Bach T. Nguyen

Stabilization of Water-in-Water Emulsions by Addition of Protein Particles

The effect of the addition of protein particles was investigated on the stability of water-in-water emulsions formed by mixing aqueous dextran and poly (ethylene oxide) solutions. Protein particles with hydrodynamic radii ranging from 15 to 320 nm were produced by heating globular proteins in controlled conditions. The structure of the emulsions was visualized with confocal laser scanning microscopy using different fluorescent probes to label the dextran phase and the protein particles. It is shown that contrary to native proteins, protein particles adsorb at the interface and can form a monolayer that inhibits fusion of emulsion droplets. In this way, water-in-water emulsions could be stabilized for a period of weeks. The effect of the polymer composition and the protein particle size and concentration was investigated.

pH-Responsive Water-in-Water Pickering Emulsions

The structure and stability of water-in-water emulsions was investigated in the presence of spherical, pH-sensitive microgels. The emulsions were formed by mixing aqueous solutions of dextran and PEO. The microgels consisted of cross-linked, synthetic polymers with a radius that steeply increased from 60 to 220 nm with increasing pH within a narrow range around 7.0. At all pH values between 5.0 and 7.5, the microgels were preferentially situated at the interface, but only in a narrow range between pH 7.0 and 7.5, the emulsions were stable for at least 1 week. The droplet size was visualized with confocal laser scanning microscopy and was found to be smallest in the stable pH range. Emulsions could be stabilized or destabilized by small changes of the pH. Addition of small amounts of salt led to a shift of the pH range where the emulsions were stable. The effects of varying the microgel concentration and the polymer composition were investigated.

Synergistic effects of mixed salt on the gelation of κ-carrageenan

The effect of the addition of calcium or sodium ions on the potassium induced gelation of κ-carrageenan (κ-car) is investigated using oscillatory shear rheology and turbidimetry. Both the gelation kinetics and the steady state shear moduli are investigated. Gelation in mixed salt solutions is compared with that in pure potassium and calcium solutions. It is shown that the elastic shear modulus increases with increasing pure KCl concentration, but decreases with increasing pure CaCl2 concentration. In mixed salts, gelation of κ-car is induced by potassium and addition of CaCl2 leads to an increase of the elastic modulus with increasing CaCl2 concentration. κ-Car gelled at low mixed salt concentrations for which it remained liquid in pure salt. At equivalent ionic strengths, the effect of adding NaCl on potassium induced gelation is much weaker. In pure KCl solutions, κ-car gels are transparent, but in pure CaCl2 they become increasingly turbid with increasing CaCl2 concentration. The turbidity of gels formed in mixed salts is intermediate.

The effect of protein aggregate morphology on phase separation in mixtures with polysaccharides.

The morphology of aggregates formed by heating the globular protein β-lactoglobulin ( β-lg) changes with the addition of a small amount of CaCl2, from small strands to larger spherical aggregates (microgels). We investigated the effect of this morphological transition on the structure of mixtures of β-lg aggregates with the polysaccharide κ-carrageenan (κ-car), using confocal laser scanning microscopy and dynamic light scattering. The change in the morphology of the β-lg aggregates strongly reduced the κ-car concentration at which the system phase separated. As a consequence a dramatic change in the structure of the mixtures occurred over a narrow range of the CaCl2 concentration. Phase separation leads to the formation of micron-sized protein rich domains that have a tendency to stick together in large flocs. There is a big difference between the protein concentrations in the two phases, but the κ-car concentration is only weakly lower in the protein rich phase. A comparison is made between mixtures prepared at room temperature, after separately heating β-lg, and heated mixtures of native β-lg and κ-car. The micro-phase separated structure of the two systems is similar, but the aggregates disperse upon dilution in the former case, while they are covalently bound within the domains in the latter case. Other, more subtle, differences were also observed. The results explain the very high sensitivity of the structure of β-lg/κ-car mixtures to calcium ions.

Structure and rheological properties of carrageenans extracted from different red algae species cultivated in Cam Ranh Bay, Vietnam

Abstractκ-Carrageenan was extracted from three subspecies of Kappaphycus alvarezii—K. alvarezii, K. striatum, and K. malesianus—and ι-carrageenan was extracted from Eucheuma denticulatum. Mild water extraction was used in order to obtain carrageenan in the native form. The yield was higher for the raw extract from K. alvarezii (42%) than for the other species (32–37%). NMR showed that the samples contained only small amounts of other carrageenan types. The structural and rheological properties were characterized after extensively dialysis against NaCl and pure water. The molar mass and the radius of gyration determined by light scattering were found to be close for all samples. The viscosity was determined as a function of the shear rate at different concentrations. The zero shear viscosity of κ-carrageenan extracted from K. alvarezii was found to be systematically larger than the other carrageenan samples. Gelation induced by adding KCl was found to occur at approximately the same gelation temperature leading to gels with approximately the same stiffness.