Effect Of Protein-polysaccharide Interactions On The Stability Of Concentrated Emulsions And Its Mechanism

Food emulsions, one of the most complex multicomponents in food industry, mainly madeup of water, oil, protein, polysaccharide and small molecular emulsifier that give thecharacteristic mouthfeel, texture and flavour to the food. Emulsions are by naturethermodynamic unstable, and can exhibit flocculation, coalescence, creaming andsedimentation of particles. The stability of emulsions is likely to be one of the most importantproblems in their application.Based on the disability of emulsions during processing and storage, and the unstablebubbles after whipping, in the present study, effects of homogenization and sterilizationprocesses on the stability and quality of emulsion were firstly investigated. Then the mixingbehavior of sodium caseinate (Na-CN) with two kinds of anionic polysaccharide i.e. xanthangum (XG) and carrageenan and a neutral polysaccharide i.e. guar gum (GG) were studied; theeffects of Na-CN-polysaccharide interactions on the stability of neutral concentratedemulsions (3wt%of protein and36wt%of fat) and its mechanism were also analyzed.Finally, response surface design was used to optimize the concentration of polysaccharides.Higher homogenization pressure decreased droplet size and surface protein concentration,enhanced surface protein content and adsorption strength, and improved apparent viscosityand elastic structure. Twice homogenization promoted the re-coalescence of fat droplets.Sterilization intensity decreased in the order of autoclaving> boiling sterilization> UHTsterilization.The characteristics of Na-CN in admixture with different amounts of polysaccharides inneutral aqueous solution were investigated by hydrodynamic diameter, ζ-potential, rheologicalbehavior and interfacial tension. The increase of hydrodynamic diameter and decrease ofζ-potential indicated the formation of Na-CN–XG complex through mainly hydrophobicinteractions. Upon the addition of Newtonian GG solutions at different concentrations, theNa-CN solution which showed slight shear-thinning behavior converted more likely to beNewtonian behavior, which implied that molecular interactions between Na-CN and GG did occur in the solution system. The hydrodynamic diameter changed slightly in Na-CN andcarrageenan mixed solutions. When the concentration of polysaccharide below a certaincritical concentration (0.04wt%for XG and GG,0.02wt%for carrageenan), thecomplexation reduced the exposure of hydrophobic groups of Na-CN on one hand and builtup a high bulk viscosity on the other hand. Both of them would decrease the mobility ofNa-CN, and hence increase the interfacial tension. When above the critical concentration, thelimited thermodynamic compatibility between Na-CN and polysaccharide resulted in anenrichment of Na-CN at the interface and decreased interfacial tension.The effects of polysaccharides on the droplet size distribution, surface proteinconcentration, microstructure, rheological properties and partial coalescence of fat werestudied. The addition of polysaccharide promoted the adsorption of Na-CN. Below the criticalflocculation concentration, the addition of polysaccharide reduced both the volume and thenumber of fat droplets. The anionic polysaccharide of XG and carrageenan accelerated theaggregation of fat droplets and increased droplet size when the concentration equal or higherthan0.04wt%. Turbid or clear serum separation was found in these emulsions depletionflocculation occurred, which caused by non-adsorbed polysaccharides in continuous phase. InXG system, flocculation retarded the motion of oil droplets and reduced the oil dropletscoalescence. The flocculation structure of carrageenan system was fairly strong and could notbe disrupted easily. All emulsion showed shear-thinning behavior and the rheological curveswere well fitted with Herschel-Bulkley model.The response surface design of Box-Behnken results showed that the interactions of GGand carrageenan significantly (P <0.05) influenced the apparent viscosity of emulsion. Theinteractions of XG with GG and carrageenan had significant (P <0.01) effect on overrun.Results of polysaccharides optimization results indicated that XG was the major factorcausing depletion flocculation, and the flocs were fragile. The flocculation structure caused byhigh concentration of carrageenan was strong. Rheological measurements showed thatemulsions were shear-thinning and weak gel properties.