Polysaccharide Charge Density Research Articles

Effects of polysaccharide charge density on the structure and stability of carboxymethylcellulose-casein nanocomplexes at pH 4.5 prepared with and without a pH-cycle

Carboxymethylcellulose (CMC) can stabilize acidic casein dispersions but the CMC charge density effect has not been studied. The objective of the present work was to study the structure and stability of CMC-casein nanocomplexes at pH 4.5 using CMC with the degree of substitution (DS) of 0.7, 0.9, and 1.2. Dispersions were prepared at 1%w/v casein and casein:CMC mass ratios from 5:1 to 1:1 with and without a pH-cycle. The calcium chelating capacity of CMC was strengthened as the DS increased, which enhanced the partial disassociation of casein micelles and therefore improved the dispersion clarity. The lowest dispersion turbidity was observed at a casein:CMC mass ratio of 3:1 for all three kinds of CMC, and the pH-cycle, by alkalizing dispersion pH to 11.3 followed by acidifying to pH 4.5, further lowered the dispersion turbidity. Electrostatic attraction was the mechanism of complex formation at pH 4.5, and the CMC on the complex surface was critical to the stability against aggregation. Furthermore, increasing the DS of CMC resulted in smaller complexes and clearer dispersions. Particularly, the dispersions with a CMC:casein mass ratio of 3:1 after the pH-cycle had the absorbance at 600 nm of 1.25, 0.85, and 0.57, and the mean hydrodynamic diameter of 379, 392, and 188 nm at the DS of 0.7, 0.9, and 1.2 respectively; the increased DS also increased the stability of complexes against aggregation and precipitation at elevated ionic strengths, from 50 mM to 200 mM NaCl. The present findings may broaden the application of caseins in acidic systems.

Effect of charge density of polysaccharide on self-assembly behaviors of ovalbumin and sodium alginate

Similarities and differences of assembly for ovalbumin (OVA) and two kinds of sodium alginate (SA1 and SA2) varying in charge densities (λSA1: λSA2 ≈ 2:1) were investigated. The assembly processes of OVA/SA mixtures were characterized by phase diagram, particle size, and microstructure. Two differences between OVA/SA1 and OVA/SA2 mixtures in the phase diagram were distinctly observed. First, due to the higher charge density of SA1, the strong interaction between OVA and SA1 caused only pHφ1 to be recorded. A higher linear charge density of SA1 narrowed the pHφ1-pHφ2 range at ratios of 2:1 and 1:1. Second, OVA/SA1 complexes formed a coacervate with a relatively strong resistance to ion-induced shielding effects. This maintained the smaller size (tighter structure) with a larger number of complexes in the coacervate without 250 mM NaCl. The regulating polysaccharides with different charge densities could control the soluble region of complexes and endow various size or morphology of the coacervate assembled by proteins and polysaccharides.

Ovalbumin-carboxymethylcellulose complex coacervates stabilized high internal phase emulsions: Comparison of the effects of pH and polysaccharide charge density

Protein-polysaccharide complex coacervates self-assembled via electrostatic interactions generally exhibit superior functional properties than the individual biopolymer. Herein, the main objective of this work was to investigate physicochemical properties of complex coacervates formed with ovalbumin (OVA) and carboxymethylcellulose (CMC) with two different charge densities (CMC0.7, CMC1.2) at pH 3.0, 3.5, and 4.0 and the properties of coacervates stabilizing emulsions with a high oil content. The results showed that the contents of protein and polysaccharide in OVA/CMC complex coacevates decreased with the increase of pH, and the microscopic network structure of the complex coacervates changed from compact to loose, which was mainly due to the weakening of the electrostatic attraction between OVA and CMC. With the higher charge density of CMC1.2, the OVA/CMC1.2 complex coacervates exhibited a denser microscopic network structure and higher storage and loss moduli and complex viscosity than OVA/CMC0.7. The stronger electrostatic attraction between OVA and CMC led to poorer oil binding ability and emulsion stability when coacervates were used to prepare emulsions with 80% v/v oil, particularly evident for OVA/CMC1.2 coacervates. With moderate electrostatic attraction at pH 4.0, OVA/CMC coacervates exhibited an outstanding ability to form and stabilize the emulsions. These findings indicate that OVA/CMC complex coacervates can be used as a new option for food texture manipulation and as a stabilizer for high oil phase food systems.

Complex coacervation of ovalbumin-carboxymethylcellulose assessed by isothermal titration calorimeter and rheology: Effect of ionic strength and charge density of polysaccharide

The objective of this study was to investigate the complex coacervation between ovalbumin (OVA) and carboxymethylcellulose (CMC) with different degrees of substitution (CMC 0.7 and CMC 1.2). Turbidity titration showed that complexes or coacervates could form between OVA and CMC by electrostatic interactions depending on pH changes. Specially, the critical pH values (pHc, pHφ1) increased with the salt ion concentration raising from 0 to 20 mM, while further increasing the ionic strength reduced the formation of complex coacervates. The ITC results also showed that when CNaCl = 20 mM, the complex coacervation between OVA and CMC exhibited the highest stoichiometric ratio and binding constant. When CNaCl≥200 mM, the complex coacervation changed from spontaneous exothermic (ΔG<0, ΔH<0) to endothermic (ΔG<0, ΔH>0) due to the shielding effect by high concentration of salt ions. In addition, dynamic rheological properties showed that the OVA/CMC coacervates displayed the strongest elastic modulus at CNaCl = 20 mM, and the elastic modulus of all samples were greater than the viscous modulus. These findings all suggested a salt-enhanced effect at lower salt concentrations or a salt-reduced effect at higher salt concentrations. On the other hand, the stoichiometric and viscoelastic properties of OVA/CMC 1.2 coacervates were greater than OVA/CMC 0.7 coacervates, indicating that CMC 1.2 had stronger protein binding capacity owing to higher charge density.

Utilizing whey protein isolate and polysaccharide complexes to stabilize aerated dairy gels

Heated soluble complexes of whey protein isolate (WPI) with polysaccharides may be used to modify the properties of aerated dairy gels, which could be formulated into novel-textured high-protein desserts. The objective of this study was to determine the effect of polysaccharide charge density and concentration within a WPI-polysaccharide complex on the physical properties of aerated gels. Three polysaccharides having different degrees of charge density were chosen: low-methoxyl pectin, high-methoxyl type D pectin, and guar gum. Heated complexes were prepared by heating the mixed dispersions (8% protein, 0 to 1% polysaccharide) at pH 7. To form aerated gels, 2% glucono-δ-lactone was added to the dispersions of skim milk powder and heated complex and foam was generated by whipping with a handheld frother. The foam set into a gel as the glucono-δ-lactone acidified to a final pH of 4.5. The aerated gels were evaluated for overrun, drainage, gel strength, and viscoelastic properties. Without heated complexes, stable aerated gels could not be formed. Overrun of aerated gel decreased (up to 73%) as polysaccharide concentration increased from 0.105 to 0.315% due to increased viscosity, which limited air incorporation. A negative relationship was found between percent drainage and dispersion viscosity. However, plotting of drainage against dispersion viscosity separated by polysaccharide type revealed that drainage decreased most in samples with high-charge-density, low-methoxyl pectin followed by those with low-charge-density, high-methoxyl type D pectin. Aerated gels with guar gum (no charge) did not show improvement to stability. Rheological results showed no significant difference in gelation time among samples; therefore, stronger interactions between WPI and high-charge-density polysaccharide were likely responsible for increased stability. Stable dairy aerated gels can be created from WPI-polysaccharide complexes. High-charge-density polysaccharides, at concentrations that provide adequate viscosity, are needed to achieve stability while also maintaining dispersion overrun capabilities.

Salt Modulates Bacterial Hydrophobicity and Charge Properties Influencing Adhesion of Pseudomonas aeruginosa (PA01) in Aqueous Suspensions

The influence on cell hydrophobicity of differential extension with ionic strength of lipopolysaccharide molecules (LPS), which exist as charged and uncharged polymers at the surface of the gram-negative bacterium Pseudomonas aeruginosa (PA01), has been investigated. Attenuated total reflection infrared (ATR-IR) spectral absorptions from a single layer of cells adsorbed to ZnSe increased in intensity with increasing NaCl concentration up to 0.1 mol L(-1). Dynamic contact angle measurements (Wilhelmy plate tensiometry) made with a ZnSe plate having an adsorbed cell layer and the adherence of the cells to hexadecane suggest that PA01 cells were most hydrophobic in contact with 0.1 mol L(-1) NaCl solutions. These data indicate a charge screening induced compression of the charged LPS polymers decreasing the cell-surface approach distance and increasing the cell hydrophobicity due to the greater surface predominance of the uncharged LPS polymers. Interestingly, adsorbed cell layers in 0.3 mol L(-1) NaCl had a lower IR absorption intensity, and PA01 cells suspended in 0.3 mol L(-1) were found to be more hydrophilic, indicating that other factors influence the cell-surface approach distance and hydrophobicity. The examination of cell electrophoretic mobility variation with NaCl concentration suggests that the compression of charged polysaccharides increases the polysaccharide charge density and may also reduce the flow of liquid through the polysaccharide layer affecting the effective potential at the interface, the cell hydrophobicity, and the cell-surface approach distance.

Polysaccharide Charge Density Regulating Protein Adsorption to Air/Water Interfaces by Protein/Polysaccharide Complex Formation

Because the formation of protein/polysaccharide complexes is dominated by electrostatic interaction, polysaccharide charge density is expected to play a major role in the adsorption behavior of the complexes. In this study, pullulan (a non-charged polysaccharide) carboxylated to four different charge densities (fraction of carboxylated subunits: 0.1, 0.26, 0.51, and 0.56) was used to investigate the effect of charge density on the properties of mixed protein/polysaccharide adsorbed layers at air/water interfaces. With all pullulan samples, soluble complexes with beta-lactoglobulin could be formed at low ionic strength, pH 4.5. It was shown that the higher was the pullulan charge density, the more the increase of surface pressure in time was retarded as compared to that for pure beta-lactoglobulin. The retardation was even more pronounced for the development of the dilatational modulus. The lower dilatational modulus can be explained by the ability of the polysaccharides to prevent the formation of a compact protein layer at the air/water interface due to electrostatic repulsion. This ability of the polysaccharides to prevent "layer compactness" increases with the net negative charge of the complexes. If charge density is sufficient (> or = 0.26), polysaccharides may enhance the cohesion between complexes within the adsorbed layer. The charge density of polysaccharides is shown to be a dominant regulator of both the adsorption kinetics as well as the resulting surface rheological behavior of the mixed layers formed. These findings have significant value for the application of complex protein-polysaccharide systems.

Use of polysaccharides to control protein adsorption to the air–water interface

In order to understand foaming behaviour of mixed protein/anionic polysaccharide solutions, we investigated the effect of β-lactoglobulin/pectin interaction in the bulk on β-lactoglobulin adsorption to the air–water interface. Adsorption kinetics were evaluated by following surface pressure development in time of several pure protein solutions and of mixed protein/polysaccharide solutions using an automated drop tensiometer (ADT). It was found that complexation of proteins with polysaccharides can slow down the kinetics of surface pressure development by at least a factor 100, and greatly diminish foam formation. In contrast, a five times acceleration in the increase of surface pressure was observed in other cases. We propose a mechanism for protein adsorption from mixed protein/polysaccharide solutions. Effects of ionic strength, pH and mixing ratio on this mechanism were studied for mixtures of β-lactoglobulin and low methoxyl pectin, whereas other proteins and anionic polysaccharides were used to explore the role of protein and polysaccharide charge density and distribution. Whereas the possibilities to change system parameters like ionic strength or pH are limited in food related systems, selecting a suitable combination of protein and polysaccharide offers a broad opportunity to control protein adsorption kinetics and with that foam formation.