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Effects of electrical pretreatment conditions on osmotic dehydration of apple slices: Experimental investigation and simulation
Abstract
Electrical pretreatments at 9 different conditions consisting of the combination of 3 different voltage gradients (20, 27, and 32 V/cm) and 3 different application times (10, 20, and 30 s) were applied on apple slices. Apple slices were osmotically dehydrated in 50% sucrose solution at 40 °C until their total dry matter content (TDM) reached to 40%. The effect of pretreatment conditions on the change of water loss and solid gain during osmotic dehydration was investigated, and effective diffusion coefficients were determined. The time needed to reach up to 40% TDM content was predicted by using the numeric solution of unsteady state mass transfer equations and diffusion coefficients via MATLAB code written. The electrical pretreatments reduced the osmotic dehydration time by in the range of 26–64%. The final water and solid distributions of apple slices were simulated in ANSYS. Modeling and simulation results were in good agreement with experimental data (p < 0.05).
Industrial relevance
Since the electrical pretreatment both shortens the osmotic dehydration time and increases the water removed per unit energy used, its application prior to osmotic dehydration processes in the commercial productions will be economical. The proposed modeling and simulation approach for assessment of the effects of electrical pretreatments on osmotic dehydration characteristics may provide valuable information on the scaling up of these conditions in the industrial scale systems.
... Pulsed electric field (PEF) as an innovative non-thermal food-processing methods, has been widely used for food preservation (Botero-Uribe et al. 2017;Liu et al. 2017). It offers advantages such as increasing rehydration rate and mass transfer properties without undesirable changes in food tissues, reducing the energy consumption, improving the sensory (color, texture and flavor) and maintaining the nutrient elements (Yildiz et al. 2016). ...
... According to the results, the D eff value was showed the clear tendency to accelerate with the increase of electric field intensity and pulse number. Similar to other study, the D eff was reported as to be more accelerated by PEF treatment before drying than untreated samples (Yildiz et al. 2016). PEF as an innovative technology resulting in a negligible ohmic heating of processed apples. ...
The influence of pulsed electric field (PEF) on freeze-drying of apple tissue was investigated. The freeze-drying was performed with different parameters of PEF treatment, and PEF treatment on the drying characters, microstructure, rehydration ratio, effective diffusion coefficient and hardness of apple tissue were discussed separately. The results indicated that PEF utilization as a pretreatment of apples enhances the drying process. The drying time was shortened by 17.73% at most, specific energy consumption decreased by 24.74% at most, and the rehydration ratio was improved by 65.22% at most for PEF treatment samples respectively, compared with the untreated samples. The effective diffusion coefficients varied from 2.60 × 10−8 m2/s to 4.20 × 10−8 m2/s for PEF pretreated samples, and was 2.40 × 10−8 m2/s for untreated samples drying at 75°C, the hardness of the untreated apple tissue was about 144.4 N which was decreased to 39.5–115.0 N after PEF treatment.
... The mass ratio of apple samples and osmotic solution was 1:15, to consider the concentration of osmotic solution with a constant value. [1,3,6,13,19,20] At the mentioned instants t i , the three samples A i,1 , A i,2 , and A i,3 were removed from the osmotic solution, washed in distilled water, dried with a paper towel, and weighed. Subsequently, the samples were taken to an oven operating at a temperature of 105°C, for 24 h, to obtain the dry mass. ...
... It is noted that the water flow direction is from the center to the extremities, as obviously expected. Similar result was obtained by Yildiz et al. [19] in the osmotic dehydration of apple slices. Figure 5d shows the spatial distribution of sucrose at 240 min from the beginning of experiment 2, on planes 44, 34, 23, and 12. ...
In the present study, experimental data of osmotic dehydration kinetics of apple, cut into slices with parallelepiped shape, were simulated using two types of diffusion models. Model 1 considers the constant values of mass diffusivities and volume of the slices. Model 2, on the other hand, considers variable mass diffusivities and also the shrinkage of the product. The numerical solution of the three-dimensional diffusion equation in Cartesian coordinates was obtained through the finite volume method, with a fully implicit formulation and boundary condition of the first kind. Process parameters were determined by optimization using the experimental data sets, through the minimization of an objective function, called χ². The results of the osmotic dehydration kinetics were compatible with those of other studies found in the literature. Process temperature and osmotic solution concentration had influence on the phenomenon, but temperature was preponderant. A study was conducted on water and sucrose distribution during the osmotic dehydration. The results obtained through the mathematical model that considered the variable diffusivity and shrinkage showed greater adequacy to the experimental data.
... Environment variables such as relative humidity, temperature, air movement, and hygiene cannot be kept under control during sun drying. Foreign substances such as dust, soil, and heavy metals can mix with the products (Demiray & Tülek, 2012;Mustayen et al., 2014;Yıldız et al., 2016), and the microbiological load may increase, especially in products dried under adverse weather conditions (Bourdoux et al., 2016). At the same time, drying time with sun drying takes much longer than in artificial systems. ...
The tomato pomace (TP), which is a by-product of the production of tomato paste, was dried in a novel custom-designed daylight simulated photovoltaic assisted dryer (DPVD). The different light applications (daylight, UV light, daylight + UV light, and without light), different air velocities (1.5 and 2 m/s), and different heating source modes (hot air and infrared) were applied to dry TP having a moisture content of 80.60 ± 0.73% to the moisture content of 7.66 ± 1.72%. The average water activity values of all dried samples were measured as 0.52 ± 0.08. Analysis was conducted to compare sun drying with the effects of process conditions on the quality (color properties, lycopene, β-carotene, and total mesophilic aerobic bacteria count) and performance (energy efficiency, exergy efficiency, specific moisture evaporation rate, and improvement potential) characteristics of TP. The effects of process conditions for each heating source mode were determined separately, and the improvement of the system performance for each mode was investigated. The effect of the process conditions on total aerobic mesophilic bacteria (TAMB) count was similar in general. In the infrared heating mode, the loss in lycopene and β-carotene contents was 59.55 ± 2.22 and 57.87 ± 2.51 minimum for 1.5 m/s air velocity without light application and for 2 m/s with ultraviolet + daylight application. In general, the performance of the system decreased in case of using ultraviolet light. The retention in the lycopene and β-carotene contents was higher in the infrared mode with light applications compared to hot air mode without light. The optimum drying conditions were air velocity of 2 m/s with “daylight” assistance in the hot air heating mode and with “ultraviolet + daylight” assistance in the infrared heating mode. All the energy and the daylight source used in drying applications were obtained from the sun, a renewable energy source, thanks to the photovoltaic panel and the solar tube units in the novel custom-designed drying system.
... Emerging technologies can be generally used as drivers of osmotic dehydration efficiency and convective drying in various agroindustrial products. Use of emerging technologies of ultrasound and electrical pulses has been successfully applied to improve the efficiency of osmotic dehydration and drying processes of fruits, vegetables, meat, and tubers such as yams, sweet potatoes, cassava, and potatoes, as well as in some polymeric matrices This generates changes in their structure and makes them more permeable with the consequent reduction in drying time of up to 50% and 70% in some cases depending on their structure, degree of hardness and porosity (Bozkir et al., 2019;Prithani & Dash, 2020;Rahaman et al., 2019;Sakooei-Vayghan et al., 2019;Sharma & Dash, 2019;Li et al., 2020;Allahdad et al., 2018;Feng et al., 2018;Goula et al., 2017;Yu et al., 2018;Dermesonlouoglou et al., 2018;Barman & Badwaik, 2017;Traffano-Schiffo et al., 2016;Yildiz et al., 2016). Figure 4 presents the statistics of the application of combined ultrasound techniques, electrical pulses, dehydration, and drying of different products from 2008-2021. ...
Background: The cassava starch industry is recognized as a source of negative externalities caused by the agroindustrial waste ‘cassava bagasse’. Even though options for bioconversion of cassava bagasse have been introduced, it is also true that hundreds of tons of this waste are produced annually with the consequent negative environmental impact. This agroindustrial context highlights the need for further research in technological proposals aimed at lowering the water contained in cassava bagasse.
Methods: We report a scoping review of studies from 2010–2021 that mention the uses of cassava bagasse, as well as the technological options that have become effective for drying fruits and vegetables. The method used for selecting articles was based on the Preferred Reporting Items for Systematic Review and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) method. Articles selected were taken from the databases of ScienceDirect, Google Scholar, Scopus and Springer.
Results : This review highlights fruit and vegetable osmotic dehydration and drying studies assisted by the combination of emerging technologies of osmotic pressure, ultrasound, and electrical pulses. Studies that take advantage of cassava bagasse have focused on biotechnological products, animal and human food industry, and development of biofilms and biomaterials.
Conclusions: In this review, we found 60 studies out of 124 that show the advantages of the residual components of cassava bagasse for the development of new products. These studies do not mention any potential use of bagasse fiber for post-industrial purposes, leaving this end products’ final use/disposal unaddressed. A viable solution is osmotic dehydration and drying assisted with electrical pulse and ultrasound that have been shown to improve the drying efficiency of fruits, vegetables and tubers. This greatly improves the drying efficiency of agro-industrial residues such as husks and bagasse, which in turn, directly impacts its post-industrial use.
... The effects described can be initiated by pulsed electric field (PEF), moderate-intensity pulsed electric field and high-intensity pulsed electric field [127]. The most studied method is PEF [35]. ...
Osmotic dehydration of fruits and vegetables is a slow process due to resistance in mass transfer (MT). This resistance can be modified by varying the osmotic solution or process conditions, or by employing non-thermal treatments (NTT). Some NTT modify the tissue structure by formation of pores in the cell membrane (such as pulse electric fields) or microchannels (such as ultrasound and osmosonication), and others increase the contact surface area with the tissue through a decrease in atmospheric pressure (such as vacuum pulses) or an increase of the system pressure (such as high hydrostatic pressures). Changes in the rate of MT caused by modifications in experimental conditions can be described with mathematical models, such as Fick’s second law. The aims of this review are to analyze the different factors that modify MT rate during osmotic dehydration, examine the application of NTT to modify MT, and to study models that describe these processes.
... Ohmic heating occurs volumetrically; the temperature distribution in the liquid product is homogeneous (Palaniappan and Sastry, 1991). Many researchers have examined ohmic heating in many studies in open literature such as heating (Cho et al., 2016;Halden et al., 2007), distillation (Gavahian et al., 2016), thawing (Cokgezme and İcier, 2019), cooking (Yildiz Turp et al., 2016), evaporation , drying (Yıldız et al., 2016), blanching (İcier, 2010), etc. It has been determined that the studies with ohmic cooking of meat and meat products are limited. ...
Turkish Sausage (Sucuk) is an important traditional meat product for Turkish society and is used in breakfast, grill, daily meals, and toast. Sucuk samples were cooked by heating up to 80 °C with ohmic heating using eight different voltage gradients. The cooking process has shortened due to the increase in the voltage during ohmic cooking. It was determined that the electrical conductivity value increased as the temperature increased during ohmic cooking, and the electrical conductivity value varied between 0.51-3.38 S m-1. At the end of the ohmic cooking process, the temperature homogeneity of the samples was examined with the help of a thermal camera and it was determined that homogeneous cooking could not be provided in low voltage gradients. As a result, it was determined that the sucuk sample could be cooked with ohmic cooking and that homogeneous cooking in a high voltage gradient can be provided.
... To circumvent with this challenge, therefore, a great deal of researches has recently devoted their efforts to use OD in conjunction with other methods to concomitantly reduce the processing time while avoid undesirable effects on food products. Specifically, OD can be accompanied with large variety of innovative technologies such as pulsed-microwavevacuum (Patel & Sutar, 2016), electrical pretreatments (Yildiz, Icier, Eroglu, & Dagci, 2016), ohmic heating (Moreno et al., 2016), high hydrostatic pressure (Nuñez-Mancilla, Pérez-Won, Uribe, Vega-Gálvez, & Di Scala, 2013), ultrasound (Feng et al., 2019;Nowacka, Tylewicz, Romani, Dalla Rosa, & Witrowa-Rajchert, 2017), pulsed electric fields (Dermesonlouoglou, Zachariou, Andreou, & Taoukis, 2016;Traffano-Schiffo et al., 2017), and vacuum (Feng et al., 2019). In this context, amongst other techniques, the use of ultrasound and vacuum seem to be common in reducing the time while saving a huge amount of energy for drying and OD processes. ...
This research seeks to understand the effects of using various vacuum pretreatments (0 and 200 mBar) and different ultrasound power levels (0 and 130 W) on osmodehydration and spouted bed drying of carrot cubes. Effects of using diverse air‐drying temperatures (55, 65 and 75°C), infrared power (0 and 250 W) and the presence of Teflon beads as inert particles (0 and 30% w/w), were also investigated. The effects of above‐mentioned parameters on drying time and physicochemical characteristics of dried carrot samples including hardness, shrinkage, total carotenoid content, rehydration ratio and total color difference (∆E) were examined. The results explicitly demonstrated that the utilization of osmodehydration in conjunction with ultrasound and vacuum (UVOD) had shorter drying time compared to those just treated with osmotic dehydration. The highest total carotenoid content and the lowest ∆E were belonged to the samples exposed to UVOD. The efficiency of artificial neural network (ANN), genetic algorithm based ANN (GANN) and Regression mathematical models were investigated to determine the nonlinear relationship between the inputs and outputs variables. To find the efficiency of all models, mean square error were compared. The results indicate that the ANN model could provide more reliable prediction over the other models.
Practical application
Combined IR‐convective drying has gained increased attention in the recent years because of the fast and efficient transfer of heat, cheap processing cost, and uniform heat transfer, which resulted in products with better organoleptic properties than common drying processes. The combination of hot air and IR radiation resulted in a synergistic effect, leading to more efficient drying than either individual convective or IR drying. Vegetables with high levels of moisture content can be effectively dried using pretreatments such as ultrasonically vacuum‐assisted osmotic dehydration, which reduced the drying time.
... Thus, treatments that modify the cell structure tend to favor mass transfer. Such treatments include the application of a pulsed electric field (McDonnell et al., 2014;Janositz et al., 2011), ohmic heating (i.e., a moderate electric field) (Hong et al., 2012;Moreno et al., 2011;Sensoy & Sastry 2004;Kusnadi & Sastry 2012;Simpson et al., 2007, Yildiz et al., 2016, and ultrasound treatment (Siró et al., 2009), in which electroporation or cavitation induces cell membrane damage, allowing and facilitating salt diffusion. Additionally, vacuum impregnation (Bampi et al., 2016;Chiralt et al., 2001;Martins et al., 2019) favors mass transfer through the replacement of the air occluded in the matrix with osmotic solution by means of pressure gradients and capillary effects (Deumier, 2004). ...
Salting is one of the oldest methods of preserving food. The main limitation of salting is its extended processing time due to slow salt diffusion. A moderate electric field (MEF) can improve the mass transfer rate through electroporation. Regularly, mass transfer processes are modeled with Fick’s second law. However, due to the anisotropic nature of food microstructures, it might be more appropriate to use an anomalous model.
The main objective of this study was to search for a phenomenological explanation for salt and water diffusion in the salmon brining process coupled with MEF.
Salmon fillets were cut into finite cylinders (0.025 x 0.025 m) and brined in two salt concentrations (6 and 24% w/w NaCl) at 6 °C for 20 h. MEFs were applied in the range of 0 to 2 V/cm. The salt and water contents of the salmon were measured during the process. Fick’s second law and anomalous model based on fractional calculus were used to describe the diffusion phenomena.
The results showed that an MEF tended to reduce the brining processing time and increase the salt content of salmon. This effect is predominantly due to an increase in the equilibrium salt concentration in the salmon tissue. Mathematical analysis shows that the anomalous diffusion model is more suitable for representing the brining process, exhibiting superdiffusion behavior (α> 1).
An MEF accelerates the salt mass transfer into salmon tissue even at lower temperatures, significantly reducing the processing time. In addition, the diffusion process can be characterized with an anomalous model.
... The computer simulation and mathematical modelling was used for prediction of PEF effects on the osmotic dehydration apple slices (Yildiz et al. 2016). The data on the osmotic dehydration times and the final water and solid distributions in apple slices were in good agreement with experimental data. ...
Cell membrane electroporation phenomena depend on the size of cells, their spatial orientation, electro-physical parameters of cells, pH of surrounding media and presence of osmotic agents. This chapter discusses fundamentals of membrane electroporation, various non-pore and pore models (condenser and conductor approximations), stages of electroporation development and simulations of electroporation in planar lipid membranes. Electroporation of single cell, Schwan’s equation for spherical and non-spherical cells are presented. Electroporation of cell ensembles, mixtures of intact and electroporated cells, and biological tissues are also discussed.
... However, it is relatively a slow and time consuming process. Hence, a few techniques have been acknowledged in order to enhance the rate of osmotically induced mass transfer that include partial vacuum [10,11], pulsed vacuum [12], high pressure [13][14][15], high intensity pulsed electric field [16,17], ohmic heating [18,19] or ultrasound [20][21][22][23]. Application of high pressure treatment accelerated the diffusion of bioactive components into the solid food due to cell membranes permeabilisation resulting in decline in the resistance to infusion [24,25]. ...
... Golden elma (Golden delicious) yerel bir marketten temin edilerek, kullanım süresi boyunca iklimlendirme kabininde 0-5°C sıcaklıkta %90-96 nisbi nemde depolanmıştır. Elmalar 4.5 x 4.5 x 0.5 cm boyutunda kesilerek kurutma işleminden önce esmerleşmesini önlemek amacıyla 5 dakika sitrik asit (%0.2 w/v) ve askorbik asit (%1 w/v) karışım çözeltisinde bekletilmiştir [41]. ...
... The computer simulation and mathematical modeling were used for prediction of PEF effects on osmotic dehydration apple slices (Yildiz et al. 2016). The data on the osmotic dehydration times and the final water and solid distributions in apple slices were in good agreement with experimental data. ...
Pulsed electric fields (PEF) are very promising for the treatment of plant (vegetable, fruits) and animal tissues. PEF-assisted processing which gives new perspectives for selective extraction and expression, dehydration, drying, freezing, and osmotic treatment can be marked out. However, the nature of electroporation in food tissues is very complicated and can include different scales starting from single membranes and cells to concentrated suspensions and real plant tissues with complex intrinsic structure and much heterogeneity in different scales. Effects of electroporation depend on the size of cells, their orientation and spatial distribution in space, passive and active electro-physical parameters of cells, pH of media, and the presence in tissue osmotic agents. Optimization of PEF treatment and PEF-assisted processing of plant tissues requires precise adaptation of PEF protocols, selection of optimal electric field strength, pulse duration, repetition time, and temperature of treatment. Mathematical modeling can provide detailed and valuable information for optimization of PEF protocols, power consumption, and preservation of the food quality. Moreover, numerical simulation can be useful for description of PEF effects in the presence of complex transport processes and changes in temperature, electrical conductivity, electric field strength, and spatial distribution of electrolyte inside the tissue during PEF treatment. In recent decades various models and simulations were applied for optimizing PEF treatment of liquid foods, geometrical optimization of the treatment chambers, and prediction of the electric field distribution, flow velocity, and temperature inside the treatment chamber.
This chapter discusses various models of PEF treatment and computer simulation of related phenomena, which accompanies PEF-assisted processing of plant tissues.
... The computer simulation and mathematical modeling were used for prediction of PEF effects on osmotic dehydration apple slices (Yildiz et al. 2016). The data on the osmotic dehydration times and the final water and solid distributions in apple slices were in good agreement with experimental data. ...
Pulsed electric fields (PEF) are very promising for the treatment of plant (vegetable, fruits) and animal tissues. PEF-assisted processing which gives new perspectives for selective extraction and expression, dehydration, drying, freezing, and osmotic treatment can be marked out. However, the nature of electroporation in food tissues is very complicated and can include different scales starting from single membranes and cells to concentrated suspensions and real plant tissues with complex intrinsic structure and much heterogeneity in different scales. Effects of electroporation depend on the size of cells, their orientation and spatial distribution in space, passive and active electro-physical parameters of cells, pH of media, and the presence in tissue osmotic agents. Optimization of PEF treatment and PEF-assisted processing of plant tissues requires precise adaptation of PEF protocols, selection of optimal electric field strength, pulse duration, repetition time, and temperature of treatment. Mathematical modeling can provide detailed and valuable information for optimization of PEF protocols, power consumption, and preservation of the food quality. Moreover, numerical simulation can be useful for description of PEF effects in the presence of complex transport processes and changes in temperature, electrical conductivity, electric field strength, and spatial distribution of electrolyte inside the tissue during PEF treatment. In recent decades various models and simulations were applied for optimizing PEF treatment of liquid foods, geometrical optimization of the treatment chambers, and prediction of the electric field distribution, flow velocity, and temperature inside the treatment chamber.
This chapter discusses various models of PEF treatment and computer simulation of related phenomena, which accompanies PEF-assisted processing of plant tissues.
... The computer simulation and mathematical modeling were used for prediction of PEF effects on osmotic dehydration apple slices ( Yildiz et al. 2016). The data on the osmotic dehydration times and the final water and solid distributions in apple slices were in good agreement with experimental data. ...
6th International CIGR Technical Symposium "Towards a sustainable food chain" (Section VI - Food process, bioprocessing & food quality management), Nantes, France, 18-20 April 2011
Food materials with high moisture content were prone to occur drying shrinkage-deformation during dying process. In-depth understanding of drying shrinkage-deformation mechanism for food materials was crucial for optimizing drying parameters as well as for obtaining better quality dried foods. Therefore, the main aim of the present work was to investigate the drying shrinkage-deformation mechanism of apple slices during process of heat and mass transfer. Hot air drying experiment of freshed apple slices was conducted and the shrinkage-deformation values of apple slices were obtained by image analysis. Simultaneously, Heat-mass transfer coupled with stress-strain mathematical model were constructed and solved by COMSOL Multiphysics software. Results showed that the mathematical models of heat-mass transfer and stress-strain can be used to simulate the hot-air drying process of apple slices effectively, and the maximum relative errors of simulated value and experimental value were less than 10%. Temperature distribution and moisture distribution of apple slices during hot-air drying process were uniform. The maximum value of thermal stress and moisture stress were occurred at the middle period of the whole drying process; and the moisture stress during the whole drying process of apple slices were higher than thermal stress, which indicated that moisture stress was the main factor causing the drying shrinkage deformation of apple slices.
Green mature jackfruits were minimally processed into cubes, dipped in solution of citric acid (0 and 1%) and ascorbic acid (0, 1 and 2%), vacuum packed at 550 mbar atmospheric pressure in 80 μm laminated low density polyethylene vacuum pouches and stored at 2-4°C for 15 days. A control was prepared, using water. Quality parameters like colour, fi rmness, pH, titratable acidity and total soluble solids were determined during storage. Colour parameters indicated increase in browning during storage. A signifi cant increase (P<0.05) in titratable acidity and signifi cant decrease (P<0.05) in pH were observed in all treatments. Texture signifi cantly decreased (P<0.05) in all treatments during storage. Combinations of the browning inhibitors were more effective than when applied individually. Citric acid and ascorbic acid when applied together resulted in non-signifi cant change (P>0.05) in microbial counts, browning, and colour lightness. Treatment of 1% citric acid and 2% ascorbic acid in combination with moderate vacuum packaging and low temperature storage was found most effective in inhibiting browning and deterioration of fresh-cut green jackfruit for up to 15 days.
Several studies have demonstrated the feasibility of pulsed electric fields (PEF) for different applications in food industry. PEF technology is therefore a valuable tool that can improve functionality, extractability, and recovery of nutritionally valuable compounds as well as the bioavailability of micronutrients and components in a diverse variety of foods. Additionally, other studies have shown the potential of PEF treatments to reduce food processing contaminants and pesticides. This opens the doors to new PEF applications in the food industry. This review focused on some of the most renowned traditional and emerging PEF applications for improvement of osmotic dehydration, extraction by solvent diffusion, or by pressing, as well as drying and freezing processes. The impact of PEF on different products of biological origin including plant tissues, suspension of cells, by-products and wastes will be analyzed in detail. In addition, recent examples of PEF-assisted biorefinery application will be presented, and finally, the main aspects of PEF-assisted cold pasteurization of liquid foods will also be described.
Tissues of apple, carrot and banana were pre-treated by pulsed electric field (PEF) and subsequently osmotically dehydrated in an agitated flask at ambient temperature using a 65% sucrose solution as osmotic medium. The effect of stirring intensity was investigated through water loss (WL) and solid gain (SG). Changes in product color were also considered to analyze the impact of the treatment. The impeller’s Reynolds number was used to quantify the agitation. The Reynolds number remained inferior to 300 thus displaying laminar flow regime. Water loss (WL) and solid gain (SG) increase with the increase of Reynolds number. Mass transfer in osmotic dehydration of all three test particles has been studied on the basis of a two-exponential kinetic model. Then, mass transfer coefficients were related to the agitation intensity. This paper shows that the proposed empirical model is able to describe mass transfer phenomena in osmotic dehydration of these tissues. It is also shown that a higher agitation intensity improves both the kinetics of water loss and solid gain.
The aim of the present work was to study the kinetics of osmotic dehydration of Caldesi nectarines (Prunus persica var. nectarina) evaluating the effect of osmotic solution concentration, type of solute, temperature, fruit/solute ratio and process time on moisture content, water loss, soluble solids content and solids gain. The process analysis was carried out experimentally and numerically through the mathematical modelling of mass transfer. Hypertonic solutions of glucose syrup and sorbitol (40 and 60% w/w) were used for dehydration, during 2 h of process at temperatures of 25 and 40ºC, with fruit/osmotic agent ratio of 1/4 and 1/10. Water loss and solids gain showed significant differences depending on the type and concentration of the osmotic agent, process time and fruit/solution ratio. The concentration interacted significantly with all variables; in addition there was an interaction between the type of osmotic agent and the relationship between fruit and the osmotic agent. The effective diffusion coefficients were obtained from the analytical solution of Fick's second law applied to flat-plate geometry and by solving the mass transfer microscopic balances by Finite Element
Method (FEM), taking into account the real geometry of the nectarine pieces. The values obtained from Fick's law varied between 1.27x10-10 and 1.37x10-08 m2 s-1 for water and from 1.14x10-10 to 1.08x10-08 m2 s-1 for soluble solids, while the values calculated by Finite Elements Method ranges were between 0.70x10-09 and 4.80x10-09 m2 s-1 for water and between 0.26x10-09 and 1.70x10-09 m2 s-1 for soluble solids. The diffusion coefficients values obtained from the numerical solution are consistent with those published in literature.
This work examined the influence of the ultrasonic pre-treatment prior to air drying on dehydration of jambo (Syzygium malaccense L.) also known as Malay apple. This study allowed the estimation of water loss and sugar gain during the pre-treatment and
the effective water diffusivity in the air-drying process for jambo subjected to ultrasonic pre-treatment. Results showed
that during the ultrasonic treatment, in distilled water, the Malay apples lost sugar, so such a pre-treatment stage can be
a practical process to produce dried fruits with lower sugar content. The water effective diffusivity increased by 28.1% (best
result) after application of ultrasound, which caused a reduction of about 27.3% in the total drying time.
The osmotic dehydration (OD) of food plant tissue constitutes an alternative step for drying. As OD is relatively slow, pulsed electric field (PEF) can be used to accelerate this process. The application of PEF induces a cell membrane permeabilisation with formation of pores accelerating the solute and water transport through the cell membrane. In this study, apple samples were subjected to a PEF pre-treatment, followed by an immersion in an osmotic solution at 25°C containing 44.5 w/w of sucrose. The impact of pre-treatment conditions (electric field intensity, pulse number) on the concentration of osmotic solution (°Brix), the water removal and the solid content in apple samples were studied. Compared to untreated apples, the application of the PEF pre-treatment resulted in decrease of sugar concentration in the osmotic solution and higher solid content in apple samples. The variation of electric field conditions showed that the optimum PEF parameters were 0.90 kV/cm and 750 pulses with a pulse duration of 100 µs, which corresponds to an energy input of 13.5 kJ/kg. A continuous method was developed to follow the evolution of OD kinetics on the apple tissue. Experiments were fitted by a two-exponential model describing the kinetics of water and solute transfer during the OD. This enables the determination of mass transfer coefficients demonstrating that PEF increased both the convection and diffusion rates.
The performance of a tray dryer system for the parsley drying process was assessed using energy and exergy analysis methods in this study. The drying temperature ranged from 40 degrees C to 60 degrees C, while the drying air velocity varied from 0.5 m/s to 1.5 m/s. The higher temperature and lower velocity led to higher exergy and energy efficiencies. The exergy efficiency value for the overall system on a product/fuel basis was found to be 3.62%. The values for Specific Moisture Extraction Rate (SMER) and Specific Moisture Exergetic index (SMExR) were obtained to be 0.08 and 2.47 kg/k Wh, respectively.
This paper is concerned with the exergy analysis of the single layer drying process of laurel leaves in a ground-source heat pump drying cabinet, which was designed and constructed in the Solar Energy Institute, Ege University, Izmir, Turkey. The effects of drying air temperature on exergy losses, exergy efficiencies and exergetic improvement potential of the drying process are investigated. The results have indicated that exergy efficiencies of the dryer increase with rising the drying air temperature. Moreover, the laurel leaves are sufficiently dried at the temperatures ranging from 40 to 50°C with relative humidities varying from 16 to 19% and a drying air velocity of 0.5 m s−1 during the drying period of 9 h. The exergy efficiency values are obtained to range from 81.35 to 87.48% based on the inflow, outflow and loss of exergy, and 9.11 to 15.48% based on the product/fuel basis between the same drying air temperatures with a drying air mass flow rate of 0.12 kg s−1. Copyright © 2006 John Wiley & Sons, Ltd.
The impact of two types of electrical treatment, electro-osmotic (EO) and moderate electric field pulses (MEFP), on intensifying juice pressing from apple cossettes has been studied. The experiments were carried out in a laboratory filter-press cell fitted with the appropriate electrical treatment system. Both EO and MEFP treatments result in a significant intensification of juice yield. Energy consumption, however, is 50–100 times less for MEFP treatment. Since a significant appearance of electrolytic processes and ohmic heating is observed with EO treatment, this treatment is very limited. MEFP treatment does not have these drawbacks and reduces the juice coloration.© 2000 Society of Chemical Industry
The mass transfer kinetics during osmotic dehydration of carrot cubes in ternary solution of sucrose, NaCl salt and water were studied. The osmotic solution concentrations used were 50 °B + 5% salt (w/v), 50 °B + 10% salt (w/v) and 50 °B + 15% salt (w/v), osmotic solution temperature used were 35, 45 and 55°C, fruit to solution ratios 1:4, 1:5 and 1:6 and the process duration varied from 0 to 240 min. Among the models applied, Azuara model well represented the experimental data for water loss; whereas solute gain was well represented by the Magee model. Effective diffusivity of water as well as solute was estimated using the analytical solution of Fick’s law of diffusion and iterative technique, which was conducted by a simple computer program, was used to solve the equation with first six terms. For above conditions of osmotic dehydration, the effective diffusivity of water was found to be in the ranged between 1.594 × 10−9 and 2.078 × 10−9 m2/s and that of solute between 1.175 × 10−9 and 1.645 × 10−9 m2/s.
Apples of Red Chief variety after stabilization at room temperature (20±2 °C) were initially washed with chlorinated water (100 ppm sodium hypochlorite) to prevent surface contamination. After manual peeling, apples were cut in to disc shaped slices of 2 and 3 mm thickness (having uniform diameter of 20 mm) and treated with 1% ascorbic acid and 1% citric acid (anti browning agents) for studying their effect on drying time and quality of apple slices. Slice size and pre-drying treatments of ascorbic acid and citric acid has resulted in significant (P<0.05) variation for drying time, rehydration, dry matter contents, firmness, quality and total colour change. Significantly minimum time (300 minutes) for drying of apple slices, maximum rehydration ratio (4.9), and maximum firmness (11.9 RI) was recorded in case of 2 mm slices treated with 1% ascorbic acid and 1% citric acid. Maximum TSS (18.9° Brix) was recorded in 2 mm slices and 3 mm slices (18.7° Brix) treated with citric acid and ascorbic acid. Ascorbic acid and citric acid were effective to stabilize the ascorbic acid content and maximum (18.0 mg/100g) was recorded in case of slices of 2 mm thickness treated with 1 % ascorbic acid and 1% citric acid. Similarly size of slices and anti browning agents were significantly effective to reduce the total colour change in apple slices and minimum colour change was observed in 2 mm slices (10.0) treated with 1 % ascorbic acid and citric acid and 3 mm slices (12.25) treated with 1% ascorbic acid and 1% citric acid. It can be concluded that apple slices of 2 mm thickness and pre drying treated with 1% citric acid and 1% ascorbic acid as anti browning agents took minimum time for dehydration with minimum changes in colour, firmness, quality, and retained maximum compositional attributes with minimum browning.
Research applications of osmotic dehydration to food processing in technology and in component transfer mechanisms are being carried out in several countries. Osmotic dehydration is a traditional process applied to food dewatering. It leads to attractive products that are ready to eat or can be applied as a pretreatment to the next process such as drying or freezing. The new osmotically dehydrated products and industrial applications require appropriate manufacturing procedures at the industrial level. Thus, an understanding of factors affecting mass transfer during osmotic dehydration is required for the process optimisation. In this review, the mechanism of osmotic dehydration is described. In addition, some factors that affect on mass transfer during osmotic dehydration such as types of osmotic agent, concentrations of osmotic agent, processing temperatures, agitation or stirring process, pretreatment methods and edible coating were reviewed.
This article uses two diffusion models to describe osmotic dehydration of guavas cut into parallelepiped shaped pieces. A one-dimensional numerical solution of the diffusion equation with boundary condition of the first kind, obtained through the finite volume method, was proposed to describe the process. The slices were cut into the following average dimensions: 9.56 x 20.03 x 30.29 mm. Two experiments involving the dipping of the product in syrup of water and sucrose were carried out in the following conditions: (1) 40 degrees Brix and 30 degrees C, (2) 50 degrees Brix and 40 degrees C. In order to determine the process parameters, an optimizer was coupled with the proposed numerical solution. A model that considers the shrinkage and variable effective moisture diffusivity well describes osmotic dehydration for two experimental conditions, and makes it possible to predict the moisture and sucrose distributions at any stipulated time.
Drying is one of the most complex and energy-consuming chemical unit operations. R&D efforts in drying technology have skyrocketed in the past decades, as new drivers emerged in this industry next to procuring prime product quality and high throughput, namely reduction of energy consumption and carbon footprint as well as improving food safety and security. Solutions are sought in optimising existing technologies or developing new ones which increase energy and resource efficiency, use renewable energy, recuperate waste heat and reduce product loss, thus also the embodied energy therein. Novel tools are required to push such technological innovations and their subsequent implementation. Particularly computer-aided drying process engineering has a large potential to develop next-generation drying technology, including more energy-smart and environmentally-friendly products and dryers systems. This review paper deals with rapidly emerging advanced computational methods for modelling dehydration of porous materials, particularly for foods. Drying is approached as a combined multiphysics, multiscale and multiphase problem. These advanced methods include computational fluid dynamics, several multiphysics modelling methods (e.g. conjugate modelling), multiscale modelling and modelling of material properties and the associated propagation of material property variability. Apart from the current challenges for each of these, future perspectives should be directed towards material property determination, model validation, more complete multiphysics models and more energy-oriented and integrated “nexus” modelling of the dehydration process. Development of more user-friendly, specialised software is paramount to bridge the current gap between modelling in research and industry by making it more attractive. These advanced computational methods show promising perspectives to aid developing next-generation sustainable and green drying technology, tailored to the new requirements for the future society, and are expected to play an increasingly important role in drying technology R&D.
Shrinkage, moisture and sucrose effective diffusivities were correlated for infinite slab shape samples of apple during osmotic dehydration in sucrose solution. Experiments were carried out in the sucrose solutions of different concentrations (30%, 40% and 50%) and temperatures (30 °C, 40 °C and 50 °C). The two parameter model, developed by Azuara et al. (1992), was used to predict water loss and solid gain at equilibrium condition. Moisture and sucrose diffusivities were estimated by fitting the experimental moisture loss and solid gain data to the modified form of Fick's second law of diffusion, considering the shrinkage of the apples during osmotic dehydration. Results showed that the volume of the samples decreased linearly with water loss (WL) and weight reduction (WR). For above conditions of osmotic dehydration, effective diffusivities without considering the shrinkage were found to be in the range of 1.36 × 10−10 m2/s–2.00 × 10−10 m2/s, and those with considering the shrinkage were in the range of 0.87 × 10−10 m2/s–1.27 × 10−10 m2/s. The values of the effective diffusivities estimated by considering the shrinkage were smaller than those without considering this phenomenon.
In a dynamic osmotic dehydration from 0 to 180 min, the influence of different edible coating materials such as Low-Methoxyl Pectinate (LMP), Carboxyl-Methyl Cellulose (CMC), Corn Starch, and an osmotic sucrose solution with two concentration of 50% and 60% (w/w) on apple rings and their physical characteristics were studied, which included: moisture loss (ML), solid gain (SG), ML/SG ratio, effective diffusivity of solute and water (Des, Dew). The qualitative characteristics such as: firmness of the texture or shear stress (SS), and Browning index (BI) were also tested. Using Fick's first and second laws, the effective diffusion coefficient of water and solute were calculated. Osmosed apples in solution with 60% sucrose concentration, showed higher ML/SG ratio than other samples and, apples coated with 2% CMC and osmodehydrated in 60% sucrose, showed not only higher ML/SG ratio but higher diffusivity coefficient as well. The results showed that in comparison with non-coated samples, coating process can cause to improve the textural structure of the apple rings. By increasing the concentration of osmotic solutions, the Browning index (BI) of both coated and uncoated samples decreased.
The influence of ohmic heating (OH) and vacuum impregnation (VI) on the osmotic dehydration (OD) kinetics and microstructure of strawberries were studied. Samples were immersed in a 65 °Brix sucrose solution and treated with OH (100 V) and VI (5 kPa). Osmotic treatments were carried out at 30, 40 and 50 °C for 300 min. Water loss, solid gain, color and firmness of the strawberries were measured. In addition, the microstructure was analyzed using electron microscopy (SEM) and (TEM). The results showed that applying OH during osmotic dehydration had significant effects on the mass transference kinetics and the microstructure of the treated samples. The greatest water loss was observed for the OD–OH treatment. The largest amount of solute gain was obtained for the VI–OH treatment. A loss in firmness was observed in the OD–OH samples at 50 °C. Color differences were related to an increase in clarity and a decrease in color chrome, and the least significant differences were observed for the samples treated at 30 °C. SEM observations showed that cell rupturing were more significant in the OD–OH than in the VI–OH samples. The application of OH and VI had beneficial effects on the acceleration of mass transference.
Osmotic dehydration kinetics of pineapple cubes (15×15×15 mm3) was studied over a range of concentration (40–70°B) and temperature (30–50°C) of osmotic solution. The effective diffusion coefficients for water and solute diffusion were determined, considering pineapple as cubical configuration, assuming osmotic dehydration to be governed by Fickian diffusion. The effective diffusion coefficients for water as well as solute were empirically correlated with concentration and temperature of osmotic solution. A high degree of correlation was observed between predicted and experimental values of the effective diffusion coefficients of water (R2=0.99) as well as solute (R2=0.96).
The influence of pulsed vacuum (PVOD) and ohmic heating (OH) on the osmotic dehydration (OD) kinetics and structure of apples was evaluated. Apple cubes (1cm3) were immersed in a 65 ºBrix sucrose solution at 30, 40 and 50°C for 300min. The PVOD treatment was conducted at 5kPa for 5min, and the OH treatment was conducted at 100V (electric field of 13V/cm). The water loss, solid gain, aw, color and firmness were measured, and the microstructure was analyzed using electronic microscopy. The largest water loss was observed with the OD/OH treatment at 50°C. The greatest amount of solute uptake and smallest firmness loss were obtained with the PVOD/OH treatment at 50°C. Color differences were associated with the loss of clarity and corresponded to the transparency gain. OH treatments led to changes in the microstructure, cell walls and tissues of the apples due to the electroporation effect, which explained the increase of mass transference.
In this study, mass transfer during osmotic dehydration of cylindrical cut green beans in salt solution was investigated. The osmotic solution concentrations used were 10%, 20% and 26.5% (w/w) NaCl, osmotic solution temperatures used were 30°C, 40°C and 50°C, the solution-to-green bean mass ratio was more than 20:1 (w/w) and the process duration varied from 0 to 6hr. A two-parameter mathematical model developed by Azuara et al. (1992) was used for describing the mass transfer in osmotic dehydration of green bean samples and estimation of the final equilibrium moisture loss and solid gain. Effective radial diffusivity of moisture as well as solute was estimated using the analytical solution of Fick's second law of diffusion in the cylindrical coordinates. For above conditions of osmotic dehydration, moisture and salt effective diffusivities were found to be in the range of 1.776×10−10–2.707×10−10m2/s and 1.126×10−10–1.667×10−10m2/s, respectively. Moisture and solute distributions as a function of time and location in the radial direction were plotted by using the estimated equilibrium moisture and solute concentrations and also moisture and solute diffusivities.
Osmotic dehydration (OD) of carambola slices in sucrose, fructose and glucose solutions has been carried out to evaluate water and solute diffusivities, as well as the final impregnation–dehydration levels of the fruit. OD kinetics was performed in sugar solutions (50g/100g) at 45, 60 and 75°C during 10h using a syrup-to-fruit mass ratio of 15:1. An analytical solution for unsteady-state mass transfer based on Fick’s second law of diffusion was used for the mathematical description of water loss and solute gain kinetics. By following a central composite design, additional OD tests were conducted to evaluate the effect of solute concentration (35.9–64.1g solute/100g solution) and process temperature (38.8–81.2°C) on the equilibrium and distribution data for both solutes and water. Under the described experimental conditions, effective water diffusivity was in the range of 1.00–3.74×10−9m2/s, whereas values for sucrose, fructose and glucose diffusivities were between 0.58–1.79×10−9, 0.56–1.34×10−9 and 0.56–1.88×10−9m2/s, respectively. Results demonstrated that sucrose can be considered a better osmotic agent than fructose and glucose for OD of carambola, favoring greater water loss-to-solute gain ratios at comparable mass transfer rates.
This paper studies the centrifugal osmotic dehydration (OD), rehydration and texture of carrot tissue treated by pulsed electric field (PEF). The influence of centrifugal acceleration (0–5430g), salt addition (NaCl/sucrose solutions 0/65, 7/44 and 15/50% w/w), and temperature (20–40°C) on the OD kinetics was investigated in the presence and absence of PEF (E=0.60kV/cm, treatment duration tPEF=0.05s). The centrifugal OD of samples untreated and treated by PEF was compared with OD under stirring (250tr/min).Centrifugal force, PEF and salt addition increase the amount of water removed during OD. However, the centrifugal force decreases the intake of solids by the product.The application of PEF enhances both water loss (WL) and solid gain (SG) during OD (under stirring or centrifugation), increases the rehydration capacity of carrot tissue, but somewhat decreases the firmness of rehydrated product.The combination of centrifugal field with PEF and salt addition can be advantageous for reducing the OD duration. The textural study shows that the product obtained with OD is less affected by thermal treatment than the directly dried product.
The osmotic dehydration mechanism of apple sample pre-treated by pulsed electric field (PEF) was investigated over a range of 44.5–65° Brix sucrose concentrations. Apple disks treated by PEF field of 0.90kV/cm during a constant time tPEF=0.75s were immersed in sucrose solution at ambient temperature with 3:1 syrup to apple ratio (w/w). Increase of the initial solute concentration and the PEF treatment resulted in acceleration of the osmotic dehydration (OD). The PEF-treated samples had a higher water loss (WL) and higher solid gain (SG) than the untreated samples. The osmotic dehydration kinetics was studied on the basis of two approaches: Fick's unsteady state diffusion equation and a two-exponential kinetic model. The coefficients of effective diffusion of water and solute were calculated. Their values were higher for samples pre-treated electrically. The effect of PEF was more pronounced for the WL comparatively to the SG. The two-exponential kinetic model permits evaluating of both convective and diffusion stages of OD. The PEF pre-treatment accelerates the kinetics of water and solute transfer during convective and diffusion stages of OD.
The effect of ohmic heating on cell membranes of cellular food material was investigated by measurement of dielectric spectra from 100 Hz to 20 kHz. Cylinders of potato were placed in a glass static ohmic heater and heated, either conventionally or ohmically, to various temperatures ranging from 30 °C to 70 °C. After cooling to 25 °C, the ohmically heated samples had significantly higher electrical conductivity than conventionally heated samples at all measurement frequencies for endpoint temperatures of 40 °C and 50 °C. At low frequencies, the apparent dielectric constant was also higher for these samples, but at high frequencies, the reverse pattern was shown. The ohmically heated samples apparently have greater membrane permeability than conventionally heated samples when heated to temperatures below 60 °C. This is reflected in the diffusion of KCl solution into the tissues, which is faster at higher endpoint temperatures.
Sterilization of solid–liquid mixtures by ohmic heating requires the assurance that all parts of the food or biomaterial in question is treated adequately to ensure inactivation of pathogenic sporeformers. Previous work has identified worst-case scenarios as being associated with a single solid piece (inclusion particle) of substantially different electrical conductivity than its surroundings. Two potentially hazardous situations were modeled, both involving an inclusion particle, but one involving a static medium surrounding the solid; and the other involving a mixed fluid, with a circuit theory analysis for the electric field. Results indicate that the mixed fluid model provides a more conservative prediction of mixture cold-spot temperatures than the static model when the cold-spot occurs within the particle; typically occurring when the medium is more conductive than the solid. However, the static fluid model provides more conservative prediction of the mixture cold-spot temperatures when the cold-spot is within the fluid; typically when the solid is more conductive than the medium. Our results underline the need to evaluate both scenarios in evaluating the potential for an underprocessed region.
To produce a safe cooked food product it is necessary to ensure a uniform heating process. The aim of this study was to develop a mathematical model of a solid food material undergoing heating in a cylindrical batch ohmic heating cell. Temperature profiles and temperature distribution of the ohmic heating process were simulated and analysed via experimental and mathematical modelling which incorporated appropriate electromagnetic and thermal phenomena. Temperature profiles were measured at nine different symmetrically arranged locations inside the cell. The material was ohmically heated imposing a voltage of 100V, while electrical field and thermal equations were solved for experimental and theoretical models by the use of FEMLAB, a finite element software. Reconstituted potato was chosen to represent a uniform solid food material and physical and electrical properties were determined prior to the experiment as a function of temperature. The simulation provided a good correlation between the experimental and the mathematical model. No cold spots within the product were detected but both experimental and model data analysis showed slightly cold regions and heat losses to the electrode and cell surfaces. The designed model could be used to optimize the cell shape and electrode configurations and to validate and ensure safe pasteurisation processes for other solid food materials.
Recently, olive leaves have a high potential for industrial exploitation in the food industry. The main process in olive leaf treatment is drying. Because drying is an energy-intensive process, analyzing and improving the drying systems is important. In this study, olive leaves were dried in a tray drier and the performance of the drying process was evaluated using energy and exergy analyses methods. The effects of the drying air temperature and the velocity on the performance of the drying process were discussed. While the energy utilization ratio values varied between 14.05% and 26.21%, the exergetic efficiencies varied between 55.95% and 75.12%. It was inferred that exergetic efficiencies increased as the drying air temperature decreased and the drying air velocity increased. They were influenced mainly by the drying air temperature.
Although olive trees cover ∼8 million ha in the whole world and scientific studies show the importance of olive leaves, there are few examples of using olive leaves industrially. The main process in olive leaf treatment is drying, and there has been no information on the thermodynamic analysis of the drying of olive leaves in the open literature according to the authors' knowledge. Thermodynamic analysis is an important step for designing and optimizing the olive leaf treatment. It is believed that such a study would contribute the industrial exploitation of olive leaves.
SummaryA two-parameter equation was developed from mass balance considerations and used to predict the kinetics of osmotic dehydration and the final equilibrium point. The model was tested using kinetic data from ten different experiments and was able to predict water loss and solids gained over long periods of drying. The final equilibrium point was estimated using data obtained during a relatively short period of time. Simple equations were obtained when the model was related to Fick's second law for non-steady one-dimensional diffusion through a thin slab, and apparent diffusion coefficients were readily estimated from these equations.
Recently, interest in olive leaves has increased because of their high phenolic contents. Although the critical process in olive leaf treatment is drying, the drying behavior of olive leaves has not been known yet in the literature. In this study, olive leaves were dried in a tray drier at the temperature range of 50–70C and velocity range of 0.5–1.5 m/s. Eleven different semitheoretical thin‐layer drying curve models were used to characterize the drying behavior of olive leaves. Effects of drying air temperature and velocity on model coefficients were determined by multiple regression analysis. The correlation coefficient ( r ), the reduced chi square ( χ ² ) and the root mean square error ( RMSE ) were taken as the statistical criteria parameters for model comparisons. It was found that the modified Henderson and Pabis model represented drying characteristics best with r = 0.9955. Effective diffusivity was computed by taking account of temperature and air velocity. The temperature‐ and velocity–temperature‐dependent activation energies were 60.97 ± 3.21 and 75.98 kJ/mol, respectively. It was concluded that olive leaves could be dried successfully by thin‐layer drying procedure, and the drying rate was influenced mainly by drying air temperature.
PRACTICAL APPLICATIONS
Olive trees are important plants covering ∼8 million hectares in the whole world. Recent studies have shown that olive leaves have higher amounts of phenolic contents such as oleuropein and hydroxytyrosol. Oleuropein is especially a very valuable component in food, medicines and in the drug industry.
Although the importance of olive leaves has been discussed in several scientific studies, there have been few studies for using olive leaves industrially. Because the main process in olive leaf treatment is drying, modeling and determining the drying behavior is an important step for designing and optimizing the olive leaf treatment. Unfortunately, there has been no study ever carried out explaining the drying behavior of olive leaves in open literature to the best of the authors' knowledge.
A study of the acid-salt equilibrium citric acid by means of the SAFES methodology during electrodialysis was performed in order to determined if the fractional composition of the acid forms affects the process due to the predominance of non-dissociated forms. Experiments at constant potential drop, feed and variable current density were performed. Total concentration of acid and its fractional forms were determined. Results showed that even though in the dilution cell, molecules of basic forms, were removed the pH did not decrease. This may indicate that when removing the salt, the acid forms did not predominate and therefore does not affect the process.
The osmotic dehydration (OD) kinetics of carrot disc untreated and treated by pulsed electric field (PEF) was studied under centrifugation (2400 × g), stirring (250 rpm) and with a salt addition (NaCl/sucrose solutions 0%/65%, 5%/60% and 15%/50%). The PEF intensity was E = 0.60 kV/cm and the treatment duration was tPEF = 0.05 s (500 rectangular monopolar pulses each of 100 μs). The water loss (WL), solids gain (SG) and water loss/solids gain ratios (WL/SG) were evaluated in the binary (sucrose + water) and ternary (sucrose + salt + water) solutions at the temperature of 20 °C during 4 h. The mass ratio of sample to solution was 1:3. The PEF treatment and salt addition enhanced the OD kinetics. WL and SG were increased under centrifugation (centrifugal OD) and under stirring (static OD). The centrifugal field enhanced the WL, however, decreased the SG comparing to the static OD. Therefore, the static OD has advantages for the higher SG (confectionary adds), while the centrifugal OD is better appropriated if the WL should be increased and the solids (sugar) uptake should be limited (dietetic products).The two-exponential kinetic model fitted well to experimental data for both static and centrifugal OD. The correlation coefficient was R2 = 0.982–0.999 and the standard error was 5–10%.
The study on osmotic dehydration of onion slices was carried out in order to remove the moisture prior to the further mechanical drying. Three salt concentration levels (5%, 12.5% and 20%), three temperature levels of osmotic solution (28 °C, 43 °C and 58 °C) and the observations on weight loss and solid gain were taken at an interval of 5 min up to first half an hour followed by interval of 10 min for next 1 h. The sample to solution ratio of 1:5, agitation of 100 shakes per minute, sample thickness of 4 mm and 0.2% potas-sium metabisulphite mixed with osmotic solution were used for the study. A two-parameter mathematical model developed by Azuara et al. was used for describing the mass transfer in osmotic dehydration of onions slices. The effect of time on mass transfer kinetics was investigated and the constants of two-parameter model and final equilibrium points for moisture loss as well as solid gain were found. The effect of solution concentration and solution temperature was also studied and it was found that equilibrium moisture loss and solid gain are related to solution concentration and solution temperature logarithmically. The optimum conditions of osmotic dehydration for further drying were found to be 20% salt concentration, 28 °C solution temperature and 1-h of osmosis.
We developed radiation heat transfer models with the combination of the Monte Carlo (MC) method and computational fluid dynamic approach and two-dimensional heat transfer models based on the fundamental quantum physics of radiation and fluid dynamics. We investigated far infrared radiation (FIR) heating in laminar and buoyancy airflow. A simple prediction model in laminar airflow was tested with an analytical solution and commercial software (CFX 4). The adequate number of photon tracks for MC simulation was established. As for the complex designs model, the predicted results agreed well with the experimental data with root mean square error of 3.8 K. Because food safety public concerns are increasing, we applied this model to the prediction of the thermal inactivation level by coupling with the microbial kinetics model. Under buoyancy airflow condition, uniformity of FIR heating was improved by selecting adequate wall temperature and emissivity.
Temperature distribution in a two-dimensional electrical heated system was determined both by computer simulation and experimental measurement. A commercial software program package was used for simulation. The experiment was conducted in a system where the vertical dimension was minimal to minimize natural convection during ohmic heating. In a rectangular static system containing a rectangular particle, computer simulation showed a trend verified by similar experimental results. Heat loss to the environment and physical property values used in simulation were major factors affecting differences between experimental and simulation results. There was no evidence for the presence of any coldest spot other than at the center of particles.
The effect of blanching by ohmic heating (OH) on the kinetics of osmotic dehydration of strawberries was studied. Ohmic heating
parameters obtained at two temperatures (65 and 85°C). The osmotic dehydration (OD) parameters are the temperature (26 and
37°C) and the sucrose in osmotic solution (30–70°B). An approximate solution of Fick’s law for unsteady state mass transfer
in spherical configuration has been used to calculate the effective diffusion coefficients of water and sucrose. Results show
that ohmic heating increases drastically the mass transfer and the effective diffusion rates. After 4h of OD (without OH)
in a sucrose solution (at 37°C and 70°B), the dry matter of the untreated strawberry halves was 20.3%; while it reached
68% when OD was combined with blanching by OH at 85°C for 3-min. Ohmic blanching permits the effective damage of cells by
combination of electrical and thermal effects. That result has an important enhancement of water and sugar transfers during
osmotic dehydration of strawberries.
KeywordsOhmic heating (OH)-Osmotic dehydration (OD)-Strawberries-Diffusion
The effects of various pre-treatments (hot water blanching, skin treatments, high pressure and high intensity electric field pulse treatment) on the dehydration characteristics of red paprika (Capsicum annuum L.) were evaluated and compared with untreated samples. Hot water blanching (100°C, 3 min) prior to dehydration (fluidised bed dryer at 60°C, 6 h and 1 m/s) resulted in the permeabilisation of 88% of the cell membranes in paprika, which in turn resulted in a higher mass and heat transfer. Skin treatments (such as lye peeling and acid treatment), as practised conventionally, increased dehydration rates but affected only the skin permeability. The application of high hydrostatic pressure (HHP, 400 MPa for 10 min at 25°C) or high intensity electric field pulses (HELP, 2.4 kV/cm, pulse width 300 μs, 10 pulses, pulse frequency 1 Hz) pre-treatments resulted in cell disintegration indexes of 0.58 and 0.61, respectively. Cell permeabilisation of these physical treatments resulted in higher drying rates, as well as higher mass and heat transfer coefficients, as compared to conventional pre-treatments.
Osmotic dehydration, due to its energy and quality related advantages, is gaining popularity as a complimentary processing step in the chain of integrated food processing. Generally, osmotic dehydration being a slow process, there has been a need for additional ways to increase the mass transfer without adversely affecting the quality. This gave the required motivation for many recent advances in this area. However, certain constraints still exist for the wide industrial adoption of osmotic dehydration, which need to be addressed in current and future research in the area. In order to compare the results of various investigators, there is a need to express research results in terms of more fundamental parameters like diffusion coefficient. Consequently, suitable methods to estimate such parameters in various foods of different size and geometry are discussed. The mechanism of osmotic dehydration proposed recently is described. Various methods to increase the rate of mass transfer, such as application of high hydrostatic pressure, high electrical field pulses, ultrasound, vacuum and centrifugal force are also presented.
Osmotic dehydration of bell peppers using sucrose and sodium chloride as osmotic agents as influenced by moderate thermal treatment (25–55 °C) and high intensity electric field pulses at varying field strengths (E=0.5–2.5 kV/cm) was studied. Two product quality indicators (vitamin C and carotenoids) were evaluated. Increasing temperature resulted in water loss from 32% to 48% and increasing field strength resulted in water loss from 36% to 50% of initial moisture content. Both conditions enhanced solid gain during osmotic dehydration of bell pepper. Air drying reduced vitamin C to approximately 5% of initial concentration while increasing temperature (25–55 °C) during osmotic dehydration decreased residual vitamin C concentration after osmotic dehydration from 20% to 4% and high intensity electric field (2.5–0.5 kV/cm) decreased it from 13% to 7% of initial value. Carotenoids reduced from 80% to 55% as a result of temperature increase and from 74% to 62% of initial fresh content as a result of high intensity electric field pre-treatment. Results obtained at field strength 2.5 kV/cm were comparable and in some cases better than those at elevated temperature of 55 °C suggesting high intensity electric field as an attractive alternative to conventional thermal processing.
This study compared mass transfer during osmotic dehydration (OD), as well as some quality indices, of untreated mango samples to those pretreated by applying high intensity electric field pulses (HELPs), high pressure (HP) or supercritical carbon dioxide having the same initial cell disintegration index (Zp) in the range 0.50–0.58. HELP and HP pretreated samples had a higher water loss (WL) and higher solid gain (Sg) than the untreated samples. Applying supercritical carbon dioxide did not improve water loss but facilitated higher sugar gain (8–42%) than in the other pretreated samples. HP and supercritical carbon dioxide pretreatments increased the red intensity (a values) of the mangos. The breaking force of HELP and HP pretreated samples increased with OD time.
Equilibrium distribution coefficient data were obtained for osmotic dehydration of apple cubes in sugar syrup of different concentrations and temperatures. The initial concentration of sucrose was varied from 30% to 70%, and temperature was varied from 22°C to 90°C. The distribution coefficients for solids ranged from 0.466 to 1.46, and for water it ranged from 0.621 to 1.80. The distribution coefficient for solids increased with the increase in temperature while it decreased with the increase in syrup concentration. However, the distribution coefficient for water decreased with increasing temperature and it increased with the increase in syrup concentration. A multiple regression analysis of experimental data was carried out to correlate distribution coefficients with dimensionless temperature and syrup concentration.
In this study, the performance of a solar assisted heat pump dryer and water heater has been investigated. A simulation program has been developed. The predicted results are compared with those obtained from experiments under the meteorological conditions of Singapore. A coefficient of performance (COP) value of 7.0 for a compressor speed of 1800 rpm was observed. Maximum collector efficiencies of 0.86 and 0.7 have been found for evaporator–collector and air collector, respectively. A value of the specific moisture extraction rate (SMER) of 0.65 has been obtained for a load of 20 kg and a compressor speed of 1200 rpm. Results suggest that the total drying time of the product decreases with the increase in drying potential. Drying potential is directly proportional to the air flow rate, drying air temperature and inversely proportional to the air relative humidity. Three important parameters that affect the system performance are solar radiation, compressor speed and the total load placed in the drying chamber. Both SMER and COP decrease with increase in compressor speed.
Vacuum impregnation (VI) and ohmic heating (OH) pre-treatments with and without addition of citric acid were used to enhance the mass transfer kinetics during osmotic dehydration (OD) of apple cubes. The VI with citric acid made the following OH treatment more homogeneous and equilibrated the fruit and syrup temperatures. The water loss (WL) and sugar gain (SG) during OD were significantly increased when the apple tissue was pre-treated with addition of citric acid. This was confirmed by calculation of water and sugar diffusivities. The VI and OH pre-treatments led to profound changes in apple fruit structure, evaluated by measurements of fruit firmness and electrical conductivity.
Osmotic dehydration processes are widely applied to obtain high quality intermediate moisture food. The study of dehydration kinetics and mass transfer mechanisms is very important for understanding and controlling the osmotic dehydration process. The internal changes and kinetics of both moisture change and mobility during osmotic dehydration of apples is reported. The effective diffusion coefficient of water was not constant during the dehydration treatment. Initially the effective diffusion coefficient calculated using a Fickian-based model was 2 ∗ 10−10 m2/s and increased to 5 ∗ 10−10 m2/s during treatment. Moreover, results showed the existence of an osmotic dehydration front that moves from the surface to the core of apple samples. It was not possible to explain the osmotic treatment process using only diffusion-based mechanisms. All layers of cells appear to be involved in the moisture transport process at the same time.