Aeration and rheology of viscoplastic bubbly liquids for buttercream frosting applications

HAYES, D; MURPHY, A; FERNANDES, RR; MEZA, BE; PERALTA, JM; ZORRILLA, SE; WILSON, DI

Buenos Aires

Congreso; WCCE11 - International World Congress of Chemical Engineering; 2023

Asociación Argentina de Ingenieros Químicos

We investigate the generation and flow behaviour of an aerated bubbly liquid based on a viscoplastic continuous phase, namely buttercream icing (frosting), using different rheometric techniques. Dispersions of bubbles in a continuous fluid phase are encountered in a great number of food applications. The incorporation of gas bubbles can create foods with a softer texture and also improve other important attributes such as spreadability, appearance, and taste whilst reducing the calories of the final product. Bubbly liquids differ from wet foams since the gas bubbles remain discrete when dispersed in a fluid continuous phase. The stability of bubbly liquids can be enhanced by adding surfactants to stabilise the dispersed phase and microstructure, or by using non-Newtonian continuous phases. Experiments were performed on buttercream icings, consisting of icing sugar mixed with unsalted butter at a 2:1 mass ratio, which were aerated using planetary mixers. Different air volume fractions were obtained by whisking the materials over time, and a simple first-order model that relates the air volume fraction to the number of revolutions of the whisking tool was shown to describe very well the evolution of the air volume fraction with time. The samples were also shown to de-aerate over a shelf-life of a few hours, suggesting that the viscoplastic nature of the continuous phase is not sufficient to promote a satisfactory level of stability to the final product. The shear yield stress of the samples was measured using a rotational stress-controlled rheometer with parallel plates and vane geometries at room temperature. This critical stress decreases linearly with the air volume fraction of the samples but did not follow the expected trends attributed to suspensions of rigid or fully deformable bubbles in viscoplastic or Newtonian continuous phases available in the literature. Flow heterogeneities, such as edge fracture and severe wall slip, prevented reliable data to be obtained past the yield point, so the flowable regime could not be obtained with these brittle yield stress fluids using conventional rheometric techniques. Squeeze flow tests under different squeeze velocities were thus performed to investigate the extensional rheology of the materials using this biaxial extensional flow configuration. Plots of biaxial viscosity as a function of the extension strain rate provided estimates of the critical yield stress in extension and were used to fit a tensorial Herschel-Bulkley fluid model. The flow parameters, namely the critical yield stress, the consistency index, and the flow exponent decreased with the air volume fraction, which is consistent with the sensorial perception that aeration leads to softer and more flowable products. We conclude by providing a comparison between the critical yield stresses measured in shear and extension.