Calcium effect on mechanical properties of model cell walls and apple tissue

Bacterial cellulose alone, with pectin added and with both pectin and xyloglucan added were produced as models for plant cell walls. The artificial cell wall materials and natural apple tissue were treated with calcium and then subjected to tests of mechanical properties; a tension test for the artificial cell wall materials and a compression test for the apple tissue. It was found that pectin and xyloglucan had a significant effect on the mechanical properties of artificial cell wall materials, making them more extensible. A high concentration of calcium increased the failure strain and decreased the failure stress of these materials. Treatment of apple tissue with calcium had similar effects on the failure stress and the secant modulus, whereas the differences of failure strain and work to failure resulted in behaviour different from that of the artificial material.     

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.     

STRUCTURAL FAILURE IN SELECTED RAW FRUITS AND VEGETABLES1

A torsion test was developed for studying the structural failure of selected raw fruits and vegetables. Apple, melon and raw potato flesh were tested at a shear strain rate of approximately 0.26s^-1 in torsion and uniaxial compression. Low strain modulus values were determined in addition to shear stresses and normal strains at failure. Results corroborated the maximum normal strain failure criterion proposed by Segerlind and Dal Fabbro (1978) for apples and suggested its application to potatoes and melons if true strains are used rather than engineering strains. The maximum shear stress theory also seemed to be a possible failure criterion for potatoes. Results comparing compressible and incompressible materials suggest that bulk strain affects the shear stress at failure. The observed failure planes supported the quantitative results for stresses at failure. Scanning electron micrographs indicated that the cellular failure occurred in the cell wall, regardless of whether it was due to tension, Varying specimen lengths or diameters had negligible effects on the uniaxial compression modulus but did affect the shear stress at failure in a manner yet to be satisfactorily explained.     

INFLUENCE OF FRICTION, SAMPLE DIMENSIONS AND DEFORMATION RATE ON THE UNIAXIAL COMPRESSION OF RAW POTATO FLESH

The influence of surface friction and lubrication on the compression behavior, at compression rates of 5–100mm min^−l, of cylindrical samples of potato flesh have been examined with an Instron Universal Testing Machine at room temperature. The samples had length/diameter ratios of 0.2–0.8. Parameters derived from compression to failure were: failure stress; failure strain; and apparent modulus of elasticity. Stress relaxation with time was studied using samples with length/diameter ratios of 0.4–0.8 following compressions of 10 and 30%. The length/diameter ratio, rate of compression, surface friction and lubrication influenced the parameters derived from both the compression and the stress relaxation tests. Relaxation times increased following sample lubrication. The effect due to surface lubrication was smaller than in previous compression and relaxation tests on Gouda cheese. This was attributed to release of fluid from the damaged cellular tissue of the potato flesh which reduced the effectiveness of the lubricant film.     

The thermomechanical properties of carrot cell wall material

The effect of temperature and moisture on the fabrication of pressed carrot cell wall specimens for Dynamic Mechanical Thermal Analysis was assessed. Results obtained from the water extractability of the material showed that more cell wall material became solubilised when moisture and temperature of the different treatments were increased. Chemical analysis revealed that this involved an increase in the water-soluble uronic acid components. Furthermore, more water-soluble neutral monosaccharides were observed, represented principally by galactose, rhamnose, arabinose and glucose. Pectic polysaccharides became more water soluble when isolated carrot cell wall was pressed at 100°C with a water content 800 g kg⁻¹ (wet weight basis). A molecular weight fraction centred at 100000 Da was observed in the severely pressed material (100°C, 800 g kg⁻¹ water) but was barely present in the mildly pressed (30°C, 500 g kg⁻¹ water) and unpressed specimens, consistent with depolymerisation and solubilisation. In contrast to the chemical modifications, the bending modulus, E′, of the pressed carrot cell wall material remained unchanged for the cell wall specimens moulded under different conditions, consistent with small changes in molecular weight. Pressed cell wall material was stiffer than pressed freeze-dried carrot which could be due to the plasticising role of the intracellular components. The stiffness of both cell wall and freeze-dried carrot specimens decreased with plasticisation by water in the range 10–500 g kg⁻¹. © 1998 Society of Chemical Industry.     

EFFECT OF MANNITOL TREATMENT ON ULTRASOUND EMISSION DURING TEXTURE PROFILE ANALYSIS OF POTATO AND APPLE TISSUE

An ultrasound acoustic emission (AE) signal was recorded during texture profile analysis (TPA) of potato and apple tissue with different texture. The acoustic sensor was in contact with the sample through solid medium. Texture of the tissue was controlled by soaking it in different mannitol solutions. Both TPA and AE parameters change with the texture of potato and apple samples. However, correlation coefficients of linear regression of acoustic parameters are higher than TPA parameters and all of them are significant at a higher confidence level. Critical stress and strain indicate micro‐cracking of the material and they increase with decreasing tissue turgor. The ratio of the critical stress to hardness allows an analysis of sound duration. The sound lasts longer when the tissue is more turgid as a result of the decrease of critical stress to hardness ratio. Counts of AE recorded in the whole TPA test decreased logarithmically with tissue osmolality. The article showed that contact AE can be used for texture evaluation of potato and apple tissue.     

EFFECT OF COLD-SET BINDERS: ALGINATES AND MICROBIAL TRANSGLUTAMINASE ON THE PHYSICAL PROPERTIES OF RESTRUCTURED SCALLOPS

Restructured scallops (Argopecten gibbys) were prepared with two cold‐set binders: 1% concentrations of alginate and microbial transglutaminase (MTG), with setting times of 2, 6, 9, 12, 18 and 24 h at 5C. Apparent modulus of elasticity, 5% secant modulus, 10% secant modulus and 20% secant modulus were used to evaluate the effect of setting times on the binding strength of restructured scallops. The binding strength as measured by the 5, 10 and 20% secant modulus was not significantly different (P > 0.05) in MTGase‐restructured scallops, but significantly different in alginate‐restructured scallops at 5 and 10% strain. Apparent modulus of elasticity showed significant differences in the binding strengths (P < 0.05) of restructured scallops prepared from both binders. The alginate gel restructured scallops achieved a binding strength of (158.7 kPa) in a 2‐h setting, while MTG‐restructured scallops reached a binding strength of (336.0 kPa) at a 24 h setting.     

STRESS-STRAIN AND FAILURE CHARACTERISTICS OF POTATO TISSUE UNDER CYCLIC LOADING

Cyclic compressive loadings between zero and a fixed peak stress were applied to cylindrical samples of potato flesh. The tissue can eventually fail under stress magnitudes insufficient to cause failure initially. Failure was apparently due to shear rupture, as under ordinary constant-strain-rate loading to failure. An empirical model for the probability distribution for the number of cycles to failure indicates that the likelihood of failure on successive load cycles remains approximately constant with the number of cycles applied. Thus, failure is apparently a random process and not an accumulative one. Normal strain at failure is significantly greater under cyclic loading. It is theorized that failure is due to a random degradation in the shear strength of the tissue. Stress-strain characteristics under cyclic loading are marked by an accumulation of plastic deformation in the first few load cycles, loss of firmness and a hysteresis between loading and unloading. An earlier model for the parenchyma cell is extended to include plasticity in the cell wall in the initial distension. This plasticity is predicted to contribute to the apparent tissue plasticity and loss of turgor pressure in the first load cycle.     

RELATIONSHIPS BETWEEN PRIMARY PLANT CELL WALL ARCHITECTURE AND MECHANICAL PROPERTIES FOR ONION BULB SCALE EPIDERMAL CELLS

This article investigates onion epidermal tissue (Allium cepa) using a combination of mechanical testing, microscopy and modeling and relates tissue mechanical properties to the known structure of the cell walls. Onion epidermal tissue has a simple, regular structure of elongated cells, which have been used to enable the contributions to mechanical properties of cell walls and of higher order structures to be separated and analyzed. Two models of wall behavior were used to explore how Poisson's ratio of cell walls parallel to the plane of the epidermal surface may vary with applied strain. In the first model, cellulose microfibrils can be reorientated in an unrestricted way with the result that the cell wall volume decreases. In the second model the volume of the cell wall remains constant, which controls the reorientation of microfibrils, hence the Poisson's ratio. Measurements made from uniaxially stretched cells show that the data most closely fits model I, therefore, it is concluded that the bulk of the matrix has little influence on the observed mechanical properties (at a test rate of 1 mm/min), allowing cellulose microfibrils to reorient through the matrix in an unrestricted way during uniaxial tests. In its mechanical attributes the primary cell wall resembles more a knitted cloth than a semisolid composite material. When biaxial stretching is applied to tissue, so that there is no re‐orientation of microfibrils, the cell wall material is still able to reach surprisingly large elastic strains of up to 12.5% and no plastic deformation was recorded. Current theory suggests that cellulose microfibrils can stretch elastically by a maximum of 7%, therefore further work is required to identify mechanisms that could account for the extra elastic strain.     

THE EFFECTS OF TURGOR PRESSURE ON PUNCTURE AND VISCOELASTIC PROPERTIES OF TOMATO TISSUE

The effects of turgor pressure on puncture and viscoelastic properties of mature-green tomato pericarp were examined using tissue discs soaked in a range of osmotica (0.0–0.6 M mannitol) for at least 36 h at 4C. Turgor pressure was estimated from the osmotic potential of soaking solutions that induced incipient plasmolysis. Based on volume changes, the osmotic potential and turgor pressure of fresh tissue were estimated to be −0.56 ± 0.08 MPa and 0.20 MPa, respectively. However, puncture and viscoelastic properties corresponded to a turgor pressure of 0.15 MPa. The discrepancy between calculated and actual turgor pressures was attributed to the presence of apoplastic solutes. The data from this study revealed a general increase in cell wall stress, strain and elasticity with increasing turgor. With increases in turgor above that of untreated tissue both wall extensibility and elasticity became limiting and thus cell wall stiffness increased. Conversely, a decrease in turgor below that of untreated tissue led to an increase in viscoelasticity. Increases in bioyield and pseudoplastic bioyield strains with a variation in turgor from that of untreated tissue were consistent with cell debonding as a dominant mechanism of tomato tissue bioyielding. The reduced failure force, deformation and firmness with increasing turgor were consistent with cell rupture as a predominant mechanism of failure of mature-green tomato pericarp tissue.     

MODELLING THE MECHANICAL AND HISTOLOGICAL PROPERTIES OF CARROT TISSUE DURING COOKING IN RELATION TO TEXTURE AND CELL WALL CHANGES

A tensile test was used to measure four mechanical properties of carrot tissue cooked under various time-temperature conditions. A kinetic model describing the changes of these mechanical properties measured during cooking was developed. The histological properties of the rupture surfaces caused by the mechanical testing were investigated. The kinetic model was found capable of predicting the changes in the rupture mechanism of the cell walls. Determining the percentage of cell wall ruptures proved to be an accurate method to assess the textural state of carrot tissue during cooking as compared to the measurement of the mechanical properties.     

DEVELOPMENT OF DYNAMIC MODULUS AND CELL OPENING OF DOUGH DURING BAKING

The dynamic shear modulus (elastic and viscous modulus) development of dough during baking was studied. Flooded parallel plate geometry was used to monitor the rheological changes of commercially available canned doughs (bread dough, bun dough and biscuit dough). The normal force exerted on the upper plate by the expanding dough was measured to study the cell‐opening event. The dough‐baking process was simulated in a rheometer oven. The morphology of baked dough was studied using a scanning electron microscope to elucidate the effect of ingredients and process parameters on the properties of the final baked product. Three stages of modulus development were observed during the baking process: bubble growth and packing, rapid expansion/starch gelatinization and final curing. The cell opening coincided with the sudden rise in modulus caused by starch gelatinization. The rate at which starch gelatinization takes place controls the temperature of the cell opening. The type and concentration of various ingredients have a greater effect on the modulus and on the cell opening than the heating rates. Frequency dependence was observed during baking, but the effect on modulus development diminished at higher frequencies.     

The effect of osmotic dehydrofreezing on the role of the cell membrane in carrot texture softening after freeze-thawing

To understand the protective mechanism of the osmotic dehydrofreezing technique on carrot texture after freeze-thawing, two mechanical texture parameters, fracture stress related to the cell wall and initial modulus related to the cell membrane, as well as cell membrane water permeability using PFG-NMR were evaluated. In particular, to understand the role of the cell membrane in texture alteration, tissue in which the cell membrane was exposed to chloroform vapor was used. Although dehydrofreezing protected texture from freezing damage, the effect was only observed with respect to fracture stress, with exhibited values close to those for raw tissue. However, there was no protective effect on initial modulus and water permeability, in which values did not differ from those of cell membrane-free tissue. More specifically, osmotic dehydrofreezing had no effect on the cell membrane induced by freeze-thawing.     

BETWEEN-SPECIES DIFFERENCES IN FRACTURABILITY LOSS: COMPARISON OF THE THERMAL BEHAVIOR OF PECTIC AND CELL WALL SUBSTANCES IN POTATO AND CHINESE WATERCHESTNUT1

This study was undertaken to determine how the mode of chemical changes taking place during the cooking of two chemically similar vegetables (potato and waterchestnut) might explain the tremendous between-species differences in cooked tissue fractura bility as determined by Instron Texture Profile Analysis. Although pectin breakdown and depolymerization by heat weakens the cell wall structure, loss of cell wall physical strength does not necessarily coincide with pectin depolymerization. Potato phosphate-soluble pectin (PSP) showed a more rapid rate of depolymerization than waterchestnut PSP as proven by both chemical and gel chromatographic studies. The unique composition (e.g., neutral sugar content) of the cell wall microstructure of a given vegetable species is the major determinant of the resistance of that plant tissue to fracturability loss during cooking. Based on GLC analyses of cell wall components, cell wall models have been proposed to illustrate the possible structural differences between potato and waterchestnut tissue. It appears that the cell wall character of edible plant tissue for a given species is determined genetically.     

Rheological characterization of fresh and cooked potato tissues (cv.?Monalisa)

 The rheological properties of fresh and cooked (15 min in boiling water) potato tissue which had been deformed to a lessor or greater extent by uniaxial compression, shear, uniaxial tension, successive cycles of stress relaxation, creep compliance and texture profile analysis were evaluated. Structural failure of fresh tissue under compression always occurred along a single plane of maximum shear stress, while fresh tensile specimens failed under tension. Tensile tests proved better methods by which to determine the failure parameters of cooked specimens since it was possible to observe two differents modes of failure with this technique. Equivalent modes of failure (causing damage to the cell wall or cell separation) and changes in structural components caused by cooking proved easier to idetify with tensile tests. The six element Burger's model incorporating two discrete Voigt-Kelvin units was the most suitable for defining tissue creep behaviour. The instantaneous elastic modulus could be related to the internal cell pressure, and gelatinization of starch and viscoelastic units appeared to reflect the viscoelastic properties of pectic sustances and hemicelluloses, respectively. Received: 1 December 1997     

BOOK REVIEW

Ratings by a trained texture profile panel were compared with the mechanical properties (shear stress at structural failure, normal strain to failure and apparent elastic modulus) determined in torsion and uniaxial compression for apple, melon and Irish potato flesh. Results indicated that, for melon and potato flesh, the structural failure stress levels were more frequent and significant indicators of sensory note magnitudes than the modulus values. The reverse was true for apple flesh. Of the sensory notes, moisture release correlated most consistently with the mechanical properties. Melon flesh failed in uniaxial compression by widespread crushing rather than along a failure plane. Correlation of the mechanical failure stress with the first bite sensory notes dominated in this case. The structural failure of potato flesh in uniaxial compression generally occurred along a single plane of maximum shear stress which left two essentially intact pieces of specimen. First bite and mastication sensory notes for potato flesh correlated with shear stress at failure about equally well. The structural failure of apple tissue in uniaxial compression was caused by general compression failure with many fragmented pieces of tissue remaining. Apple flesh produced the most frequent correlations of both first bite and mastication texture notes with the instrumental modulus and failure values.     

EFFECTS OF STORAGE TIME AND STATIC PRELOADING ON THE RHEOLOGY OF POTATO TISSUE 1

Potatoes held in cold storage were periodically sampled over a period of 7 weeks. At each sampling, tissue specimens were soaked in mannitol solutions (0-0.6 M) or statically preloaded (0-1.20 MPa) and then loaded to failure at constant strain rate. Tissue stiffness, cell turgor pressure, and cell-wall stress-strain relation were affected by storage time. Cell turgor pressure decreased during the first 14 days of testing and then increased. Cell wall stiffness increased with time. Statically preloaded tissue did not undergo structural failure during preloading, but exhibited large unrecovered strain, loss of water, a reduction in tissue stiffness, and an increase in failure stress. The effects of preloading were not significantly different at different storage times. Failure strain varied nonmonotonically with preload level, possibly as a result of interactions between loss of turgor, cell wall plasticity, and cell reorientation during preloading.     

A MEMBRANE MODEL FOR ELASTIC DEFLECTION OF INDIVIDUAL PLANT CELL WALLS

Deformation of plant cells by a fine probe can provide a measure of cell wall stiffness which is responsible for many of the mechanical properties of the plant. An example system of potato tuber parenchyma cells is used, and cell wall deformation approximated as the axisymmetric deformation of a circular membrane made from a nonlinear-elastic material with a Mooney strain energy function. A system of differential equations describing the shape of the membrane is derived and solved. The effects of turgor and initial cell wall strain on the force required to produce a given deflection and on the shape of the deformed cell are examined. Values for the elastic constants of the wall material are proposed, guided by statistical data for cell size and stiffness obtained from micropenetration tests.     

Mechanical characteristics of artificial cell walls

Artificial plant cell walls were produced from bacterial cellulose and cell wall constituents. The artificial cell walls were stored at low, medium and high relative humidity, and then subjected to micro-mechanical tests. From chemical composition and microstructure analysis it was found that, among all artificial cell wall materials produced, the most representative analogue of natural apple cell wall was based on bacterial cellulose supplemented with xyloglucan and pectin. Uniaxial tensile tests revealed that the different cell wall materials differed in their mechanical properties; increasing the humidity during storage resulted in a decrease in the value of the secant modulus. The cell wall model material obtained may be used for the simulation of the effect of external factors on the physical and chemical properties of cell walls.     

Instrumental Textural Characteristics of Restructured Carrot Cubes

Studies were carried out on the instrumental textural evaluation of restructured carrot cubes. The experiment was conducted by incorporating different levels of alginate, glucono delta lactone (GDL), and calcium salt to the carrot pulp. Investigations showed that as pulp level increased from 0 to 90%, there was a corresponding decrease in failure stress, failure strain, and deformability modulus. Instrumental textural profile analysis (TPA) parameters viz. hardness, springiness, gumminess, cohesiveness, chewiness, and resilience also showed a similar trend. Effect of formulation variables, i.e., alginate, GDL, and calcium salt on hardness (response variable) were evaluated by the application of response surface methodology. All the three ingredients showed a significant (P < 0.05) influence on hardness of carrot gel. Heat treatment of restructured carrot samples resulted in an increased hardness, cohesiveness, gumminess, and chewiness while springiness, cohesiveness, and resilience decreased. The data indicated that the shrinkage during thermal treatment may be responsible for the change in textural attributes. The authors concluded that a thermally stable restructured product with appreciable textural integrity can be obtained from carrot pulp.