A Study of the Rheology of Low-in-Fat O/W Emulsions with Emphasis in Flow Behavior Modeling

G. LORENZO; N. ZARITZKY; A. CALIFANO

Napoles, Italia

Congreso; Third International Symposium on FOOD AND AGRICULTURAL PRODUCTS: PROCESSING AND INNOVATIONS; 2007

International Commission of Agricultural Engeenering, CIGR

To obtain low-in-fat O/W emulsions, it must be considered that when oil content is reduced, and droplet concentration decreases, creaminess velocity rises. Thus, hydrocolloids incorporation is a suitable alternative to stabilize emulsions droplets against creaming. However gum concentration as well as their structural features must be carefully selected in order to obtain a stable product with appropriate viscoelastic behaviour. The objective of the present work was to study the effect of composition on rheological properties and stability of low-in-fat O/W emulsions. Continuous phase was formulated using a synergistic ratio of xathan and guar gum (7:3) in an acidic aqueous solution (2% acetic acid) with 2% ClNa. Polyoxyethylene-sorbitan monooleate (Tween 80) was used as emulsifier and sunflower oil as the disperse phase. A central composite design involving oil fraction (10 – 30%) and gum content (0.5 – 2%) was used. Thus, nine formulations were tested replicating the central point three times. Dynamic Rheological measurements were performed in a Controlled Stress Rheometer (Haake, RS600). Storage (G’) and loss (G’’) moduli were obtained in the linear viscoelastic range. The steady flow behaviour (viscosity vs. shear stress) was also studied in both emulsions and aqueous dispersions. Droplet size distribution was measured by light scattering in a Malvern Mastersizer 2000 and complemented with microscopic observations. In all cases Sauter diameter values between 1.7 y 1.8 μm were obtained. Micrographs revealed that hydrocolloids caused the flocculation of oil droplets by depletion mechanism. Frequency sweeps showed that both G’ and G’’ increased with oil content. However, the viscoelastic behaviour was mainly governed by the hydrocolloid content. At 1.25% or over, a weak gel structural network was observed (G’>G’’) due to droplet flocculation. Below 1.25%, the viscoelastic behaviour corresponded to polymeric dispersion where G’ y G’’ curves intersected within the range of tested frequencies. Both aqueous dispersions and emulsions showed a shear-thinning behaviour with high viscosities at low shear stress, which was successfully modelled with Meter equation. Mooney equation, which relates relative viscosity with oil fraction, was used to obtain the shape factor and maximum packaging volume fraction. Zero shear viscosity was also predicted based on emulsion composition by response surface methodology. Visual inspections of the formulated emulsions showed that they remained stable after six months and were adequate to be used as pourable dressings.