How to deal with visco-elastic properties of cellular tissues during osmotic dehydration

In this work, vacuum impregnated apple discs with different isotonic solutions (sucrose and trehalose) were equilibrated during osmotic dehydration (55°Brix glucose at 40 °C). Changes in sample composition (water and soluble solid contents), weight and volume are analysed. A mathematical model is proposed to describe and quantify the outflow of water from the protoplast as well as the visco-elastic behaviour of the cell. Good correspondence between simulated and measured data of non impregnated samples and samples impregnated with isotonic solutions of sucrose or trehalose during long term osmotic dehydration is obtained. Fitted values of the cell permeability correspond well with tabulated values. Furthermore, also the obtained values of the parameters describing the mechanical properties of the cell wall and Hectian strands seem to reflect the observed structural development of these structures for the different treated samples well.     

Analysis of structure-property relationships in isolated cells during OD treatments. Effect of initial structure on the cell behaviour

Isolated apple cells were osmotically dehydrated in sucrose solutions (25%, 35% and 45% (w/w)) inside a heating/cooling stage with temperature control (30 °C), and examined by video-microscopy. The purpose of this work was to analyse the progress of cell microstructural changes by direct observation and further study of the digital images captured during the treatments. Cell features were identified and measured. Protoplast shrinkage increased as did sucrose concentration; however, the cell wall underwent more deformation at the lowest concentration assayed. High sucrose concentration led to an almost instantaneous protoplast separation since the membrane-to-wall linkers did not resist the stretching of the contracting protoplast; this response frequently resulted in membrane lysis and loss of cellular compartmentation. Cells structure was characterized and was proved to be related to the phenomenon undergone by the cell (shrinkage, complete plasmolysis, or membrane lysis). The response of the cell was found to be a function not only of the osmotic solution concentration but also of initial cell microstructural features. Results also suggested that a better preservation of membrane integrity and membrane-to-wall connections might be achieved if cells are dehydrated in increasingly concentrated OS solutions.     

ARTIFICIAL CELL STUDIES IN SIMULATED APPLE AND POTATO STARCH CELL COMPLEX DURING OSMOTIC DEHYDRATION

ABSTRACT

Visking tubing was employed as an artificial cell system to investigate the behavior of water in simulated apple and potato starch cell complex during osmotic dehydration. The study involved a factorial experimental design at three levels of temperature (32.2, 40 and 55C) and five levels of solute concentration solutions (four sucrose concentrations: 40, 50, 60 and 70%; one sodium chloride concentration: 0.5%). A time period of 0–3 h was implemented with 30‐min sampling period for every treatment. Three phases of a sharp decrease in the initial weight of the visking tubing in the first 30 min were observed for all temperatures and solute concentrations. In all the treatments, there was an incremental increase in water loss with increased temperature and sucrose and salt concentrations. Water loss was higher for simulated apple and 1% potato starch solutions compared with 15% potato starch solution. Starch in the visking tubing competed for the removal of water during osmotic dehydration. The amount of starch contributed to the differences in the rate of water loss in the visking tubing.

PRACTICAL APPLICATIONS

In order to better understand the behavior of water in simulated apple and potato starch cell complex during osmotic dehydration of plant materials, the Visking tubing was used as an artificial cell in simulating biological membranes. Simulated apple and potato starch was studied as representatives of cellulosic and starchy plant materials, respectively. Notably, there was antagonistic effect between starch and salt solutions, starch and sugar solutions similar to sugar and salt solutions as a result of differences in their molecular weights. This study is industrial applicable to achieve efficient osmotic dehydration processes for plant materials that are cellulosic or starchy in nature.     

Mass Transfer in Strawberry Tissue During Osmotic Treatment I: Microstructural Changes

ABSTRACT: Microstructural changes in strawberry tissue were quantified during osmotic treatment (OT) with sucrose solutions in concentrations of 15 to 65 (% w/w). The response-surface method was used to determine how the process variables (processing time and concentration) influence cellular shrinkage and cell destruction. With regard to the concentration of the osmotic solution, the kinetics of cell destruction followed a 1st-order model. The diffusional approach for modelling water transport in a layer of cells inside the strawberry tissue showed that water diffusivity is related to the concentration of the sucrose solution. Water diffusivity ranged from 16 ± 2 10⁻¹² to 6.2 ± 0.6 10^12.2 m²/s with sucrose solutions in concentrations between 15 and 65 (% w/w).     

Osmotic concentration of potato.

A study was conducted to determine what conditions define the equilibrium state between potato and osmosis solution for an osmosis concentration process. It was shown that at equilibrium, there is an equality of water activity and soluble solids concentration in the potato and in the osmosis solution. Rinsing the surface of the potato after osmotic concentration was shown to significantly reduce solids gain and soluble solids concentration in the potato, thus resulting in a sizeable increase in the potato water activity.
When water loss, solids gain, change of water activity and economics are considered, the optimal conditions for an equilibrium osmosis with sucrose would use a 50% solution at a solution/solids ratio of 4. Uptake of solids during sucrose-based osmosis results in 75% of the soluble solids in the equilibrated potato coming from the osmosis solution. A comparison of various osmosis solutions at a 60% total solids level shows that mixed sucrose-salt solutions give a greater decrease of water activity than the pure sucrose solution, even though the mass transport data are similar, this undoubtedly being due to the uptake of salt.
A model has been developed for calculation of osmosis mass transport data and water activity for osmotic concentration to equilibrium in sucrose solutions for the concentration range 10–70% and solutionlsolids range of 1–10. The mass transport data can be calculated with an average error < 4%. Water activity can also be predicted with good accuracy for the range of parameters normal for osmosis concentration processes. The proposed model was also able to predict osmosis mass transport data and water activity data for short, non-equilibrium osmosis times.     

Microscopic features, mechanical and thermal properties of osmotically dehydrated guavas

The effects of an osmotic dehydration process using sucrose and maltose solutions at 40 and 60 °Brix on microscopic features and some mechanical and thermal properties of guava tissue were studied. Also the addition of calcium lactate to the sugar solutions, aiming at preserving the structure of the processed fruits, was investigated. The guava texture (stress at failure) and the structure as observed by light microscopy were both evaluated, and differential scanning calorimetry (DSC) was used to verify the interaction between calcium ions and cell wall pectin in the guava tissue. The calcium content of the differently treated samples was also related to microscopic features, mechanical and thermal properties of guavas. The osmotic process using sucrose and maltose solutions caused severe structural damage to the guava tissue, and this effect was intensified at higher sugar concentrations and by the use of sucrose solutions. The addition of calcium lactate promoted maintenance of the guava structure, showing turgid cells with well-defined cellular contours, resulting in an increase in hardness and indicating bonding between the Ca²⁺ and cell wall pectin, which was confirmed by the DSC experiments.     

Analogical Cellular Structure Changes in Solid-Liquid Contacting Operations

Mass transfer characteristics are closely related to the cell structures. When cellular material comes in contact with a solution of high concentration, changes in physical properties of cellular material occurred throughout the process at a different degree depending on the process conditions and material matrix characteristics. Six possible situations, Cases I-VI, of changes in cell structure immersed in solution of high concentration are proposed. In this study, the specific influence of red beet tissue shrinkage on mass transfer in 600 g/kg sucrose solution at 25°C is discussed. The changes of cell unit and an analogical cellular structure are discussed. In the solid-liquid contacting period, the situation of volume changes of cell unit of red beet material remained in the no cell volume change (Cases I and II) condition. In most cases, the volume of extracellular space of cellular material decreased with water transfer out of tissue, then material matrix began to collapse, resulting in the changes in porosity. The porosity changes in the no cell volume change cases (Cases I and II) were considered as special conditions without the consideration of cellular material tissue shrinkage. The tissue structure shrinkage cases (Cases III-VI) were considered to occur more often when a fresh, soft cellular material tissue such as fruits and vegetables was contacted with osmotic solution of high concentration.     

Use of pectinmethylesterase and calcium in osmotic dehydration and osmodehydrofreezing of strawberries

Strawberry samples of two varieties (Camarosa and Elsanta) were dehydrated using different osmotic solutions (60% glucose, fructose, sucrose and raftilose) and subsequently frozen by rapid and high-pressure shift freezing (HPSF). The effect of pectinmethylesterase (PME) and calcium (Ca⁺⁺) added to the osmotic solutions on several compositional parameters and the textural/structural quality of dehydrated and osmodehydrofrozen-then-thawed samples was studied. Due to the presence of PME and Ca⁺⁺ in the osmotic solutions, weight reduction upon dehydration was slightly decreased, which was correlated to a small positive effect on the net uptake of sugars and depression of the initial freezing point. Except for the Camarosa samples treated with sucrose, PME and Ca⁺⁺ in osmotic sugar solutions positively affected the relative hardness of dehydrated fruits, which was ascribed to the effect of PME and Ca⁺⁺ on the cell wall strength of the tissue. No cell wall damage and tissue particle alterations were observed upon dehydration. The effect of osmotic dehydration (OD) using different sugar solutions without PME and Ca⁺⁺ on the texture and structure of frozen-then-thawed samples was limited and sometimes negative. The added PME and Ca⁺⁺ however positively influenced the volume and shape of the thawed samples, which could be related to slightly higher relative hardness values and, for the Elsanta strawberry fruits, also to the reduced (up to 81%) drip loss upon thawing. Upon freezing the dehydrated fruits, no cell wall disruption was observed. Tissue distortion caused by freezing and indicated by a decrease in particle size, convexity and roundness, was compensated by the use of PME and Ca⁺⁺ during OD.     

A study on the rehydration ability of isolated apple cells after osmotic dehydration treatments

A study on rehydration of isolated apple cells is presented. Isolated cells previously dehydrated in 35% and 25% sucrose solutions were rehydrated in 5% sucrose under the microscope with the aim of analyzing the phenomena that take place during rehydration. Cells response to rehydration was found to be more heterogeneous than their response to hypertonic treatments. Cells showed different degrees of delay in their response, which was related to differences in the formation and preservation of membrane-to-wall connections. Results confirmed that rehydration success is based on the preservation of the structures along both, dehydration and rehydration treatments. During swelling, Hechtian strands are reincorporated to the protoplast as far as they are formed and preserved during dehydration and rehydration; their absence or shortage leading to a loss of rehydration capacity or even membrane lysis. Different stages have been identified during rehydration, mass transfer being coupled with deformation–relaxation phenomena once the protoplast reaches the cell wall. Phenomenological coefficients for water transfer indicated that rehydration kinetics is faster than water transfer during dehydration.     

Changes in apple liquid phase concentration throughout equilibrium in osmotic dehydration

ABSTRACT:  Previous results on apple tissue equilibration during osmotic dehydration showed that, at very long processing times, the solute concentrations of the fruit liquid phase and the osmotic solution were the same. In the present study, changes in apple liquid phase composition throughout equilibrium in osmotic dehydration were analyzed and modeled. Results showed that, by the time osmosed samples reached the maximum weight and volume loss, solute concentration of the fruit liquid phase was higher than that of the osmotic solution. The reported overconcentration could be explained in terms of the apple structure shrinkage that occurred during the osmotic dehydration with highly concentrated osmotic solutions due to the elastic response of the food structure to the loss of water and intake of solutes. The fruit liquid phase overconcentration rate was observed to depend on the concentration of the osmotic solution, the processing temperature, the sample size, and shape of the cellular tissue.     

Cell fusion of Saccharomyces cerevisiae fragile mutants

Fragile mutants of Saccharomyces cerevisiae are defective in the structure of the cell wall and plasma membrane. The mutant cells lyse in hypotonic solutions but grow exponentially when osmotic stabilizer is included in the medium. These mutants display a general increase in the permeability of the plasma membrane. We show here that fragile yeast cells of the same mating type can fuse without protoplast formation. The frequency of cell x cell fusion is lower than that observed for protoplast x protoplast fusion and can be significantly increased if the cells of one partner are converted to protoplasts. Microscopic observations and genetic analysis demonstrate that the hybrids obtained are fusion products. The fusion between fragile cells is explained in terms of the existence of local defects on their surface where the cell wall is thinner (or even missing), thus allowing a direct contact of cells by means of their plasma membranes.     

EQUILIBRIUM DATA FOR OSMOTIC CONCENTRATION of POTATO IN NACL-WATER SOLUTION

Equilibrium data needed in modeling of osmotic dehydration are reported for unblanched diced potato in contact with aqueous solutions of sodium chloride. Diced potato, 1 cm cubes, were equilibrated with 0–12% (w/w) aqueous NaCl solutions at three temperatures (8, 21 and 35°C). the mass fraction of total solids nonsalt (w^F₃) in potato at equilibrium was modeled as a function of mass fraction of total solids in the fresh material at full turgor (w^F₃), concentration of NaCl in the external solution (w_IB) and temperature. Using w^F₃ and w_IB, and assuming that the activity of NaCl in the potato tissue and the external solution at equilibrium are equal, equilibrium densities were calculated based on the principle of volume additivity. the predicted and experimentally measured equilibrium densities were in close agreement.     

Transmembrane Mass Transfer in Carrot Protoplasts during Osmotic Treatment

Minimized experiments with Confocal Scanning Laser Microscopy were used to describe mass transfer of isolated carrot protoplasts from at the usual conditions of the Osmotic Treatments (OT). Carrot protoplasts during OT with 30, 40 and 50% sucrose solutions were monitored. The ratio of cellular volume before and after OT with 30, 40 and 50% sucrose solutions was 0.86 ± 0.12, 0.41 ± 0.04 and 0.17 ± 0.02, respectively. Trans‐membrane water flux was determined from cellular shrinkage, and the coefficient for water membrane permeability was (5.2 ± 0.9) 10^‐6 mol²/Jm²s. To describe water transport in protoplasts at transient conditions, the diffusional approach was used. The effective water diffusivity during OT with 50% sucrose solutions was in the (0.8‐1.8) 10^‐12 m²/s range.     

Evaluation of Mass Transfer Mechanisms During Osmotic Treatment of Plant Materials

ABSTRACT: The proportion of intact, damaged, and ruptured (non-intact) cells (Zp) due to osmotic stress during osmotic treatment of potato was monitored using electrophysical measurement based on electrical impedance analysis. Osmotic stress on potato cell culture made cell membranes shrink thereby damaging the cells. The proportion of the ruptured and shrunk cells within the samples increased with the increase in concentration of solute in the osmotic solution. The osmotic removal of water from thin potato slices started at a critical osmotic pressure. Once the critical osmotic pressure was exceeded, mass transfer was rapid and the cells lost substantial amounts of water due to rupture of cell membranes.     

RHEOLOGY OF APPLE AND POTATO TISSUE AS AFFECTED BY CELL TURGOR PRESSURE

Experiments were performed on two varieties of apple tissue and one variety of potato tissue, in which cell turgor pressure was varied and the concomitant mechanical properties of the tissue were tested. Turgor pressure was controlled by immersion in solutions of various mannitol concentrations, and the degree of swelling or contraction of the tissue samples was measured. Under constant-strain-rate loading, mode of failure varied among the different commodities, and underwent a transition as the turgor pressure was varied. Rate of loading also caused a transition in failure mode. The stress and strain at failure and tissue stiffness were related to mode of failure, turgor pressure, and strain rate. From the osmotic swelling data, estimates were made of the cell wall stress-strain behavior and of the probability distribution for cell-wall strength in Ida Red apple tissue.     

Osmotic dehydration kinetics of banana slices considering variable diffusivities and shrinkage

This article studied the use of diffusion models to describe variation of water quantity and sucrose quantity during osmotic dehydration of bananas cut into cylindrical slices. Bananas with radius of 1.7 cm and 18 °Brix (on average) were cut into 1.0 cm of thickness. A solution was proposed for the diffusion equation in cylindrical coordinates using the finite volume method, with fully implicit formulation. The diffusion equation was discretized assuming diffusivities and dimensions with variable values for the banana slices. Boundary conditions of the third kind have also been considered. The osmotic dehydration experiments were conducted in binary solutions (water and sucrose) under conditions of 40 and 60 °Brix and temperatures of 40 and 70°C. Mathematical modeling was proposed to describe the processes presented good results for water quantity and sucrose quantity, with good statistical indicators for all fits.     

INFLUENCE OF CELL SIZE AND CELL WALL VOLUME FRACTION ON FAILURE PROPERTIES OF POTATO AND CARROT TISSUE

This article presents the influence of cell size and cell wall volume fraction on the failure parameters of potato tuber and carrot tissue. Confocal scanning laser microscope was used for obtaining images of the cell structure of the tissues. The mean cell face area and the cell wall volume fraction obtained from the images was compared with work to failure, failure stress, failure strain and secant modulus obtained in a compression test of potato and carrot tissue at two strain rates. Bigger cells and less amount of cell wall material weakened the tissue, which was visible as a linear decrease in the parameters: work to failure, failure stress and failure strain. There were differences between potato and carrot in the secant modulus. For carrot, the secant modulus changed with microstructural parameters, whereas for potato, the secant modulus did not depend on these values. The strain rate decreases all the failure properties for potato. For carrot, only the work to failure was affected by the strain rate.     

OSMOTIC DEHYDRATION OF STRAWBERRIES IN A BATCH RECIRCULATION SYSTEM

Physical and chemical characteristics of two cultivars of strawberries during osmotic dehydration in sucrose and glucose solutions were investigated. Temperature was found to have a significant effect on the water and sugars (glucose, fructose and sucrose) exchange between strawberry and the osmotic solution. Mass transfer was found not to be significantly different between cultivars. Glucose gain was found to be higher than sucrose for the strawberries osmotically dehydrated in glucose and sucrose solutions at the same mole fraction, respectively. Sugars other than the osmotic sugar were found to decrease in concentration during the osmotic process. The combination of 63% sucrose solution with 25C process temperature for 2 h was able to remove more than 40% of moisture and load less than 0.1% of sucrose in the strawberries.