural modifications. Therefore, for accurate interpretation of this coupled heat and mass transport operation, moisture transport characteristics and physical property data is essential. This paper reviews more salient aspects of convective drying of foodstuffs.
A shorter drying time, together with acceptable product quality, is preferred in food processing. To accomplish this requirement, the present work was studied on the effect of temperature on drying kinetics and quality of cooked rice. The long grain-jasmine rice was chosen as a raw material. The experiments were carried out at temperatures of 50, 60, 80, 100 and 120°C. Effective diffusion coefficient was determined experimentally using a method of slopes. An empirical equation was developed to describe the effective diffusion coefficient as a function of temperature and moisture content. The developed equation was utilized to predict moisture content as the drying progressed and the prediction agreed with the experimental result relatively well. For the quality, the drying temperature affected the color of cooked rice, but it insignificantly affected shrinkage and rehydration capability of dried cooked rice. As examined by scanning electron microscope, morphologies of cooked rice dried at temperatures were similar, but they were relatively different to that of freshly cooked rice, with slightly more porous material after processing.
A mathematical model was developed to predict heat and moisture transfer, drying-induced stresses and the material shrinkage of viscoelastic food materials during drying. Potato starch was selected as the model food material and physical and material properties such as moisture diffusivity, thermal conductivity, and viscoelastic properties were measured as a function of temperature and moisture concentration. To verify the numerical model, results of the drying experiments were compared with the numerically ohtained results and were found to be in good agreement with each other.
A review of the recently reported moisture diffusivity experimental data in food materials is presented in this work. Values are classified and analysed statistically to reveal the influences of temperature and moisture content. Empirical models relating moisture diffusivity with the above variables, along with relevant graphs for various materials, are included. Other factors affecting moisture diffusivity prediction are also discussed.
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
An experimental air dryer was used to investigate the effects of air temperature, sphere diameter and puffing with CO2 on the drying of potato spheres. Accordingly, the experimental results showed only falling-rate behaviour and hence drying completely controlled by internal mass transfer was interpreted on the basis of Fick's diffusional model for non-steady state diffusion. Drying rate increased with increasing air temperature, and also increased with decreasing diameter of sphere. By considering the diffusion coefficients at different diameters, it was established that the drying occurred by a diffusion mechanism as opposed to a capillary mechanism. It was also found that CO2 puffing had a positive effect on the drying rate.
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 5.7kgwaterkgdrysolid−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 0.04kgwaterkgdrysolid−1. Drying characteristics of dahi were investigated under varying conditions of dahi thickness (0.003m, 0.004m and 0.005m) and drying air temperatures (45°C, 50°C and 55°C) and velocities (1.5ms−1, 2.0ms−1 and 2.5ms−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 2.52×10−10m2s−1 and 1.3×10−9m2s−1 under experimental drying air temperatures and sample thicknesses at air velocity 2.5ms−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 2.5ms−1 and varying temperatures and sample thickness.
The effective moisture diffusivity of yogurt was estimated by the method of slopes of the drying curve and the regular regime technique using the isothermal drying data obtained by microwave vacuum drying. The use of microwave vacuum drying seemed advantageous for obtaining the isothermal data. The method of slopes of the drying curve was found useful for estimating effective moisture diffusivity from the drying data of yogurt, because the model obtained from this method simulated the drying curve better than the one from the regular regime technique and because the method of slopes is simple to handle.
Equilibrium and dynamic water sorption properties of sponge cakes with varying porosity (86–52%) and fat content (0–0.30g/g d.b.) were determined using a water vapour sorption microbalance. Contrary to porosity, addition of fat decreased equilibrium moisture contents. The effective moisture diffusivity (Deff) was identified from a numerical solution of Fick’s second law, taking into account an external mass transfer coefficient and the swelling of the solid matrix. Deff increased from 1.61 to 8.67×10−10m2/s with moisture content, reached a threshold at moisture content 0.15g/g d.b. and then decreased until water saturation. Deff decreased from 8.67 to 2.97×10−10m2/s with decreasing porosity. This effect was attributed to a change of water diffusion mechanism, from predominant vapour to liquid. Deff decreased from 8.67 to 2.12×10−10m2/s with increasing fat content. Addition of fat had an effect on the water diffusion in two ways, decreasing porosity (sagging of the foam) and increasing tortuosity.
A mathematical model describing moisture migration by diffusion in a solid sphere with variable diffusion coefficient is proposed. An analytical expression for dependence of the diffusion coefficient with moisture content was derived based on the assumption that the activation energy for diffusion varies linearly with the desorption energy.The expression for moisture dependence of diffusion coefficient was used to simulate drying of parboiled rice in the temperature range 50–90°C. The mathematical model shows good agreement between observed and predicted drying rate curves.
Moisture diffusivity is an important parameter needed in the analysis, design and optimization of drying processes for food and other materials. Published data on moisture diffusivities of food materials are scarce and, sometimes, inconsistent due to a lack of a precise and repeatable experimental technique. Most experimental data are limited to low and moderate drying temperature (<70°C), whereas in the food industry hot air of up to 100°C is usually used in the falling rate period to speed up the drying processes. In this study, the effective moisture diffusivities of Red Delicious apple tissues were determined from drying curves produced with a Perkin Elmer thermogravimetric analyzer, using the slope method. The experiments were conducted at four temperatures 60, 80, 100 and 120°C. Two well defined falling rate periods were observed. The effective moisture diffusivity, for the four temperature levels ranged from 3.2 x 10-7 to 7.9 x 10-8 m2/s for the first falling rate period and 3.8 x 10-8 to 4.7 x 10-9 m2/s for the second falling rate period. The temperature dependence of the effective diffusivity can be described with an Arrhenius-type equation.
Gelatinized food systems develop a porous structure during dehydration; therefore, the moisture diffusivity calculated from such experiments should be considered as an effective diffusion coefficient. In this work, the effective moisture diffusivity of gelatinized starches was determined from drying experiments using the method of slopes. A methodology is presented which permits the calculation of the effective moisture diffusivity from the diffusion coefficients of the two phases (air-solid) and the porosity of the system. The diffusion coefficient of the homogenous solid phase (gel) was estimated from the calculated values of the effective diffusion coefficient, using non-linear regression analysis. Its value depends on the moisture content and temperature and varies in the range 1·4 × 10−11 to 2·6 × 10−10 m2/s for Amioca and 4·1 × 10−11 to 3·2 × 10−10 m2/s for Hylon 7.
Effects of sodium salts of pyro- (SAPP), tripoly- (STPP), and hexametaphosphate (SHMP) on cell numbers, enterotoxin A (SEA) and extracellular protein production (ECP) of Staphylococcus aureus strain 196A were studied in 4% N-Z amine broth plus 1% yeast extract after 24 hr at 30°C. At pH 7.0, concentrations lower than 56 mM (1.2%) SAPP, 27 mM (1%) STPP or 3 mM (0.4%) SHMP had no antibacterial effects and a bacteriostatic effect showed at slightly higher concentrations. SAPP was the least effective inhibitor at neutral pH, but displayed enhanced inhibitory effects at pH 5.5. Reduction in SEA and ECP paralleled cell growth suppression; their concentrations were 1400 ng/mL and 1.5 mg/mL, respectively, in phosphate free broth (9 × 109 CFU/mL), and nondetectable when cell numbers were ≤106 per mL.
Tylose gel (the Karlsruhe test substance) is a meat analog commonly used for studying heat transfer during freezing and thawing operations. It has thermal properties similar to lean beef, which are well defined in literature; however, moisture diffusivities for Tylose gel are not available in the literature. In this article, the mass transfer properties of Tylose were investigated. This included determining a moisture isotherm for Tylose and measuring effective moisture diffusivity as a function of temperature using 2 different methods: a drying method and a concentration-distance method. The moisture isotherm was fit best using the Guggenheim-Anderson-de Boer (GAB) equation. The resulting moisture diffusivities were fit to an Arrhenius model for temperature dependence from 2.5 °C to 12 °C. The moisture diffusivities measured for Tylose ranged from 4.00 × 10-11 to 9.75 × 10-11 m2/ s, depending on temperature. The drying method resulted in moisture diffusivities that were found to have higher dependence on temperature than when the concentration-distance method was used. The concentration-distance method gave much more variation among measurements.
The effective diffusivity of water (D) in hydrated mixtures of two granular starches and three sugars (glucose, sucrose and dextrin) was investigated in the temperature range 40-100°C. Spherical samples (2-cm diameter) of the hydrated mixtures were air-dried at controlled conditions and the D values at various moistures were estimated from the slopes of the drying curves. The sugars reduced significantly the D values in proportion to their molecular size. The changes in water diffusivity were related to the porosity of the samples. Temperature had an Arrhenius-type effect, and the estimated energies of activation for diffusion of water increased significantly by the addition of sugars.
Ripe banana, cut to 10mm thick slabs were osmotically treated in sugar solutions of 35, 50 and 65° Brix for 36h. The initial moisture content fell from a value of 3.13kg H2O DM to 2.19, 1.63 and 1.16kg H2O kg−1 for treatment in the three solutions, respectively. These slabs, with Total Soluble Solids (TSS) contents of 26, 34 and 39° Brix, respectively, as well as freshly cut but untreated slabs (15° Brix) were air dried in a cabinet type tray drier to near equilibrium conditions at fixed temperatures from 40 to 80°C and at a constant air speed of 0.62m s−1. Drying was found to occur in the falling rate period only for both banana types and two drying constants K1 and K2 were established for a first and second falling rate period of drying. Increasing the drying air temperature significantly enhanced the drying rate and the K-values, except at 80°C when the rates fell, possibly because of case hardening of the slabs. Reducing the slab thickness also improved the drying rate, but increasing the air speed to 1.03m s−1 did not have any profound effect. As the sugar content of the banana slabs increased through the osmotic treatment, drying rates fell. Calculated apparent moisture diffusivities at 60°C ranged from 34.8× 10−10 m2 s−1 (fresh slab) to 8.8×10−10 m2 s−1 for dried (39° Brix) slabs. The moisture diffusivity was significantly lowered as the moisture content dropped in drying and with increased levels of sugar. Previously osmosed and then air dried banana slabs showed appealing colour and texture compared to the fresh banana.
The goals of this study were to measure moisture migration in a porous solid matrix simulating a real food (flour dough) and to accomplish heating of the solid matrix. An off-line technique and a microwave temperature controller system were developed for measuring moisture concentration under isothermal heating condition. A temperature profile of a cylindrical dough sample was achieved by accurate control of microwave energy input and preheated carrier gas temperature. Results showed a simplified 1st order kinetic model adequately predicted moisture loss in dough samples. Effect of temperature on the rate constant was adequately modeled by the Arrhenius relationship. The rate constant was found to be affected by porosity of the dough sample.
A numerical procedure was developed for simulating moisture migration in multi-component breakfast cereal systems. The transport of moisture within cereals and raisins was described by equations of unsteady state diffusion. The surface moisture concentration of raisins and cereals was assumed to be in equilibrium with the package void space moisture concentration and was obtained from the corresponding GAB moisture isotherm equations. Using diffusion coefficients determined from regression analysis of the series solution for gain or loss of moisture, the simulation accurately predicted the moisture gain by cereal at 25 °C and 75% relative humidity and loss from raisins when stored alone at 25 °C and 11% relative humidity. Simulated curves were obtained using an explicit finite difference scheme to model unsteady state moisture diffusion within raisins and cereals when stored in an impermeable package at 25 °C. These simulations allowed visualization of the effect of increasing or decreasing the moisture diffusion coefficient in raisins on the moisture migration pattern within the system.
Equilibrium moisture content (EMC) and effective moisture diffusion (Deff) were experimentally determined for cooked rice during drying and were compared with the models described in the literature. Arrhenius-type of equation has been developed to define the effect of temperature on diffusion coefficient (Deff). A two-parameter-combined equation was developed to describe the Deff in terms of moisture and temperature as the drying progressed. Shrinkage effect was also considered in transforming these Deff values to Dreference values. The equation developed in this study can be used to predict the Deff of cooked rice at given conditions
The moisture diffusion during freezing of porous medium has been widely studied for low porosity matrix (i.e. concrete, rocks, …), whereas very few works are available for materials with high porosity. This work focuses on the evaluation of moisture diffusion of a model food (sponge) with a high porosity (in the range of porosity 0.90–0.94). The drying data has been obtained at positive temperatures (10–30 °C) for selected water content (25–142 g H2O/100 g DM). Then, the effective diffusivities have been estimated by using two different methods. Finally, moisture diffusivity in the sub-zero domain has been extrapolated by using an Arrhénius’s model. During drying, the diffusivity increases as the average moisture content decreases. The temperature plays a significant role on the diffusivity. One of the two methods gives better results for the estimation of the effective diffusivity.
The effect of radiation intensity and thickness of slab on the moisture diffusion characteristics of potato during FIR drying is investigated. The standard solution to the non-stationary state diffusion equation was used as a mathematical tool. A model fitting procedure was applied to the experimental drying data to determine the diffusion coefficients. The diffusivity was found to vary with radiation intensity. The diffusivity also increased with the thickness of slab at a constant radiation intensity level. It has been indicated that decrease in energy of activation caused an increase in the drying rate. The diffusion coefficient values predicted with the proposed model were in close agreement with the experimental.
Air-drying of banana slabs has been investigated and the influence of experimental parameters such as temperature, relative humidity and slab thickness has been studied. This was in part re-investigated because of inconsistencies in previous studies, particularly in relation to derived water diffusion coefficients. In addition, it is shown that harvest season and hence initial moisture content has a very marked influence on the drying kinetics. By contrast banana maturity (ripeness) has little influence on the kinetics despite there being significant differences in morphology and chemical composition between green and ripe bananas. The effect of these two variables on the drying kinetics has not previously been studied.
The objective of this study was to investigate the influence of structure, and component interactions, on the sorption and transport properties of water in starch-based systems. We compared the effective diffusivity (Deff) of water in two starches, with differing amylose-amylopectin ratios, using either kinetics of water adsorption or analysis of drying curves (water desorption) to estimate Deff. The effect of incorporating small sugar molecules into the granular or gelatinized starch matrices on Deff was measured by drying curve analysis. To investigate the possible mechanisms of water transport, the porosity and microscopic appearance of the samples at different stages of drying were determined.
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