Sodium Caseinate-polysaccharide Interactions At The Oil-water Interface: Effect On Emulsion Stability

Protein and polysaccharide are essential functional ingredients usually coexist in food emulsions. Their functionality plays a central role in controlling the texture and stability of food colloids. Being surface-active, proteins can function as effective emulsifying agents, and also protect the droplets against flocculation by a combination of electrostatic and steric mechanisms. The long-term stability can be further enhanced by the addition of polysaccharides to control the rheology and network structure of the continuous phase, hence retarding phase separation and gravity-induced creaming. The interfacial properties of the adsorbed proteins play a crucial role in determining the stability of emulsions. With charged polysaccharides, strong attractive electrostatic complexes are commonly formed between the pKa of the polysaccharide and the pI of the protein. Thus, the mechanical stabilizing role of the added polysaccharide is complicated due to the protein-polysaccharide interactions. Especially, the protein-polysaccharide interactions induced at the interface are sufficiently strong to influence the viscoelastic properties of the adsorbed protein layer, which in turn affect the emulsion stability.Xanthan gum (XG) and carboxymethylcellulose (CMC) are two important hydrocolloids in food industry. In the present work, the interactions between sodium caseinate (CN) and XG or CMC at the oil-water interface were studied based on their interactions in aqueous solutions. The effects of polysaccharide on the adsorption behaviour and dilatational viscoelasticity of the CN films adsorbed at the oil-water interface and the relationship with emulsion stability were fully discussed.In this work, the phase behaviour and microstructures of CN in admixture with different amounts of XG or CMC in aqueous solution was investigated by visual observation and confocal laser scanning microscopy (CLSM). The results showed that the segregative phase separation was absent in the protein-polysaccharide mixed systems while the concentrations of biopolymers were low (1 wt% CN, XG or CMC≤0.5 wt%). At acidic pH, the interactions between CN and XG were improved by the addition of NaCl, and the solutions appeared cloudy but sponge-like precipitate was absent by visual observation beyond 150 mM NaCl. Moreover, the region to form a stable dispersion system increased gradually with NaCl concentration, and the system remained stable at 500 mM NaCl. For CN-CMC mixed system. stable solution can be obtained at 0-150 mM NaCl, but serious phase separation occured beyond 150 mM NaCl.To understand the nature of protein-polysaccharide interactions, as well as the characteristics of the protein-polysaccharide complex, the hydrodynamic diameter (DH),ζ-potential, and rheological properties of the new formed complex were investigated. The formation of CN-XG complex through mainly hydrophobic interactions had an effect on the rheological properties of the mixed solution. Then due to the increasing electrostatic interactions between CN and XG, more CN molecules bind to XG molecules and led to a formation of micron-size complex (～2μm) at pH4, with tolerance to high salinity. On the other hand, the electrosorption of CMC onto CN occurred at pH below 5.5. Complex with DH less than 1μm was formed at pH4, but the complex was dissociated as a result of screening of charges beyond 150 mM NaCl.The effects of polysaccharide on the adsorption behavior and dilatational properties of protein adsorbed film at the oil-water interface were studied using dynamic drop shape analysis combined with oscillating drop technique. The results showed that the final values ofπere slightly lower for CN-XG mixtures compared to those of CN adsorbed films due to protein-plysaccharide interactions at pH 5-7. Moreover, the presence of XG led to a decrease in dilatational modulus (E), dilatational elasticity (Ed), and dilatational viscosity (Ev). The addition of CMC had no obvious effect on the adsorption behavior and dilatational properties of CN adsorbed film at the oil-water interface at pH 7. However, the abilities to decrease interfacial tension and resist mechanical deformation were improved due to the formation of protein-polysaccharide through electrostatic attractions.The effect of polysaccharide on the particle size distribution,ζ-potential, microstructure, creaming stability, and surface protein concentration and composition of the oil-in-water emulsions coated by CN was studied. The result showed that XG and CMC cannot be adsorbed onto the droplet interface at pH 6 and 7. Obvious creaming was found to occur in these emulsions due to depletion flocculation of the droplets caused by the unadsorbed polysaccharides. After storage for a few days, these emulsions were separated into an opaque droplet-rich phase (cream layer) at the top and a transparent droplet-free phase (serum layer) at the bottom. On the other hand, XG and CMC adsorbed onto the CN-coated droplets at pH 4, 5 which can protect the oil droplets against flocculation and coalescence upon acidification. This stability was mainly assigned to the thickening of the interfacial layer, accompanied by an increase of the steric repulsions, as well as a decrease of the van-der-Waals attraction between droplets. Moreover, the repulsion flocculation between proteins and anionic polysaccharides was avoided.