Application of Ultrasound and Ultrasound-Assisted Osmotic Dehydration in Drying of Fruits

This work examines the influence of ultrasonic and ultrasonic assisted osmotic dehydration pretreatments on the dehydration of eight fruits (banana, genipap, jambo, melon, papaya, pineapple, pinha, and sapota). An overview of the effects of ultrasound application on water loss, sugar gain, and effective diffusivity of water during the dehydration process is presented. The results showed significant differences for water loss and sugar gain among the fruits that were studied, which were analyzed based on the changes observed on the tissue structure of the fruit. The results also showed that the effective diffusivity of water in the fruit increased after application of ultrasound reducing air-drying time.     

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.     

Pulsed Electric Field Pretreatment for Osmotic Dehydration of Apple Tissue: Experimental and Mathematical Modeling Studies

The aim of this study was to analyze the influence of pulsed electric field pretreatment (PEF) on the osmotic dehydration of apple tissue. Osmotic dehydration was carried out in sucrose solution at 40°C and 100 rpm in a water-bath shaker. PEF pretreatment was performed using varying field strength of 5 and 10 kV/cm and 10 and 50 pulses. On the basis of electric conductivity measurement, the cell disintegration index was calculated. The course of osmotic dehydration was described by means of water loss, solid gain, weight reduction, and water content changes. Moreover, the course of the process was described by different mathematical models that are commonly used in the literature. PEF application before osmotic dehydration significantly increased water loss after 60 minutes of the process. In turn, no significant differences were found in the case of solid gain. The highest osmotic dehydration efficiency ratio (WL/SG) was noticed for samples treated by PEF at the electric field strength of 5 kV/cm and 10 pulses. The statistical analysis of mathematical modeling of the process showed the equations utilized generally exhibit a good fit to the experimental data.     

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。     

Dehydration of potato slices following brief dipping in osmotic solutions: Effect of conditions and understanding the mechanism of water loss

A novel variant of osmotic dehydration, named here as postdipping dehydration—where a material is dipped in a salt or sugar solution for a very short time followed by simple exposure to ambient conditions was explored with the aim of lowering water content of potato slices but at the same time not gain a high level of sugar/salt. The rate of water loss, which was rapid initially, was found to approach equilibrium. This article also explored whether the water loss process could subsequently be kick started once again, by employing a multistage process, where each stage consisted of osmotic solution dipping followed by ambient holding of the potato slices that had reached equilibrium in the earlier stage. Water loss values comparable to conventional osmotic dehydration could be achieved thus, but with significantly lower overall solid gain (<50%)—which can potentially yield a significantly healthy product option.     

Use of Ultrasound as Pretreatment for Dehydration of Melons

This work examined the influence of the ultrasonic pretreatment prior to air drying on dehydration of melon (Curcumis melo L.). Ultrasonic pretreatment for air drying of fruits was studied and compared with osmotic dehydration. This study allowed estimate of the effective diffusivity water in the air-drying process for melons submitted to ultrasonic pretreatment. Results show that the water effective diffusivity increases after application of ultrasound causing a reduction of about 25% in the drying. During ultrasonic treatment the melons lost sugar, so such a pretreatment stage can be a practical process to produce dried fruits with lower sugar content. Compared to osmotic dehydration, the use of ultrasonic pretreatment performed better when large amounts of water need to be removed from the fruit, since the combined processing time (pretreatment and air drying) is shorter.     

EQUILIBRATION OF APPLE TISSUE IN OSMOTIC DEHYDRATION: MICROSTRUCTURAL CHANGES

The changes in density and porosity and the microstructural development of apple tissue throughout the equilibration in osmotic treatments were analyzed for different osmotic solution concentrations. The effect of vacuum impregnation of the sample with the osmotic solution, before the osmotic treatment was also studied. Two periods were differentiated in the process. For relatively short term treatments (24-48 h) cell walls shrunk or separated from plasmalemma because of the cell water loss while compositional pseudoequilibrium was achieved. Afterwards for long term treatments, a bulk flux of osmotic solution into the fruit tissue occurred in line with the relaxation of mechanical energy accumulated in the deformed cell wall matrix. So, the true equilibrium (chemical plus mechanical), achieved when no changes either in sample composition or weight occurred, implied cell wall sphericity and sample volume recovery. However, some degree of irreversibility was observed in cell wall recovery, especially for osmotic treatments at high concentration without previous vacuum impregnation of sample.     

EQUILIBRATION OF APPLE TISSUE IN OSMOTIC DEHYDRATION: MICROSTRUCTURAL CHANGES

ABSTRACT

The changes in density and porosity and the microstructural development of apple tissue throughout the equilibration in osmotic treatments were analyzed for different osmotic solution concentrations. The effect of vacuum impregnation of the sample with the osmotic solution, before the osmotic treatment was also studied. Two periods were differentiated in the process. For relatively short term treatments (24–48 h) cell walls shrunk or separated from plasmalemma because of the cell water loss while compositional pseudoequilibrium was achieved. Afterwards for long term treatments, a bulk flux of osmotic solution into the fruit tissue occurred in line with the relaxation of mechanical energy accumulated in the deformed cell wall matrix. So, the true equilibrium (chemical plus mechanical), achieved when no changes either in sample composition or weight occurred, implied cell wall sphericity and sample volume recovery. However, some degree of irreversibility was observed in cell wall recovery, especially for osmotic treatments at high concentration without previous vacuum impregnation of sample.     

Influence of Ultrasound-Assisted Osmotic Dehydration and Freezing on the Water State,Cell Structure,and Quality of Radish (Raphanus sativus L.) Cylinders

Ultrasound-assisted osmodehydrofreezing technique is a partial dehydration technique prior to freezing to diminish the tissue damage and preserve the quality by quickly removing part of the water from vegetable tissues. In this study, radish cylinders with three different water contents (85%, 80% and 75%, w/w) were dehydrated by osmotic dehydration (OD) and ultrasound-assisted osmotic dehydration (UOD). The effects of OD and UOD pretreatment on the characteristics (e.g., latent heat of fusion of ice, freezable water and microstructure) of dehydrated products and quality attributes (e.g., freezing time, firmness, drip loss and ascorbic acid content) of osmodehydrofrozen products were investigated. Ultrasound application significantly shortened the time of dehydration and subsequent freezing. Compared to OD products with equal water content, UOD products exhibited less freezable water content and better preservation on firmness and microstructure. After freezing/thawing, frozen products of UOD also displayed less drip and ascorbic acid losses and better firmness than that of OD.     

Osmotic dehydration of blueberries pretreated with pulsed electric fields: Effects on dehydration kinetics,and microbiological and nutritional qualities

Fresh blueberries were treated by pulsed electric fields (PEF) at 2?kV/cm before osmotic dehydration in 70% cane sugar syrup. The changes in water loss, solids gain, populations of native microorganisms, antioxidant activity, contents of anthocyanins, predominant phenolic acids and flavonols, and total phenolics in blueberries were investigated after PEF pretreatment and during osmotic dehydration at 40°C. Compared with non-PEF-pretreated blueberry samples, PEF pretreatment reduced the osmotic dehydration time of blueberry samples from 120 to 48?h at target moisture content (3.0?g/g initial dry matter). No significant differences (p?>?0.05) in total phenolics, antioxidant activity, anthocyanins, and predominant phenolic acids and flavonols were observed between the PEF-pretreated and non-PEF-pretreated blueberry samples. The PEF-pretreated blueberry samples had lower microbial populations than those which were not subjected to PEF pretreatment. Regardless of pretreatment, decreases in anthocyanins, predominant phenolic acids and flavonols, total phenolics, and antioxidant activity in blueberries were observed during the osmotic dehydration process. The results demonstrate that PEF pretreatment significantly reduced the dehydration time and enhanced the microbiological quality of blueberries without affecting their nutritional quality.     

Influence of osmotic dehydration process parameters on the quality of candied pumpkins

The influence of osmotic dehydration process parameters on the efficiency of water loss and sucrose gain of pumpkins and the influence on the quality of the final product are essential for production of superior quality candied pumpkins. Mass transfer kinetics during osmotic dehydration of pumpkins were modelled by assuming Fickian diffusion of sucrose and water in unsteady state conditions, which described very well the experimental results for water loss and sugar gain. Water and sucrose effective diffusion coefficients increase significantly with temperature. Temperature and sucrose concentration had a significant influence on both water and sugar diffusion, increasing as the solution temperature increased. Significant increase in the effective water diffusivity and decrease in the sucrose effective diffusivity was observed when the sucrose solution concentration increased from 40 °Brix to 50 and 60 °Brix. From 50 to 60 °Brix, no difference in the effective diffusivities was observed. Candied pumpkins with higher sucrose content have a higher breaking stress, less breaking strain and lower work to fracture. Moreover, the product becomes harder and less elastic with increasing sucrose content, resulting in more brittle products, which seems to be related with reinforcement of the pumpkin cell wall matrix.     

Degradation of epoxy coatings in humid environments: the critical relative humidity for adhesion loss

An abrupt loss of adhesion at interfaces between epoxy and inorganic substrates has been shown to occur whenever the epoxy is equilibrated in air whose relative humidity exceeds a critical value. We report that the critical relative humidity marking the onset of adhesion loss is associated with (1) a sudden increase of water solubility in the epoxy, (2) a corresponding increase in the volume of the epoxy, and (3) a strong decrease in the mobility of absorbed water. Possible mechanisms such as capillary condensation, osmotic cell formation, and a decrease in glass transition temperature induced by water are ruled out on the basis of available data. Instead, it is proposed that the observed behavior is linked to water 'condensation' into traps or clusters in the polymer above the critical humidity. The solubility behavior of a liquid diol that models the diol-terminated epoxy chain tails suggests that the excess sorption above the critical humidity involves trapping of the water by hydroxyl groups which become available as inter-chain hydrogen-bonded structures are broken above the critical relative humidity. Pronounced irreversibility of solubility and swelling is due to a reaction of the absorbed water with residual oxirane groups, leading to the formation of diols. This explains the upward shift of the solubility and swelling curves toward asymptotic behavior upon relative humidity cycling. We also show that osmotic cells that form around artificially introduced water-soluble impurities can cause a loss of adhesion at a predictable critical relative humidity.     

Kinetic Aspects, Texture, and Color Evaluation of Some Tropical Fruits during Osmotic Dehydration

Osmotic dehydration of some tropical fruits like guava, melon, and papaya using sucrose and maltose solutions is presented in this article. The influence of sugar type and concentration, process temperature, and calcium salt addition on osmotic solution was investigated. Water loss and sugar gain up to 6 h of processing were evaluated and the effect of osmotic dehydration on fruit quality parameters like color and texture was analyzed. All studied variables affected the osmotic process kinetics, while for quality parameters the influence of sugar type or solution concentration and temperature was dependent on fruit and process conditions.     

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.     

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.     

DRYING CHARACTERISTICS OF OSMOTICALLY DEHYDRATED FRUITS COATED WITH SEMIPERMEABLE EDIBLE FILMS

This paper deals with key issues of the coating technology applied to plant materials prior to osmotic dehydration and convective drying. Coatings created with solutions of starch products and pectin were examined. It was shown that coating of foods to be dehydrated influences mass transfer in a plant tissue during osmotic pre-treatment as well as during convective drying. Each coating should be individually examined. The properties of coatings depend on their composition and the method used for the fabrication of a coating. Coating of foods with an artificial barrier on the surface may efficiently hinder the penetration of solute inside the food, not affecting much the rate of water removal. The coated plant materials had a greater water loss/solids gain ratio then the uncoated ones. Osmotic dehydration seems to be one of possible pre-treatment methods before drying. The conditions of convective drying are changed upon osmotic dehydration of plant materials. It was shown that changes in drying rate depend on the kind of coating substance. Bigger differences were observed at higher water content and these differences narrowed as water content in dried material decreased. The positive effect of coatings on the physical properties of dried fruits was found.     

DRYING CHARACTERISTICS OF OSMOTICALLY DEHYDRATED FRUITS COATED WITH SEMIPERMEABLE EDIBLE FILMS

This paper deals with key issues of the coating technology applied to plant materials prior to osmotic dehydration and convective drying. Coatings created with solutions of starch products and pectin were examined. It was shown that coating of foods to be dehydrated influences mass transfer in a plant tissue during osmotic pre-treatment as well as during convective drying. Each coating should be individually examined. The properties of coatings depend on their composition and the method used for the fabrication of a coating. Coating of foods with an artificial barrier on the surface may efficiently hinder the penetration of solute inside the food, not affecting much the rate of water removal. The coated plant materials had a greater water loss/solids gain ratio then the uncoated ones. Osmotic dehydration seems to be one of possible pre-treatment methods before drying. The conditions of convective drying are changed upon osmotic dehydration of plant materials. It was shown that changes in drying rate depend on the kind of coating substance. Bigger differences were observed at higher water content and these differences narrowed as water content in dried material decreased. The positive effect of coatings on the physical properties of dried fruits was found.     

Air-Drying Kinetics of Osmotically Dehydrated Fruits

ABSTRACT

The air drying kinetics of fresh and osmotically dehydrated fruits (apples) was determined. Two sugars, glucose and sucrose, were used as osmotic dehydration agents. Three levels of sugar concentration (15%, 30% and 45%) and several times of immersion into the sugar solution were used. Following the osmotic preconcentration, the fruit samples were dried at 55°C and the weight of material was recorded. The effective water diffusivity of samples treated under various osmotic conditions was estimated and the results were related to the sugar content and the bulk porosity of the samples. The effective water diffusivity, resulting from the application of the diffusion equation to the drying kinetics of the apples was found to decrease significantly for the samples pretreated by a concentrated sugar solution (e.g. 45%), evidently due to the lower porosity and other physicochemical factors. The low diffusivity may be beneficial in the storage stability and utilization of dehydrated fruits.     

Optimization of Osmo-convective Dehydration Process for the Development of Honey-ginger Candy Using Response Surface Methodology

Osmotic dehydration of ginger with honey is an interesting alternative for the development of confectionary-based functional food with extended shelf life. Response surface methodology (RSM) was used to investigate the effects of process variables on solid gain, water loss, and overall acceptability of honey-ginger candy. The process variables included blanching time (6–10 min), osmotic solution temperature (30–50°C), immersion time (90–150 min), and convective drying temperature (50–70°C). The honey to ginger ratio was 4:1 (w/w) during all the experiments. Ginger cubes were blanched before osmotic dehydration to increase the permeability of the outer cellular layer of tissue. After osmotic concentration of ginger with honey, convective dehydration was done to final moisture content of 3–5% (w.b.) to make it a shelf-stable product. Finally, osmo-convectively dried ginger was coated with sucrose for candy preparation. The optimum osmo-convective process conditions for maximum solid gain, water loss, and overall acceptability of honey-ginger candy were 7.07 min blanching time, 50°C solution temperature, 150 min immersion time, and 60°C convective drying temperature.     

Effect of Drying on Microstructure of Plant Tissue

Drying causes many chemical and physical changes in plant tissue. Physical changes are mostly due to stresses developed in the tissue and are pronounced by macro- and microalterations of size, shape and internal structure. The extent and direction of these changes depend on the mode of drying. Convective drying of apple tissue results in numerous breaks of cell walls and formation of many microcavities. In consequence, the dominating cross-sectional area of cells moves toward values smaller than those observed in raw apple. Cavities formed during convective drying can be pictured in 2D plane as pentagons joined together or ellipses with large ratio between axes. Puff-drying forms porous structure with cavities twice as large as in raw apple. The process homogenizes the structure, and spread of cell sizes is even smaller than that observed in raw apple. Broken cell walls are clearly visible in microphotographs of sliced samples. Many large cavities with cross-sectional area exceeding 40,000 µm² are characteristic of puff-dried apple while such areas are hardly found in raw apple. The structure of freeze-dried apple is similar to that formed during puff-drying. Large irregular cavities and broken cell walls are clearly seen in microphotographs. However, larger cavities are present in freeze-dried apple than in puff-dried one. Cross-sectional area of cavities in freeze-dried apple can be simulated by pentagons joined together or elongated ellipses.