30E-13 |
Modeling viscoelastic behavior of oil-in-water emulsions stabilized with gums and starch |
J. M. QUINTANA1, A. N. Califano1, N. E. Zaritzky2, P. Partal3, and J. M. Franco3. (1) CIDCA, CONICET, Fac. Ciencias Exactas, Universidad Nacional de La Plata, 47 y 116, 1900. La Plata, 1900, Argentina, (2) CIDCA, CONICET, Fac. Ciencias Exactas, Universidad Nacional de La Plata, Dep. Ing. Qca., Fac. Ingenierķa, UNLP, (3) Dep. Ingenieria Quimica, Universidad de Huelva, Escuela Politecnica Superior, La Rabida, Palos de La Frontera, Huelva, Spain Polysaccharides are usually added to the aqueous phase of oil-in-water food emulsions to improve stability. The objective was to analyze and model the viscoelastic behavior of oil-in-water emulsions stabilized by different thickening agents. Food emulsions were prepared with commercial sunflower oil (40%oil-in-water) and stabilized with 1%Tween 60. The tested thickeners were: 1% xanthan gum (XG), 5% potato starch (PS), 5%PS + 0.5%XG, 1% guar gum (GG), and 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 vs. shear rate), oscillatory shear tests (linear viscoelasticity) and transient flow analysis were performed. Nonlinear viscoelasticity was also studied from stress relaxation curves at different shear strains. 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. The 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; those with GG correspond to a polymeric solution. The viscoelastic linear behavior was described according to the Maxwell generalized model. The discrete relaxation spectra and relaxation times were estimated from the experimental values of G“ and G““. Damping functions were calculated from nonlinear viscoelastic tests and the Soskey-Winter parameters were determined. Transient flow viscosities at different shear rates were measured and compared to the values estimated from stress relaxation measurements. Constitutive rheological models are useful to describe viscoelastic phenomena in small and large deformation conditions and to simulate operating parameters in food processing design.
Session 30E, Food Engineering: Rheology and texture
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