Examining the time-dependent changes in the bubble structure of whole egg and egg white foams and batters using small strain shear oscillatory rheology, large strain shear flow rheology and image analysis techniques.

摘要

Foam structure is responsible for texture and mouth-feel in a variety of foods. The stability of foams during processing greatly affects the appearance and the texture of the resulting product. Angel food and sponge cake textures are ultimately determined by the air bubbles entrained into the foam (batter) and their stability over time. Gravity-driven drainage and pressure-difference-driven diffusion of the gas alter the numbers and sizes of air bubbles in the foam and directly affect foam rheology. Whole egg and egg white foams (and their corresponding batters) were prepared according to typical sponge cake and angel food cake recipes, respectively. Eight mix times ranging from 0--1065 s were used to achieve a range in densities (1225 to 235 kg m-3 ) within the four different systems. Ten consecutive frequency sweeps (0.1267--62.81 rad s-1), at a controlled stress of 0.02 Pa (a stress within the linear viscoelastic regime of all systems), were performed to monitor changes in G' and G" over time. Procedures described by Cohen-Addad et al. (1998) scaled rheological data successfully onto a single master curve. A plot of the rheological scaling factor versus time allowed characterization of the time-dependent evolution of the foam or batter. For short mix times (where foams and batters were dilute emulsions of bubbles) slopes of these plots approached +1, indicative of drainage dominating rheological changes. For long mix times a slope of -0.5 was approached, so that disproportionation was the dominant factor. Thirty consecutive shear rate ramps (1.452--145.2 s.-1) were used to monitor changes in yield stress and shear-thinning viscosity as a function of time. For whole egg foams, viscosity increased as more air was entrained with longer mix time. For egg white foams, viscosity also increased, but these foams began to exhibit a yield stress at a bubble fraction of ~0.65. Image analysis was used to monitor bubble size distribution changes over the same time period to confirm mechanisms of foam evolution. Models previously employed on industrial foams and emulsions and similar soft glassy materials were adapted and applied to the foam and batter systems. Ultimately, using the three experimental procedures and the models designed for similar systems, the role of surface-active ingredients and bubble packing density can assist in understanding foam and batter deterioration so that cake appearance and texture can be optimized.