Steady and Dynamic Shear Rheological Properties, and Stability of Non-Flocculated and Flocculated Beverage Cloud Emulsions

Abstract

Rheological properties of single-phase, and emulsions containing modified starch and gum arabic as surface active hydrocolloids, as well as xanthan and tragacanth gums as stabilizers were evaluated under steady and dynamic shear testing conditions using a control stress rheometer. Emulsions were formed by 9% and 14% gum concentrations with oil concentration maintained at 9% thus giving a 1:1 and 1.5:1 surface active agent to oil ratio, respectively. The rates of droplet coalescence and creaming, for a total of 8 emulsions, as a function storage time before and after dilution in a simulated fruit beverage were then investigated. Steady shear (flow curve) were well described by the Carreau model at shear stress ranging from 0.01 to 100 Pa. All prepared water phases indicated a zero-shear viscosity plateau followed by shear thinning behavior with flow behavior index (n) ranging from 0.51 to 0.79 for 14% starch-0.3% xanthan and 14% gum arabic-0.8% tragacanth stabilized emulsions, respectively. The water phase flow property data were well fitted by the Einstein equation and its expansions. The dynamic rheological properties of water phase and emulsions were also evaluated for G′(ω) and G″(ω) from 1 to 50 rad/s. Similar curves were obtained with varying degrees of deviations (G′ from G″) for different emulsions. Starch-xanthan emulsion and associated water phase at 1.5/1 agent to oil ratio demonstrated viscoelastic behavior (G′ ≥ G″) with lower droplet coalescence and creaming rates. On the other hand, gum arabic-xanthan emulsion at 1:1 agent to oil ratio showed the highest rate of droplet coalescence and a greater degree of creaming. It was speculated that the lower stability of gum arabic-xanthan emulsion could be related to the denaturation of proteinaceous part in the gum and loss of emulsification capacity due to lower pH and pasteurization.