Power Laws in the Elasticity and Yielding of Plant Particle Suspensions

The yielding and flow behaviour of plant suspensions are perhaps the most important rheological properties in process and product design for applications in paper, biofuel and food industries. Studies are reported here on the yield properties and flow behaviour of suspensions of plant particles with different shapes (clusters of cells, individual cells and cell fragments). Carrot and tomato were selected as model plant systems to prepare suspensions at particle dry mass concentrations ranging from 0.010 to 0.065. The flow behaviour was characterised by an apparent yield stress and shear thinning. The Herschel-Bulkley yield stress obtained from up and return flow curves was compared to the yield stress calculated from oscillatory measurements. The dependence of the yield stress values on particle dry mass concentration is approximately a power-law, with a fitted exponent of 3 ± 0.5 for all the suspensions, independently of the plant origin and particle shape. This same power-law behaviour was found for the elastic modulus G′, and in this case the exponent was 3 for carrot and 4 for the tomato suspensions. The yield strain, calculated from oscillatory measurements, decreased slightly with dry mass fraction, but did not follow a power-law. We discuss possible explanations for power law behaviour, and provide a model for G′ based on folded elastic sheets, which predicts an exponent of 3, similar to the values obtained for these suspensions.     

Polymer–Magnesium Aluminum Silicate Composite Dispersions for Improved Physical Stability of Acetaminophen Suspensions

The aims of this study were to characterize the morphology and size of flocculates and the zeta potential and rheological properties of polymer–magnesium aluminum silicate (MAS) composite dispersions and to investigate the physical properties of acetaminophen (ACT) suspensions prepared using the composite dispersions as a flocculating/suspending agent. The polymers used were sodium alginate (SA), sodium carboxymethylcellulose (SCMC), and methylcellulose (MC). The results showed that SA, SCMC, and MC could induce flocculation of MAS by a polymer-bridging mechanism, leading to the changes in the zeta potential of MAS and the flow properties of the polymer dispersions. The microscopic morphology and size of the flocculates was dependent on the molecular structure of the polymer, especially ether groups on the polymer side chain. The residual MAS from the flocculation could create a three-dimensional structure in the SA–MAS and SCMC–MAS dispersions, which brought about not only an enhancement of viscosity and thixotropic properties but also an improvement in the ACT flocculating efficiency of polymers. The use of polymer–MAS dispersions provided a higher degree of flocculation and a lower redispersibility value of ACT suspensions compared with the pure polymer dispersions. This led to a low tendency for caking of the suspensions. The SCMC–MAS dispersions provided the highest ACT flocculating efficiency, whereas the lowest ACT flocculating efficiency was found in the MC–MAS dispersions. Moreover, the added MAS did not affect ACT dissolution from the suspensions in an acidic medium. These findings suggest that the polymer–MAS dispersions show good potential for use as a flocculating/suspending agent for improving the rheological properties and physical stability of the suspensions.     

Rheological properties of agar microgel suspensions prepared using water-in-oil emulsions

Agar microgel suspensions containing sugar fatty acid esters were prepared using a water-in-oil (w/o) emulsion system. The volume fraction of microgel particles was adjusted by centrifugation to obtain high volume fraction suspensions. The relationship between the apparent equilibrium shear modulus and the volume fraction was analyzed using a hard-sphere model. The rigidity of the microgel particles never affected the rheological property under the random close-packed-volume fraction, but it affected the fracture of the structure composed of packed microgel particles. The length of fatty acid emulsifiers also affected the rheological properties of the condensed suspensions. The stress growth under step shear flow was observed to study the lubrication effect of close-packed microgel particles containing ER-190 (C 22:1) or O-170 (C 18:1). Although both suspensions showed strain softening, the tendency was different in each. Such a w/o microgel suspension is a likely candidate for use in low-fat foods.     

Design of Model Apple Cells Suspensions: Rheological Properties and Impact of the Continuous Phase

The objective of this work is twofold: to develop a relevant model system to study plant cells suspensions’ rheology and to evaluate the impact of the continuous phase composition and viscosity on the rheological behaviour of apple cells suspensions. Model suspensions of individual or clustered apple cells were developed. Rheological behaviours of both type of suspensions were observed separately, suspending from 0.145 g/100mL to 3.48 g/100mL of particles in five model media and in the original apple serum. Our results show that model suspensions successfully reproduce the rheological behaviour of apple purees, following three concentration domains. In particular, cell clusters greatly reproduce the behaviour of bimodal apple purees, suggesting that clusters dominate the rheological behaviour of the whole puree. One of our main result is that continuous phase does not affect elastic properties of suspensions in the concentrated domain since they are essentially governed by particle interactions: G’ values are similar whatever the continuous phase. If the continuous phase has the main impact on diluted suspensions’ viscosity, its effect becomes smaller as particle concentration increases. A lubricating effect was observed in the concentrated domain for continuous phases containing polymers. Presence of polymers may help in structuring the network in the intermediate domain.     

Low Shear Rheology of Concentrated Tomato Products. Effect of Particle Size and Time

Time-dependent rheological properties of three tomato paste suspensions in the concentration range of 200–1,000 g paste/kg suspension have been investigated by using the vane geometry at shear rates . Creep tests were conducted to analyze the influence of the level of stress on the rheological behavior of the samples before and after homogenization. The experimental results indicate that the suspensions exhibit an elastic behavior at long times and relatively low stresses, which proves that this type of material can be characterized by a yield stress (σ _y). Applying stresses just beyond the yield stress, an initial rheopectic behavior appeared. This increase in viscosity at low deformations was markedly larger after homogenization, and this difference was attributed to changes in the aspect ratio, shape, and orientation of the particles induced by homogenization. These structural changes were also reflected in the transient viscosity when the samples were subjected to larger stresses (σ >> σ _y): before homogenization the suspensions exhibited a steady-state viscosity at large deformations, whereas after homogenization, the transient viscosity continuously decreased. That behavior was attributed to flocculation of the particles. This work was partially presented at the “4th International Symposium on Food Rheology and Structure” in February 19–23, 2006, Zürich, Switzerland.     

Regenerative medicine bioprocessing: Concentration and behavior of adherent cell suspensions and pastes

Regenerative medicines based on human cells demand their harvesting, culture, and processing. Manufacturing processes are likely to include cell concentration and subsequent controlled dosing of concentrates, for example, to the patient or tissue construct. The integrity and functionality of the cells must be maintained during these processing stages. In this study the performance of two different cell concentration protocols (involving centrifugation and resuspension) are compared and consideration given to possible causes of cell loss. Further studies examine cell size and rheological behavior of anchorage‐dependent mammalian cell suspensions, and the effect of capillary flow stress (0.5–15 Pa, laminar flow regime) on cell number and membrane integrity as quantified by flow cytometry. The cell concentration protocols achieved maximum cell volume fraction of around 0.3 and the improved protocol exhibited intact cell yield of 80 ± 13%, demonstrating proof‐of principle for achieving tissue‐like cell concentrations by a process of centrifugation and orbital shaking. Volume mean cell diameter (cell diameter at the mean cell volume) for the rat aortic smooth muscle cells (CRL‐1444) used in this study was 22.4 µm. Concentrated cell suspension rheology approximated to power law behavior and exhibited similar trends to reports for plant and yeast cells. Capillary transfer at 2–15 Pa (wall shear stress) did not significantly affect cell number or membrane integrity while losses observed at low shear (0.5, 1.0 Pa) were probably due to surface attachment of cells in the apparatus. Biotechnol. Bioeng. 2009;103: 1236–1247. © 2009 Wiley Periodicals, Inc.     

An elasto-visco-plastic model of cell aggregates

Concentrated cell suspensions exhibit different mechanical behavior depending on the mechanical stress or deformation they undergo. They have a mixed rheological nature: cells behave elastically or viscoelastically, they can adhere to each other whereas the carrying fluid is usually Newtonian. We report here on a new elasto-visco-plastic model which is able to describe the mechanical properties of a concentrated cell suspension or aggregate. It is based on the idea that the rearrangement of adhesion bonds during the deformation of the aggregate is related to the existence of a yield stress in the macroscopic constitutive equation. We compare the predictions of this new model with five experimental tests: steady shear rate, oscillatory shearing tests, stress relaxation, elastic recovery after steady prescribed deformation, and uniaxial compression tests. All of the predictions of the model are shown to agree with these experiments.     

Rheology of concentrated suspensions of deformable elastic particles such as human erythrocytes

New models for the viscosity of concentrated suspensions of deformable elastic particles are developed using the differential effective medium approach (DEMA). The models are capable of describing the rheological behavior of un-aggregated suspensions of human red blood cells (RBCs). With the increase in shear rate, a shear-thinning behavior is predicted similar to that observed in the case of un-aggregated suspensions of RBCs. A decrease in relative viscosity and an enhancement of shear-thinning behavior is predicted when either the particle rigidity (elastic modulus) is decreased or the continuous medium viscosity is increased. These predictions are similar to those observed in suspensions of human RBCs. The proposed models are evaluated using experimental data on normal and hardened human RBC suspensions in protein-free saline.     

Rheological,Mass Transfer,and Mixing Characterization of Cellulase-Producing Trichoderma reesei Suspensions

Steady and dynamic shear measurements are utilized to characterize the rheological behavior of Trichoderma reesei RUT-C30 fungal suspensions during batch growth on xylose (soluble substrate) or cellulose (particulate solid substrate) at three different fermentor impeller speeds (250, 400, and 550 rpm). Biomass concentrations versus time were unimodal on xylose and bimodal on cellulose. This behavior is consistent with relatively rapid, early growth on easily metabolized growth medium components (yeast extract), followed by a second, slower growth phase due to hydrolysis of recalcitrant cellulose by increasing cellulase concentrations. Critical dissolved oxygen (DO) concentration for T. reesei growth on cellulose was found to be 0.073 mmol/L. The DO was kept above this level by supplementing the air feed with pure oxygen, implying that mass transfer limitations were not the cause of bimodal cell growth. Steady shear rheological data showed shear thinning behavior and a yield stress for all broth samples regardless of substrate. Casson and Herschel−Bulkley constitutive equations fit steady shear data well. Dynamic shear measurements on broth suspensions indicated “gel-like” behavior at low strains, with microstructural breakdown at larger displacements. Time variations of the Casson model parameters (yield stress and Casson viscosity) and dynamic moduli (elastic and viscous modulus) followed both cell mass and morphology: a single maximum in all rheological variables resulted when cells were grown on xylose or on cellulose at impeller speeds of 400 or 550 rpm, and dual maxima were observed for cellulose-grown cells at 250 rpm.     

Packing of human erythrocytes in a centrifugal field

The rate of packing of human erythrocytes in whole blood and washed ones in aqueous suspension was investigated in a centrifugal field of 250 g. The Voigt-Kelvin rheological model was found to be well suited to describe the packing process. The ratio of the elastic modulus to viscosity was evaluated from this model. Its value suggests that the flexibility of the cell plays a minor role compared to other viscosity factors. Also the model suggests that the rate of packing is a complicated function of various viscoelastic factors. Empirical parameters describing the rate of packing are sensitive to drastic changes in cell flexibility, such as caused by formaldehyde treatment, whereas no fluidizing effect of procaine on cell membrane was detected. The rate of packing is not affected by decreasing the pH from 7.4 to 6.5. The method of mild centrifugation could be of some use for rapid evaluation of substantial flexibility changes in washed blood cells.     

Binding of water, oil, and bile acids to dietary fibers of the cellan type

Dietary fibers (DF) of the "cellan" type (consisting mainly or exclusively of undestroyed cells) were prepared as ethanol-dried materials from apple, cabbage, sugar-beet, soybean hulls, wheat bran, and suspension cultures of Chenopodium album L. and investigated with respect to their interactions with water, water-oil dispersions, bile acids, and oil. Water binding and retention capacities were found to be especially high in cellans obtained from thin-walled raw material. Water damp sorption by dry cellans, when analyzed according to the GAB and BET equations, shows a considerable fraction of monolayer water. At a water activity of 0.98, the cell and capillary spaces outside the walls remained in the air-filled state but the cell wall pores are filled with water. When the water content of a concentrated aqueous cellan suspension was equal to or below the water binding capacity, its rheological behavior was found to be of pseudoplastic nature. At a given dry weight concentration, yield stress and viscosity of such concentrated suspensions were highest for cellans with the highest water binding capacity. Dry cellan particles absorbed fatty oils without swelling but swell in a detergent-stabilized oil/water emulsion with a similar liquid absorption capacity as in water. In contrast to the dry or alkane-saturated cell wall, the hydrated wall is not permeable to oils in the absence of a detergent. Oil droplets may be entrapped within the cells, yielding a stable dispersion of oil in water. As DF of the cellan type absorb bile acids, preferentially glycoconjugates, from diluted solutions, they may have a potential to decrease the serum cholesterol level.     

Influence of Particle Size and Concentration on Rheological Behaviour of Reconstituted Apple Purees

This work investigates the impact of structural parameters on the rheological behaviour of apple purees. Reconstructed apple purees from 0 g/100 g up to 2.32 g/100 g of insoluble solids content and varying in particle size were prepared. Three different particle size distributions were obtained by mechanical treatment only, to modify both size and morphology of the particles without modifying the intrinsic rigidity of the cell walls. Rheological measurements showed that the insoluble solids content have a first order effect on the rheological behaviour of the suspensions: three concentrations domains were observed in both dynamic and flow measurements. A model is proposed for each domain. The existence of a weak network between particles is clearly shown over a critical concentration of insoluble solids (cell walls) depending on particle size distribution (semi-diluted domain). In a concentrated domain, particles are on close packing conditions and their apparent volume begin to shrink. Particle size and shape also play an important role on the rheological behaviour of reconstructed apple puree. Due to their irregular shape, cell clusters clog the medium at lower concentration compared to individual cells.     

Effect of Oppositely Charged Polymer and Dissolution Media on Rheology of Spray-Dried Ionic Complexes

The purpose of this research was to address the utility of rheological study in understanding the influence of oppositely charged polymers on release of naproxen sodium encapsulated in chitosan particles. The interaction between oppositely charged κ-carrageenan (κ-Ca) and chitosan leads to relatively higher gel strength, which is proportional to the ability to retard the drug release at acidic pH. The oscillatory tests within the linear viscoelastic range where the stress is proportional to the applied strain were performed on the hydrated sample matrices containing chitosan-naproxen sodium spray-dried complexes and k-Ca or hydroxypropyl methylcellulose (HPMC) in various ratios. It was observed that the effect of pH change on the dynamic moduli in spray-dried complexes containing κ-Ca was much stronger than that with HPMC reflecting presence of strong ionic interaction between κ-Ca and chitosan. The combination of oppositely charged polymers in different ratios proved to be useful in modulating the rheological properties of the hydrated formulations and their release-retarding properties. Dynamic moduli can be used to measure gel strength and are significant for the interpretation of oral sustained release spray-dried complexes.     

Temperature-induced changes of the packing of double-stranded linear DNA molecules in particles of liquid-crystalline dispersions

The circular dichroism spectra of liquid-crystalline dispersions obtained by phase exclusion of linear double-stranded DNA molecules from aqueous saline solutions of polyethylene glycol (120 ≤ C_PEG ≤ 300 mg/mL) have been investigated. The formation of liquid-crystalline dispersions at polyethylene glycol concentrations ranging from 120 to 200 mg/mL was accompanied by the emergence of an abnormal negative band in the spectrum of circular dichroism; this is indicative of cholesteric packing of the double stranded DNA molecules in the particles of the dispersion. Liquid-crystalline dispersions formed at PEG concentrations higher than 220 mg/mL and room temperature did not show any abnormal bands in the circular dichroism spectra; this is indicative of hexagonal packing of double-stranded DNA molecules in the particles of the dispersions. Heating of optically inactive liquid crystal dispersions induced a transition of the dispersions into a different state accompanied by the emergence of an abnormal negative band in the spectrum of circular dichroism. This transition is considered within the concept of the transformation of a hexagonal packing of DNA molecules into a cholesteric packing. A qualitative mechanism of such a transition is proposed that is formulated in the terms of the “quasinematic” layers of double-stranded DNA molecules that change their spatial orientation under the competing influences of the osmotic pressure of the solvent, orientational elasticity of the cholesteric packing, and thermal fluctuations.     

Influence of Extraction Method on the Rheological Properties of Jackfruit (<Emphasis Type="Italic">Artocarpus heterophyllus</Emphasis>) Seed Starch Dispersions

Starches are widely used in food processing, and their rheological properties are affected by thermal conditions imposed during the process. The objective of the present study was to assess the rheological behavior of jackfruit seed starch (JSS) dispersions, with a particular interest on the effects of the starch extraction techniques using either water or an alkali solution (0.1 M sodium hydroxide) as solvents. The analyses on the starches were performed using small amplitude oscillatory shear tests, including frequency, temperature, and time sweeps, and determining flow curves at different temperatures (10–40 °C). JSS dispersions were classified as weak-intermediary gels, and those prepared with JSS extracted with water showed higher values of G’ and G” in all of the tested samples. All gelatinized samples exhibited viscoelastic behavior, and with increasing temperatures the storage and loss moduli increased until a maximum value before reaching a plateau. Gelatinization temperatures were slightly lower for dispersions prepared with JSS extracted with water. Gelatinization kinetics showed that increasing temperatures caused a reduction in the time to achieve gelatinization. The JSS dispersions displayed thixotropic behavior, which was influenced by their concentration and the extraction method employed. The flow curves exhibited pseudoplastic behavior. The Power Law model was used to describe the JSS dispersions rheological behavior, which enabled to assess the effects of starch concentration and extraction method on their rheological properties.     

Oscillatory shear stress and hydrostatic pressure modulate cell-matrix attachment proteins in cultured endothelial cells

Summary Endothelial cells (ECs) may behave as hemodynamic sensors, translating mechanical information from the blood flow into biochemical signals, which may then be transmitted to underlying smooth muscle cells. The extracellular matrix (ECM), which provides adherence and integrity for the endothelium, may serve an important signaling function in vascular diseases such as atherogenesis, which has been shown to be promoted by low and oscillating shear stresses. In this study, confluent bovine aortic ECs (BAECs) were exposed to an oscillatory shear stress or to a hydrostatic pressure of 40 mmHg for time periods of 12 to 48 h. Parallel control cultures were maintained in static condition. Although ECs exposed to hydrostatic pressure or to oscillatory flow had a polygonal morphology similar to that of control cultures, these cells possessed more numerous central stress fibers and exhibited a partial loss of peripheral bands of actin, in comparison to static cells. In EC cultures exposed to oscillatory flow or hydrostatic pressure, extracellular fibronectin (Fn) fibrils were more numerous than in static cultures. Concomitantly, a dramatic clustering ofα ₅β₁ Fn receptors and of the focal contact-associated proteins vinculin and talin occurred. Laminin (Ln) and collagen type IV formed a network of thin fibrils in static cultures, which condensed into thicker fibers when BAECs were exposed to oscillatory shear stress or hydrostatic pressure. The ECM-associated levels of Fn and Ln were found to be from 1.5-to 5-fold greater in cultures exposed to oscillatory shear stress or pressure for 12 and 48 h, than in static cultures. The changes in the organization and composition of ECM and focal contacts reported here suggest that ECs exposed to oscillatory shear stress or hydrostatic pressure may have different functional characteristics from cells in static culture, even though ECs in either environment exhibit a similar morphology.     

Effects of Pulsed Electric Field on the Viscoelastic Properties of Potato Tissue

We have investigated whether transient permeabilization caused by the application of pulsed electric field would give rise to transient changes in the potato tissue viscoelastic properties. Potato tissue was subjected to nominal field strengths (E) ranging from 30 to 500 V/cm, with a single rectangular pulse of 10⁻⁵, 10⁻⁴, or 10⁻³ s. The changes on the viscoelastic properties of potato tissue during pulsed electric fields (PEF) were monitored through small amplitude oscillatory dynamic rheological measurements. The elastic (G′) and viscous moduli (G″) were measured every 30 s after the delivery of the pulse and the loss tangent change (tan-δ) was calculated. The results were correlated with measurements of changes on electrical resistance during the delivery of the pulse. Results show a drastic increase of tan-δ in the first 30 s after the application of the pulse, followed by a decrease 1 min after pulsation. This response is strongly influenced by pulsing conditions and is independent of the total permeabilization achieved by the pulse. Our results, supported by similar measurements on osmotically dehydrated control samples, clearly show that PEF causes a rapid change of the viscoelastic properties of the tissue that could be attributed to a partial loss in turgor pressure. This would be an expected consequence of electroporation. The recovery of tan-δ to values similar to those before pulsation strongly suggests recovery of cell membrane properties and turgor, pointing at reversible permeabilization of the cells. A slight increase of stiffness traduced by a negative change of tan-δ after application of certain PEF conditions may also give an indication of events occurring on cell wall structure due to stress responses. This study set the basis for further investigations on the complex cell stress physiology involving both cell membrane functional properties and cell wall structure that would influence tissue physical properties upon PEF application.     

Elastic Networks of Protein Particles

This paper describes the formation and properties of protein particle suspensions. The protein particles were prepared by a versatile method based on quenching a phase-separating protein–polysaccharide mixture. Two proteins were selected, gelatin and whey protein. Gelatin forms aggregates by means of reversible physical bonds, and whey protein forms aggregates that can be stabilized by chemical bonds. Rheology and microscopy show that protein particles aggregate into an elastic particle gel for both proteins. Properties similar to model systems of synthetic colloidal particles were obtained using protein particle suspensions. This suggests that the behaviour of the particle suspensions is mainly governed by the mesoscopic properties of the particle networks and to a lesser extent on the molecular properties of the particles.     

Effects of cell-seeding methods of human osteoblast culture on biomechanical properties of porous bioceramic scaffold

Many studies have been performed to accelerate osteoinduction and osteoconduction into porous ceramic scaffolds by seeding them with cells. In this study, we compared available cell-seeding methods on a porous β-tricalcium phosphate (β-TCP) scaffold and evaluated the effects of cell-seeding on the mechanical properties of the porous β-TCP scaffold. Three types of porous bioceramic scaffolds were used: dry scaffold, scaffold wetted with media, and scaffold cultivated with normal human osteoblasts (NHOs). Cell-seeding into the porous β-TCP scaffolds was performed by conventional, centrifuge, high-density, and vacuum methods. After confirming cell proliferation with MTT assay and cell staining, a compressive test was performed after 2 and 4 weeks of cell culture. The vacuum method based on the high-density cell culture inserted effectively NHOs into the β-TCP scaffolds. The compressive elastic modulus of wetted β-TCP scaffolds decreased significantly (p < 0.05) about 20∼30% after 2 and 4 weeks of incubation in comparison with that of the dry scaffold. However, the compressive strength of the scaffolds cultivated with NHOs for 3 weeks was significantly (p < 0.05) higher than that of scaffolds without NHOs. The vacuum with the high-density of cell-seeding seems to be a suitable method for seeding cells into complex porous ceramic scaffolds. Cell proliferation and uniform distribution in the scaffolds can change the initial mechanical properties of porous ceramic scaffolds.     

Technological problems in cultivation of plant cells at high density

Using Cudrania tricuspidata cells as model plant cells which have high sensitivity to hydrodynamic stress, technology problems in the cultivation of the plant cells at high density were investigated. Using “shake” flasks on a reciprocal shaker and Erlenmeyer flasks on a rotary shaker and with a high supply of oxygen on order to obtain high cell densities in shaken cultures, particles breakdown and damage to the largest cell aggregate group (above 1981 μm in diameter) occurred and normal cell growth became impeded. The mass-transfer coefficient (K)for a model solid–liquid system (β-naphthol particles and water) in place of a system of plant cells and a liquid medium was proposed as an intensity index of hydrodynamic stress effects on plant cells in subsequent cultures under various conditions in the bioreactor systems. Normal cell growth was obtained under culture conditions for K values less than about 4.4 × 10^?3 cm/sec. The characteristics of various bioreactors used until now were investigated by considering the three main technological factors (capacity of oxygen supply, intensity of hydrodynamic stress effects on plant cells, and intensity of culture broth mixing and air-bubble desperation). The most suitable bioreactor for culturing plant cells at high density was ajar fermentor with a modified paddle-type impeller (J-M). The yield of cell mass in the 10-liter J-M (working volume 5 liter) was about 30 g dry weight per liter of medium.