Viscoelastic characterization of fluid and gel like food emulsions stabilized with hydrocolloids

G. LORENZO; N. ZARITZKY; A. CALIFANO

Atenas

Congreso; 11th International Congress on Engineering and Food (ICEF 11); 2011

Justification: Food emulsions exhibit a great diversity of rheological characteristics, ranging from low-viscosity Newtonian liquids to viscoelastic and plastic materials. Hydrocolloids are usually key components in food emulsions to deal with creaming instability. There is a growing emphasis on understanding the colloidal basis of the rheology of food emulsions. Viscoelastic measurements are appropriate tools for obtaining information about the microstructure of the system related to the organization of the macromolecules in the medium. The existence of a broad distribution of relaxation times in polymeric materials can be represented by the mechanical relaxation spectrum derived from experimental values of the dynamic moduli G´ (storage modulus) and G´´(loss modulus). Objectives: The objectives of the present work were: a) to determine the viscoelastic behavior of two different low in fat oil-in-water food emulsions: a gel like and a pourable fluid type emulsions stabilized with hydrocolloids (gellan gum and xanthan-guar mixtures respectively) b) to model and predict the mechanical relaxation spectrum for both emulsions and continuous aqueous phases. Methodology: Low-in-fat emulsions (20g/100g) were analyzed in two distinct formulations, a pourable fluid dressing type emulsion and an emulsion-filled gel Oil-in-water emulsions were prepared using sunflower oil (20 wt.%) and Tween 80 (1 wt.%). Both type of emulsions were prepared using an Ultraturrax at 11500 rpm during 4 minutes. Gel type emulsions containing gellan gums were heated up to 90ºC maintaining a constant stirring, then 5mM CaCl2 was added followed by rapid cooling down to 20ºC. High acyl gellan gum (Kelcogel, CA) concentration ranged between 0.1-0.5%. Fluid emulsions containing xanthan and guar gums were formulated using a synergistic ratio 7:3, with total hydrocolloid concentration ranging between 0.5 to 2 wt%. The aqueous phases contained NaCl (2 wt.%) and acetic acid (2 wt.%). The rheological measurements were performed on the continuous phases and on the emulsions using a Controlled Stress Rheometer Haake RS600 (Thermoelectron, Germany). The effect of hydrocolloids was studied using oscillatory measurements (G´ and G´´ vs. frequency) within the linear viscoelastic range (LVR) previously determined by stress-sweeps. Results: Time-Concentration Superposition principle was applied to find the master curves that describe the mechanical spectra of the viscoelastic materials. Superposition allows to obtain a wide spectrum of nearly ten decades of frequencies in emulsions containing xanthan–guar mixtures, whereas gellan gum systems did not show a significant frequency displacement. Viscoelastic behavior of the systems was satisfactorily modeled using Baumgaertel-Schausberger-Winter (BSW) equation. This empirical model was used to predict the mechanical relaxation spectrum for both emulsions and continuous aqueous phases. Validation of the predicted spectra was carried out through creep compliance data for emulsion-filled gels and steady-state flow curves for emulsions containing xanthan–guar mixtures. Implication: With a precise knowledge of the viscoelastic behavior of a wide variety of food emulsions, ranging from fluid dressings with a much higher viscous contribution, to highly structured systems stabilized by a three-dimensional gel structure, caused by physical entanglements among polymeric chains, it is possible to control the desired rheological properties in a particular final product and its stability.