VACUUM OSMOTIC DEHYDRATION OF FRUITS

ABSTRACT

Vacuum osmotic dehydration leads a special behaviour of mass transfer in fruit-sugar soluion system.Vacuum treatments intensify the capillary flow function and increase water transfer ratio.but have no significant influence on sugar uptake.Fruits such as pineapples which have higher porosity are more suitable to be treated under vacuum during the osmotic dehydration.     

声空化强化渗透脱水

引　言渗透脱水是指果蔬在一定温度下 ,放入高渗透压介质中 ,利用膜两边的浓度不同产生的渗透压除去其中部分水分的一种方法 .它是基于天然果蔬细胞壁作为半透膜 ,在渗透脱水中 ,主要存在两个相反过程 ,一方面果蔬中的水分向溶液中传递 ,另一　　方面溶液中的溶质逐渐渗入果蔬 ,最终达到渗透平衡 .渗透脱水可以在较短时间内除去果蔬的水分而不损坏果蔬的组织结构 .从生产的角度来说 ,经渗透脱水的果蔬再进行干燥 ,产品的干燥时间可缩短10 %～ 15 % [1] .通常 ,渗透脱水是一非常缓慢的过程 ,因此在不影响果蔬品质的前提下有必要采用一定方法…     

声空强化渗透脱水过程质扩散研究

An experimental study on intensifying osmotic dehydration was carried out in a state of nature and with acoustic cavitation of different cavitating intensity (0.5A, 0.7A and 0.9A) respectively, in which the material is apple slice of 5mm thickness. The result showed that acoustic cavitation remarkably enhanced the osmotic dehydration, and the water loss was accelerated with the increase of cavitating intensity. The water diffusivity coefficients ranged from 1.8×10-10m2·s-1 at 0.5A to 2.6×10-10m2·s-1 at 0.9A, and solute diffusivity coefficients ranged from 3.5×10-11m2·s-1 at 0.5A to 4.6×10-11 m2·s-1 at 0.9A. On the basis of experiments, a mathematical model was established about mass transfer during osmotic dehydration, and the numerical simulation was carried out. The calculated results agree well with experimental data, and represent the rule of mass transfer during osmotic dehydration intensified by acoustic cavitation.     

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。     

OSMOTIC DEHYDRATION OF PINEAPPLE

The effects of sugar type, sugar concentration, immersion time and temperature on the mass transfer of osmotic dehydration were studied using pie shape slices (7 mm thick with its center cork thrown away) of fresh pineapple (Queen variety, 90% maturity). The dehydration process was performed using two type of sugar as an osmotic agent (glucose and sucrose), three levels of sugar concentration (50, 60, and 70%), three levels of temperature (30, 50, and 70 °C), and three levels of immersion time (3, 6, and 9 hours). Following the osmotic dehydration process, the pineapple was dried at 70 °C for 48 hours. The mass transfer was then calculated and analyzed statistically. Sugar type, concentration, temperature, and length of immersion, have a significant effect on the mass transfer of osmotically dehydrated pineapple. The highest mass transfer of pineapple was found by using sucrose at the concentration of 70%, temperature 50 °C and 9 hours of immersion time.     

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.     

Ultrasound-assisted osmotic process on quality of microwave vacuum drying sweet potato

The effects of pretreatment before microwave vacuum drying (MVD) on texture, color, expansion, rehydration, drying rate, microstructure, sensory evaluation, and other properties of sweet potato were investigated in this study. The pretreatment consisted in five processing conditions, using blanching; osmotic dehydration at 35°Brix of sucrose (OD); ultrasound in distilled water (US); ultrasound in distilled water before osmotic dehydration (US?+?OD), and ultrasound-assisted osmotic dehydration (USOD). Pretreatments of sweet potato before MVD have shown success in reducing drying time with US treatment relatively more effective regarding drying time than other treatments. Compared with other treatments, US showed the highest rehydration ratio values. The osmotic group pretreatment exhibited a pronounced effect on water loss and solid gain, improved the color, aroma, and taste of dried sweet potato, whereas sucrose impregnation resulted in a hard texture observed with OD sample. USOD samples had a higher expansion ratio, lower hardness and color difference values, appeared less cell damaged, and recorded better overall quality than the other samples. There was a slight difference between USOD and US?+?OD samples. Combining osmotic dehydration with ultrasound as a pretreatment can significantly accelerate the heat transfer rate, reducing the dried time accordingly and increasing energy efficiency.     

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.     

Air drying and colour characteristics of chestnuts pre-submitted to osmotic dehydration with sodium chloride

Air drying kinetics of chestnuts (Castanea sativa Mill.) submitted previously to osmotic dehydration with sodium chloride solutions (22%, w/w, 25 °C, 8 h) were experimentally determined. Drying experiments were carried out at 45, 55 and 65 °C during 32 h. Before osmotic dehydration operation, shell and tegument tissue were carefully removed in all samples. Rough external surface was maintained (peeled samples) or also removed (cut chestnuts) in different tests in order to evaluate the effect of the presence of this layer in the mass transfer rate. A diffusional model involving shrinkage was employed to simulate the drying kinetics of the all studied systems and values of the effective coefficient of diffusion of water were obtained. These coefficients were successfully correlated with temperature by means of an exponential equation and were employed to evaluate the additional mass transfer resistances due to the presence of layers and by the acquisition of osmotic solute in cut and peeled samples. The experimental determination of local salt and moisture contents during drying showed the existence of a drying front for water that moves along from surface to the centre of the sample and that salt accumulates near surface but also achieve the centre during the osmotic dehydration. Additionally, the analyses of the surface colour of air dried samples indicated that osmotic dehydration with salt keeps the characteristics observed for non-treated cut samples and deteriorates the colour quality in the case of peeled samples.     

Influence of Process Conditions on the Mass Transfer Kinetics of Pulsed Vacuum Osmotically Dehydrated Mango Slices

The pulsed vacuum osmotic dehydration of mango slices was studied using a 2^5–1 fractional factorial design. The process responses were water loss, solids gain, water activity, and the effective diffusivities of the water or solids. Statistical analyses revealed that temperature and solution concentration were significant for all the responses studied. Vacuum time was significant for solids gain and the effective diffusivity of water. Diffusion coefficients were determined using an analytical solution of Fick's unidirectional diffusion equation for flat plates, showing a good fit to the experimental data. Osmotic recirculation and vacuum pressure had no effect on any of the responses studied.     

EFFECT OF OSMOTIC AGENT ON OSMOTIC DEHYDRATION OF FRUITS

Mass transfer phenomena were investigated during osmotic dehydration of apple, banana and kiwi in glucose and sucrose osmotic solution. A complete set of experiments was performed for a wide range of temperature, sample size, speed of agitation, osmotic agent concentration and immersion time. An empirical model, based on a first order kinetic equation, was fitted satisfactorily to experimental data. Furthermore, the effect of solute molecular weight on mass transfer phenomena during the osmotic treatment was evaluated. The results showed that low molecular weight solute (glucose) leads to higher water loss and solids uptake than high molecular weight solute (sucrose), of osmodehydrated fruits under the same solution concentration.     

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.     

EFFECT OF OSMOTIC AGENT ON OSMOTIC DEHYDRATION OF FRUITS

ABSTRACT

Mass transfer phenomena were investigated during osmotic dehydration of apple, banana and kiwi in glucose and sucrose osmotic solution. A complete set of experiments was performed for a wide range of temperature, sample size, speed of agitation, osmotic agent concentration and immersion time. An empirical model, based on a first order kinetic equation, was fitted satisfactorily to experimental data. Furthermore, the effect of solute molecular weight on mass transfer phenomena during the osmotic treatment was evaluated. The results showed that low molecular weight solute (glucose) leads to higher water loss and solids uptake than high molecular weight solute (sucrose), of osmodehydrated fruits under the same solution concentration.     

OSMOTIC DEHYDRATION OF COD FILET WITH SKIN IN A STAGNANT BRINE

Salt transport models and diffusion coefficients have been studied during osmotic dehydration of fish muscle in stagnant, saturated brine. Attempts have been made to estimate mass transfer resistance through skin and through stagnant boundary layer at surface. These resistances have been converted to equivalent product thickness or “length” in order to apply analytical solutions to the diffusion equations (i.e. Fick's second law). A similar approach has been used to estimate the extra mass transfer resistance due to the stagnant brine, as most experiments in literature have been made under agitated conditions, in contrary to industrial practice, which is in stagnant brine.     

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.     

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

Mass transfer of apple cylinders during osmotic dehydration was quantitatively investigated under continuous medium flow conditions. The influences of the main process variables (solution concentration, operation temperature, contact time, and solution flow rate) were determined. A second-order polynomial regression model was used to relate weight reduction (WR), moisture loss (ML), solids gain (SG), and mass diffusivity (D _m and D _s ) to process variables. The conventional diffusion model using a solution of Fick's unsteady state law involving a finite cylinder was applied for moisture diffusivity and solute diffusivity determination. Diffusion coefficients were in the range of 10^?9–10^?10 m²/s, and moisture diffusivity increased with temperature and flow rate, increased with solution concentration (> 50°Brix), and decreased with increasing solution concentration (< 50°Brix), but solids diffusivity increased with temperature and concentration and decreased with increasing flow rate. A continuous-flow osmotic dehydration (CFOD) contactor was developed to be a more efficient process in terms of osmotic dehydration efficiency: time to reach certain weight reduction (T _w ) and moisture loss (T _m ) were shorter than that of conventional osmotic (COD) dehydration processes. Effectiveness evaluation functions used in this study could be widely applied to osmotic dehydration system evaluation.     

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.     

Mathematical modeling of moisture and solute diffusion in the cylindrical green bean during osmotic dehydration in salt solution

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 6 hr. 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 × 10⁻¹⁰ m²/s and 1.126 × 10⁻¹⁰-1.667 × 10⁻¹⁰ m²/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.     

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.