LINEAR AND NONLINEAR VISCOELASTIC BEHAVIOR OF OIL‐IN‐WATER EMULSIONS STABILIZED WITH POLYSACCHARIDES

Abstract

ABSTRACTThe rheological behavior and stability of oil‐in‐water emulsions stabilized by different thickening agents were analyzed. Food emulsions were prepared with commercial sunflower oil (40% w/w oil‐in‐water) and stabilized with 1% emulsifier. The tested thickeners were: (1) 1% w/w xanthan gum (XG), (2) 5% w/w potato starch (PS), (3) 5% PS + 0.5% XG, (4) 1% w/w guar gum (GG), and (5) 0.5% XG + 0.5% GG. Mean droplet size and droplet size distribution (DSD) of emulsions were determined by static light scattering. Steady flow (viscosity versus shear rate), transient flow (viscosity versus time) and oscillatory shear tests (linear viscoelasticity) were performed. The addition of thickening agents improved the stability of the emulsions, the effect was less marked in systems containing only GG. DSD was not significantly modified in emulsions containing starch or hydrocolloids. Microscopic observations showed that all the tested emulsions were flocculated due to the presence of hydrocolloids. The observed shear thinning behavior was attributed to the molecular structure of the polysaccharides and to the flocculation/deflocculation process; viscosity data were satisfactorily fitted to the Cross model. Frequency sweeps showed that emulsions with PS or XG have a weak gel structural network (G’ > G); those with GG correspond to a polymeric solution where G’ and G″ curves intersect within the range of tested frequencies. The viscoelastic linear behavior was described according to the Maxwell generalized model. The discrete relaxation spectrum and relaxation times were estimated from the experimental values of G’ and G″ for emulsions with PS, PS + XG, and XG. Nonlinear viscoelasticity was also studied from stress relaxation curves at different shear strains. The damping function was calculated and the Soskey‐Winter parameters were determined. Transient flow viscosities at different shear rates were comparable to the values estimated from stress relaxation measurements.