Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k _m and k _s ) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k_m and k_s) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

Osmotic Dehydration of Apple Cylinders: I. Conventional Batch Processing Conditions

Osmotic drying was carried out, with cylindrical samples of apple cut to a diameter-to-length ratio of 1:1, in a well-agitated large tank containing the osmotic solution at the desired temperature. The solution-to-fruit volume ratio was kept greater than 30. A modified central composite rotatable design (CCRD) was used with five levels of sucrose concentrations (34–63°Brix) and five temperatures (34–66°C). Kinetic parameters weight reduction (WR), moisture loss (ML), solids gain (SG) were considered. A polynomial regression model was developed to relate moisture loss and solids gain to process variables. A conventional diffusion model involving a finite cylinder was also used for moisture loss and solids gain, and the associated diffusion coefficients were computed. The calculated moisture diffusivity ranged from 8.20 × 10^?10 to 24.26 × 10^?10 m²/s and the solute diffusivity ranged from 7.82 × 10^?10 to 37.24 × 10^?10 m²/s. Suitable ranges of main parameters were identified for OD kinetics further study.     

Osmotic Dehydration of Apple Cylinders: I. Conventional Batch Processing Conditions

Osmotic drying was carried out, with cylindrical samples of apple cut to a diameter-to-length ratio of 1:1, in a well-agitated large tank containing the osmotic solution at the desired temperature. The solution-to-fruit volume ratio was kept greater than 30. A modified central composite rotatable design (CCRD) was used with five levels of sucrose concentrations (34-63°Brix) and five temperatures (34-66°C). Kinetic parameters weight reduction (WR), moisture loss (ML), solids gain (SG) were considered. A polynomial regression model was developed to relate moisture loss and solids gain to process variables. A conventional diffusion model involving a finite cylinder was also used for moisture loss and solids gain, and the associated diffusion coefficients were computed. The calculated moisture diffusivity ranged from 8.20 × 10^-10 to 24.26 × 10^-10 m²/s and the solute diffusivity ranged from 7.82 × 10^-10 to 37.24 × 10^-10 m²/s. Suitable ranges of main parameters were identified for OD kinetics further study.     

Effect of Blanching by Ohmic Heating on the Osmotic Dehydration Behavior of Apple Cubes

The effect of blanching by ohmic heating (OH) on the damage to apple tissues and subsequent osmotic dehydration kinetics was investigated. Apple cubes were heated ohmically to various blanching scales. Heating temperature and duration were, respectively, 60–95 ± 2°C and 0–6 min. After cooling, the treated samples were put into sucrose solutions (70 °B) for the osmotic dehydration (OD). The equilibrium state of osmotic dehydration was estimated using the Azuara model. Ohmic heating leads, even for short treatments, to significant changes in the cellular structure of apples and to the enhancement of mass transfer during osmotic dehydration.     

Assessment of Osmotic Process in Combination with Coating on Effective Diffusivities during Drying of Apple Slices

The effect of carboxymethyl cellulose (CMC) coating on the mass exchanges during the osmotic dehydration of apples and its effect on the quality of final product were studied. Coating semi-rings of apple with CMC solution (1%) was found to prevent solute uptake and to reduce salt diffusion coefficient from 4.35 × 10^-10 m²/s to 2.86 × 10^-10 m²/s. However, coating did not significantly affect the diffusivity of water. The effective diffusivity of salt and consequently solids gain were found to be depended on the concentration of CMC solution in the range of 0-1%. Increasing the concentration of CMC further from 1% had no effect on the mass exchanges during the osmotic process. Maximum performance ratio, defined as water loss/solids gain, and the lowest solids diffusion was observed for coated samples (with 1% CMC solution) treated with an osmotic solution containing glucose syrup (50%) and NaCl (2%).     

果蔬渗透脱水过程动力学研究

结合植物组织结构与流体传输过程机理建立了渗透脱水过程的一维质量传递数学模型。模型以植物细胞为传输过程的基本单元，考虑了各组分在细胞内、细胞外、通过细胞膜及胞间连丝的质量扩散，和由于体积收缩而导致的集流传输。以土豆为实验物料，在40℃恒温条件下，采用40％（质量百分比）的蔗糖溶液作为渗透液，进行渗透脱水实验，得到的实验结果与模拟结果十分接近，验证了模型的有效性。通过数值模拟可详细描述渗透脱水过程中土豆细胞内外水和蔗糖的质量浓度分布。并就能源与生产效率方面对“渗透-干燥”与“无预处理干燥”过程作了比较。     

Mass Transfer during Osmotic Dehydration in a Multicomponent Solution Rich in Grape Phenolics with Antioxidant Activity

Osmotic dehydration has been assessed as an operation for obtaining solid foodstuffs supplemented with grape phenolics. However, mass transfer in multicomponent osmotic solutions during OD needs to be described appropriately if the infusion of target solutes, in this case phenolics with antioxidant activity, is to be better controlled. The effective diffusion coefficients of moisture, total phenolics, and the major individual phenolics of low-molecular-weight (caftaric acid, coutaric acid, caffeic acid, coumaric acid, ferulic acid, and rutin) were evaluated during the OD of a model food with a concentrated red grape must. An increase in the concentration of soluble solids above 50° Brix decreased the level of penetration of phenolics in the model food.     

Mass Transfer during Osmotic Dehydration in a Multicomponent Solution Rich in Grape Phenolics with Antioxidant Activity

Osmotic dehydration has been assessed as an operation for obtaining solid foodstuffs supplemented with grape phenolics. However, mass transfer in multicomponent osmotic solutions during OD needs to be described appropriately if the infusion of target solutes, in this case phenolics with antioxidant activity, is to be better controlled. The effective diffusion coefficients of moisture, total phenolics, and the major individual phenolics of low-molecular-weight (caftaric acid, coutaric acid, caffeic acid, coumaric acid, ferulic acid, and rutin) were evaluated during the OD of a model food with a concentrated red grape must. An increase in the concentration of soluble solids above 50° Brix decreased the level of penetration of phenolics in the model food.     

Optimization of Osmotic Dehydration of Kiwifruit

Mass transfer rates were quantitatively investigated during osmotic dehydration of kiwifruit slices using response surface methodology with the sucrose concentration (20-80%, w/w), temperature of sucrose solution (15-75°C), osmotic time (60-420 min), and slice thickness (2-10 mm) as the independent process variables. Quadratic regression equations are obtained to describe the effects of independent process variables on the water loss (WL), sucrose gain (SG), and ascorbic acid loss (AAL). It was found that all factors had significant effect on the WL during osmotic dehydration of kiwifruit. Effects of temperature, time, and slice thickness were more pronounced on SG than the effect of concentration of sucrose solution. The osmotic solution temperature was the most significant factor affecting the AAL, followed by slice thickness and duration of treatment. The optimal conditions for osmotic dehydration were: 60% sucrose concentration, 30-40°C osmotic temperature, 150 min osmotic time, and 8 mm slice thickness.     

Ultrasound-Assisted Osmotic Dehydration of Strawberries: Effect of Pretreatment Time and Ultrasonic Frequency

Pretreatment of fruits prior to drying has shown success in reducing drying time and costs. In this work, ultrasound-assisted osmotic dehydration has been implemented as a method to increase water diffusivity and reduce drying time in strawberries. Strawberry halves were immersed in distilled water and in two different concentrations of sucrose solutions while pretreatment time and ultrasonic frequency levels were varied to determine their effect on drying time, water loss, and soluble solids gain. A microscopic analysis was carried out to evaluate the formation of microchannels and other changes to the fruit tissue structure. Greater sucrose concentration used in ultrasound-assisted osmotic dehydration resulted in greater water loss with greatest loss observed for the strawberry halves pretreated for 45 min in a 50% w/w sucrose solution. The pretreatment carried out for 30 min employing an osmotic solution of 50% w/w of sucrose resulted in the highest drying rate among the pretreatments. Osmotic dehydration used alone during pretreatment increased total processing time, whereas osmotic dehydration combined with ultrasonic energy during pretreatment reduced total processing time and increased effective water diffusivity. Cell distortion and breakdown were observed not only in pretreatments employing ultrasound-assisted osmotic dehydration but in conventional osmotic dehydration. Formation of microchannels through ultrasonic application and effects of osmotic pressure differential were considered to be largely responsible for reducing drying time for strawberry halves.     

Artificial Neural Network Modeling of Osmotic Dehydration Mass Transfer Kinetics of Fruits

Artificial neural network (ANN) models were developed for the prediction of transient moisture loss (ML) and solid gain (SG) in osmotic dehydration of fruits using process kinetics data from the literature. ANN models for ML and SG were developed based on data over a broad range of operating conditions and ten common processing variables: temperature and concentration of osmotic solution, immersion time, initial water and solid content of the fruit, porosity, surface area, characteristic length, solution-to-fruit mass ratio, and agitation level. The trained models were able to accurately predict the outputs with associated regression coefficients (r) of 0.96 and 0.93, respectively, for ML and SG. These ANN models performed much better than those obtained from linear multivariate regression analysis. The large number of process variables and their wide ranges considered along with their easy implementation in a spreadsheet make them very useful and practical for process design and control.     

Artificial Neural Network Modeling of Osmotic Dehydration Mass Transfer Kinetics of Fruits

Artificial neural network (ANN) models were developed for the prediction of transient moisture loss (ML) and solid gain (SG) in osmotic dehydration of fruits using process kinetics data from the literature. ANN models for ML and SG were developed based on data over a broad range of operating conditions and ten common processing variables: temperature and concentration of osmotic solution, immersion time, initial water and solid content of the fruit, porosity, surface area, characteristic length, solution-to–fruit mass ratio, and agitation level. The trained models were able to accurately predict the outputs with associated regression coefficients (r) of 0.96 and 0.93, respectively, for ML and SG. These ANN models performed much better than those obtained from linear multivariate regression analysis. The large number of process variables and their wide ranges considered along with their easy implementation in a spreadsheet make them very useful and practical for process design and control.     

A Comparison of Some Mathematical Models Used for the Prediction of Mass Transfer Kinetics in Osmotic Dehydration of Fruits

Mass transfer in osmotic dehydration of fruits at atmospheric pressure has been modeled either by a phenomenological approach using Crank's model (which is a Fick's law solution) or empirically, using models developed from polynomial equations, mass balances, or relations between process variables (i.e., Magee's, Azuara's, and Page's models). For all of these models, experimental data are required to determine the values of their adjustable parameters for specific processing conditions. From a wide variety of published data, the parameters for moisture loss (ML) and solid gain (SG) were calculated for the above-mentioned models, and a comparison of their correlation capability was made. Azuara's and Page's models yield better correlations (with mean absolute errors less than 1.26% for ML and 0.46% for SG) than Magee's and Crank's models (with mean absolute errors of up to 2.98 and 1.68% for ML and SG, respectively).     

Osmotic Dehydration Pretreatment in Drying of Fruits and Vegetables

Several vegetables and fruits, apple, ginger, carrot, and pumpkin were dehydrated under various osmotic conditions using sucrose and salt as the permeating agents. The dehydrated materials were then dried. The influence of solute concentration, process temperature and the type of solute on osmotic dehydration and further thermal drying were investigated. The nutrition loss during the osmotic process was measured using carotene as the nutrition index. The effect of calcium chloride present in osmotic solution on the product quality was also studied. A first order kinetic model was chosen to describe the mass transfer phenomena of the osmotic process. The equilibrium value of water loss, solute gained, kinetic constants K_WL and K_SG under various conditions are successfully predicted by the model. The relationship between the equilibrium value and four major factors that influence osmotic process of carrot was obtained based on the experimental data. The relations between the loss constant of carotene and the solute concentration in carrot and pumpkin were obtained based on the experimental data. The qualities of dried products are better for the osmotic dehydration pretreated samples than those dried directly.     

Mathematical Equations to Predict Mass Fluxes and Compositional Changes During Osmotic Dehydration of Cherry Tomato Halves

Cherry tomato halves (L. esculentum var. Cerasiforme cv. Cocktail) were dehydrated by immersion in binary solutions of salt or sugar and ternary salt-sugar solutions at 30, 40, and 50°C. Compositional changes (moisture, sugar, and salt content) as well as mass fluxes were controlled at different process times. The influence of temperature and osmotic agents (salt, sucrose, and the combination of them) on mass transfer phenomena was studied, and the experimental results were modeled in order to establish a set of equations that would facilitate the prediction not only of final product composition, which is related to its quality and its stability, but also the mass fluxes involved in the process associated to the yield of the process.     

On the Development of Osmotically Dehydrated Seabuckthorn Fruits: Pretreatments,Osmotic Dehydration,Postdrying Techniques,and Nutritional Quality

Osmotic dehydration of whole seabuckthorn berries, followed by convective or vacuum drying, was investigated. First, different pretreatments were applied to the fruits in order to accelerate the rate of osmotic dehydration: immersion in liquid nitrogen, steam blanching, or freeze cycles. Immersion in liquid nitrogen was found to be the best pretreatment to maximize dehydration rate and to increase sugar gain during osmotic dehydration. An evaluation of moisture loss and sugar gain kinetics during osmotic dehydration of seabuckthorn fruits pretreated with liquid nitrogen, followed by vacuum or hot-air drying, was then performed. Loss of nutritional compounds due to processing was also measured. Sugar intake and partial dehydration of seabuckthorn samples increased with osmosis time and reached an equilibrium value after 4 h treatment. The finish drying methods (vacuum or convective) applied after OD showed a marked impact on the remaining moisture content of seabuckthorn samples. Concentration of some nutritional compounds was, however, dramatically reduced after the combined osmotic dehydration/drying processes.     

连续渗透汽化过程工艺设计与优化

介绍了两种渗透汽化膜分离过程工艺及膜的分离性能,提出了连续渗透汽化逐级级联计算,并对中试装置的运行结果进行了验算;其次对醇水混合物的连续分离进行了过程优化计算,计算了两种级联方式的级联数和加热段数。结果表明,在相同条件下,等温级联换热过程比等膜面积换热级联过程更有利于对醇水混合物的连续分离。本文提出的级联计算方法可用于渗透汽化膜分离醇水混合物的工业过程放大设计。     

Diesel Oil Removal by Froth Flotation Under Low Interfacial Tension Conditions II: Continuous Mode of Operation

Abstract

The objective of this study was to investigate the relationship between interfacial tension (IFT) and foam characteristics and the efficiency of diesel oil removal from water in a continuous froth flotation column. The effects of operational parameters, including surfactant concentration, salinity, oil-to-water ratio, foam height, air flow rate, and hydraulic retention time (HRT) on the oil removal were investigated in the continuous mode of a froth flotation operation and compared to batch operation results. Unlike the batch system, for the continuous system used in the present study, having only branched alcohol propoxylate sulfate sodium salt surfactant (C_14–15(PO)₅SO₄Na) and NaCl present in the solution yielded such poor foam characteristics that a stable froth which overflowed the flotation column could not be produced, so the addition of sodium dodecyl sulfate (SDS) as a froth promoter was used to improve the foam stability. Unlike the batch froth flotation system with only C_14–15(PO)₅SO₄Na, the continuous froth flotation with the mixture of C_14–15(PO)₅SO₄Na and SDS, it was not possible to find a SDS and a NaCl concentration at which both ultralow IFT and good foaming were both achieved. Foam formation, stability, and production rate were found to be crucial parameters to the froth flotation efficiency. The continuous froth flotation system offers a high diesel oil removal of 96% in the single stage unit. Demonstration of efficient operation in the continuous mode in this work is important to the practical application of froth flotation in large scale processing.