Biomedical subjects

R Tuinier

Publications and source records attributed to R Tuinier.

14 recordsLinked to original sources

PubMed2003

Depletion-induced phase separation in colloid-polymer mixtures.

Phase separation can be induced in a colloidal dispersion by adding non-adsorbing polymers. Depletion of polymer around the colloidal particles induces an effective attraction, leading to demixing at sufficient polymer concentration. This communication reviews theoretical and experimental work carried out on the polymer-mediated attraction between spherical colloids and the resulting phase separation of the polymer-colloid mixture. Theoretical studies have mainly focused on the limits where polymers are small or large as compared to the colloidal size. Recently, however, theories are being developed that cover a wider colloid-polymer size ratio range. In practical systems, size polydispersity and polyelectrolytes (instead of neutral polymers) and/or charges on the colloidal surfaces play a role in polymer-colloid mixtures. The limited amount of theoretical work performed on this is also discussed. Finally, an overview is given on experimental investigations with respect to phase behavior and results obtained with techniques enabling measurement of the depletion-induced interaction potential, the structure factor, the depletion layer thickness and the interfacial tension between the demixed phases of a colloid-polymer mixture.

Journal Article↗

PubMed2003

Segment-sphere size ratio influence on the stability of a polymer-colloid mixture.

The influence of the sphere to segment size ratio on the effective depletion layer thickness around a sphere and the resulting phase behaviour of a colloid-polymer mixture is considered. An analytical expression for the excluded volume between an ideal freely jointed chain and a sphere [L. Lue, J. Colloid Interface Sci. 202, 558 (1998)] allows the effect of polymer characteristics (chain length, segment/sphere size ratio) on the depletion interaction in polymer-colloid mixtures to be calculated. For both small and intermediate polymer-to-colloid size ratios the respectively gas-solid and gas-liquid coexistence curves shift to larger polymer coil volume fractions as the relative segment size increases. It is further demonstrated that the conditions for optimal (protein) crystallization are affected when the segment to sphere size is altered.

Journal Article↗

PubMed2002

Interaction potential between two spheres mediated by excluded volume polymers.

The interaction between two spheres in a solution of nonadsorbing polymers, with excluded volume interaction, is calculated from the depletion layers around the spheres using the generalized Gibbs adsorption equation. By combining the bulk correlation length with the curvature-dependent interfacial tension between a sphere and the surrounding polymer solution [Hanke, Eisenriegler, and Dietrich, Phys. Rev. E 59, 6853 (1999)], the depletion layer thickness around a sphere is obtained. The resulting contact potential agrees with a scaling prediction of de Gennes in the semidilute regime.

Journal Article↗

PubMed2001

Effects of structural modifications on some physical characteristics of exopolysaccharides from Lactococcus lactis.

The relationship between the primary structure and the chain stiffness of exopolysaccharides (EPSs) and modified EPSs produced by two strains of Lactococcus lactis subsp. cremoris was investigated. The molar mass and radius of gyration of these exopolysaccharides were analyzed by multiangle static light scattering after size-exclusion chromatography. From these results and the chemical structure of the repeating units of the investigated EPSs, the Kuhn lengths could be calculated. We found that the initial Kuhn lengths of the two native EPSs are similar. Modification of the EPSs by removing parts of the side groups resulted in a decrease in both the absolute value and the normalized value of the Kuhn length. It is therefore concluded that partial removal of the side groups of these polysaccharides could make them less efficient as thickeners if no specific interaction with other components occurs.

Carbohydrate Conformation↗

PubMed2000

Viscoelastic properties of an exocellular polysaccharide produced by a Lactococcus lactis.

The viscoelastic properties of a well-characterized exocellular polysaccharide (EPS) produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain B40 were investigated. Dynamic rheological measurements were made as a function of frequency and EPS concentration. The bead-spring model of Rouse could reasonably describe the dynamic properties. Concentrated EPS solutions have a significant elasticity (G' > G") at high frequencies. The relatively high G' values at high concentrations and high frequencies are indicative of significant normal stress differences, and we put forward a hypothesis that suggests that these normal stresses may explain the contribution of EPSs to the ropy behavior of yogurts.

Algorithms↗

PubMed2000

Influence of different substrate limitations on the yield, composition and molecular mass of exopolysaccharides produced by Lactococcus lactis subsp. cremoris in continuous cultures.

The type of substrate limitation had a remarkable influence on the molecular mass of exopolysaccharides (EPS) produced by Lactococcus lactis subsp. cremoris NIZO B40 and NIZO B891. Under glucose/energy limitation, the molecular mass was much smaller than under leucine or phosphate limitation, resulting in a marked decrease of the intrinsic viscosity of this EPS. The sugar composition of EPS produced by both strains, and the phosphate content of EPS produced by NIZO B40, were not affected by the type of nutrient limitation. Both strains produced comparable amounts of EPS under leucine and glucose limitation, but the efficiency of EPS production was highest under glucose limitation. The EPS yields of the phosphorylated B40 EPS as well as the unphosphorylated B891 EPS were reduced, with about 40% under conditions of phosphate limitation.

Carbohydrate Sequence↗

PubMed1999

Phase Separation, Creaming, and Network Formation of Oil-in-Water Emulsions Induced by an Exocellular Polysaccharide.

We have investigated the effect of an exocellular polysaccharide (EPS) on the phase behavior and the rheology of oil-in-water emulsions. Already at low EPS concentrations the phase separation occurs. The phase line can be described by depletion interaction theory. At high EPS concentrations and dispersed phase volume fractions above 10% there is a stable "gel"-like region in the phase diagram. A kinetic study showed that the rate of creaming decreases with increasing oil content due to hydrodynamic effects. This rate depends strongly on the concentration of EPS, which is related to the strength of the depletion interaction and the viscosity of the continuous phase. At low EPS concentration the creaming rate strongly increases with EPS concentration because of the stronger attraction. At higher EPS concentrations creaming is slowed down by the viscosity increase of the continuous phase and of the particle network which is formed. At high EPS concentrations this network becomes so strong that the gel prevents creaming. The rheological behavior of the "gel" was studied by measuring flow curves, which could be interpreted by a theoretical model for weakly aggregating particles. Copyright 1999 Academic Press.

Journal Article↗

PubMed1999

Isolation and physical characterization of an exocellular polysaccharide.

The physical properties of a polysaccharide produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain NIZO B40 were investigated. Separation of the polysaccharide from most low molar mass compounds in the culture broth was performed by filtration processes. Residual proteins and peptides were removed by washing with a mixture of formic acid, ethanol, and water. Gel permeation chromatography (GPC) was used to size fractionate the polysaccharide. Fractions were analyzed by multiangle static light scattering in aqueous 0.10 M NaNO3 solutions from which a number- (Mn) and weight-averaged (Mw) molar mass of (1.47 +/- 0.06).10(3) and (1.62 +/- 0.07).10(3) kg/mol, respectively, were calculated so that Mw/Mn approximately 1.13. The number-averaged radius of gyration was found to be 86 +/- 2 nm. From dynamic light scattering an apparent z-averaged diffusion coefficient was obtained. Upon correcting for the contributions from intramolecular modes by extrapolating to zero wave vector a hydrodynamic radius of 86 +/- 4 nm was calculated. Theoretical models for random coil polymers show that this z-averaged hydrodynamic radius is consistent with the z-averaged radius of gyration, 97 +/- 3 nm, as found with GPC.

Carbohydrate Sequence↗

PubMed1999

Concentration and shear-rate dependence of the viscosity of an exocellular polysaccharide

The viscosity of an exocellular polysaccharide (EPS) produced by the bacterium Lactococcus lactis subsp. cremoris B40 was studied in aqueous solution at an ionic strength of 0.10M. First, the zero-shear viscosity was determined as a function of the concentration. From the data in the low concentration range, the intrinsic viscosity was determined. In addition, the shear-thinning behavior was measured at several concentrations. By combining existing theories, a new equation is proposed that describes and predicts the intrinsic viscosity and the concentration dependence of the (zero-shear) viscosity of B40 EPS solutions from the molar mass and the hydrodynamic radius of the polysaccharide. Based on the Rouse theory, the shear-rate dependence of the viscosity also could be described and predicted from the molecular characteristics, i.e., molar mass and radius of gyration. It is shown that these equations can be applied to all random coil polysaccharides. Copyright 1999 John Wiley & Sons, Inc.

Journal Article↗

PubMed1999

Depletion interaction of casein micelles and an exocellular polysaccharide.

Casein micelles become mutually attractive when an exocellular polysaccharide produced by Lactococcus lactis subsp. cremoris NIZO B40 (hereafter called EPS) is added to skim milk. The attraction can be explained as a depletion interaction between the casein micelles induced by the nonadsorbing EPS. We used three scattering techniques (small-angle neutron scattering, turbidity measurements, and dynamic light scattering) to measure the attraction. In order to connect the theory of depletion interaction with experiment, we calculated structure factors of hard spheres interacting by a depletion pair potential. Theoretical predictions and all the experiments showed that casein micelles became more attractive upon increasing the EPS concentration.

Caseins↗

PubMed1998

Effective Viscosity of Polymer Solutions: Relation to the Determination of the Depletion Thickness and Thickness of the Adsorbed Layer of Cellulose Derivatives.

The diffusion of silica particles with radii ranging from 12 to 510 nm in dilute solutions of carboxymethyl cellulose (Mw = 180 to 1200 kg mol-1, cCMC = 5 to 1000 mg l-1) was investigated by means of dynamic light scattering at pH 5 in 0.01 mol l-1 NaCl. The viscosity of the polymer solution as experienced by the silica probes (the "microscopic" or effective viscosity, etaeff) differs from the viscosity as determined by capillary viscometry (etap). For small particles etaeff nearly equals the viscosity of the solvent (eta0). The effective viscosity increases with the size of the probe particles and the polymer concentration but remains less than etap. The effective viscosity is interpreted in terms of a model in which the particle is surrounded by a layer of polymer free solution (eta = eta0). The thickness of the polymer-free layer is assumed to be equal to the thickness of the depletion layer (lambdad). Applying this model, a decrease in lambdad as a function of CMC concentration is observed. At low concentration lambdad equals the radius of gyration. The hydrodynamic layer thickness (deltah) of cellulose derivatives (carboxymethyl cellulose and hydroxyethyl cellulose) adsorbed on inorganic oxide surfaces (alpha-Fe2O3 and SiO2) is also investigated by dynamic light scattering. Upon using etap maxima in deltah are found. However, these maxima are a consequence of an incorrect choice of the viscosity. When the viscosity is used as obtained from inert probe diffusion, no anomalies are observed. Copyright 1998 Academic Press.

Journal Article↗

PubMedDate not supplied

Electrosorption of pectin onto casein micelles.

Pectin, a polysaccharide derived from plant cells of fruit, is commonly used as stabilizer in acidified milk drinks. To gain a better understanding of the way that pectin stabilizes these drinks, we studied the adsorption and layer thickness of pectin on casein micelles in skim milk dispersions. Dynamic light scattering was used to measure the layer thickness of adsorbed pectin onto casein micelles in situ during acidification. The results indicate that the adsorption of pectin onto casein micelles is multilayered and takes place at and below pH 5.0. Renneting, i.e., cleaving-off kappa-casein from the casein micelles, did not alter the adsorption pH. It did, however, show that pectin arrests the rennet-induced flocculation of casein micelles below pH 5.0. From the findings we concluded the attachment of pectin onto casein micelles is driven by electrosorption. Adsorption measurements confirmed the multilayered nature of the adsorption of pectin onto casein micelles. Both the adsorbed amount and the layer thickness increased with decreasing pH in the relevant range 3.5-5.0. The phase behavior of a casein micelles/pectin mixture was determined and could be explained in terms of thermodynamic incompatibility being relevant above pH 5.0 and adsorption, leading to either stabilization and bridging, being relevant below pH 5.0. The results confirm that electrosorption is the driving force for the adsorption of pectin onto casein micelles.

Adsorption↗

PubMedDate not supplied

On the long-range attraction between proteins due to nonadsorbing polysaccharide.

The effect of a nonadsorbing polysaccharide (dextran) on the structure factor of a solution of lysozyme was studied using small-angle neutron scattering (SANS) experiments. By choosing the appropriate water/deuterium ratio as solvent, we made the scattering signal from dextran invisible for the SANS measurements. Dextran induces a weak long-range attraction between the lysozyme molecules. This attraction is described using a depletion interaction potential from theory for two spheres in an ideal polymer solution. Incorporation of the theory in a mean-spherical approximation shows that the wave vector below which the structure factor increases depends on the polymer size. The theoretical prediction is in fair agreement with the measured structure factor of lysozyme, as affected by nonadsorbing dextran.

Adsorption↗

PubMedDate not supplied

Exopolysaccharides produced by Lactococcus lactis: from genetic engineering to improved rheological properties?

Over the last years, important advances have been made in the study of the production of exopolysaccharides (EPS) by several lactic acid bacteria, including Lactococcus lactis. From different EPS-producing lactococcal strains the specific eps gene clusters have been characterised. They contain eps genes, which are involved in EPS repeating unit synthesis, export, polymerisation, and chain length determination. The function of the glycosyltransferase genes has been established and the availability of these genes opened the way to EPS engineering. In addition to the eps genes, biosynthesis of EPS requires a number of housekeeping genes that are involved in the metabolic pathways leading to the EPS-building blocks, the nucleotide sugars. The identification and characterisation of several of these housekeeping genes (galE, galU, rfbABCD) allows the design of metabolic engineering strategies that should lead to increased EPS production levels by L. lactis. Finally, model development has been initiated in order to predict the physicochemical consequences of the addition of a EPS to a product.

Genes, Bacterial↗