E73 Aeration - Biscuits

Topic

The aeration of this cake occurs in two successive stages: first, the mechanical incorporation of bubbles into the mixture during beating, and second, the expansion of those same bubbles during baking, driven by both the heating of the gas and the evaporation of water within the mixture. The final result of these two stages is a solid foam structure characterized by the specific porosity of the baked cake.

During beating, the rate at which bubbles are incorporated into the mixture depends on both the agitation speed and the concentration of available protein to stabilize the bubbles; bubble coalescence—a process that also plays a role in this system—constantly competes with the incorporation of new bubbles. Consequently, gentle yet prolonged beating produces smaller, more uniform bubbles, whereas vigorous beating may incorporate excess air in the form of large, unstable bubbles. Vigorous beating allows bubble coalescence to compete more effectively against the incorporation of new bubbles.

Inside the oven, each bubble expands according to the ideal gas law: the gas within each bubble increases significantly in volume solely due to the rise in temperature experienced as the bubble heats up. Additionally, the vapor generated by the free water in the mixture contributes to this expansion, accounting for a significant portion of the baked cake's final volume. The fact that this mixture contains more available free water means that the steam generated inside the oven contributes more to the final expansion of each bubble.

The final porosity of the cake ultimately determines its lightness: the Gibson-Ashby model shows that the cake's elastic modulus—the same modulus defined by another model in this set—scales sharply with the ratio between the density of the aerated cake and the density the mixture would have if unaerated. Consequently, a more porous cake with lower density is tenderer (having a lower elastic modulus) but also more fragile. A more porous cake with lower density has a lower elastic modulus and is, as a result, more fragile to handle.

For this reason, the model—based on the final porosity and elastic modulus established at the outset—indicates that the effective thermal conductivity of the aerated cake (also defined by another model in this set) is significantly lower than that of the unaerated mixture, thereby slowing the cooking of the cake's interior; this explains why a denser cake with lower porosity cooks faster than a highly aerated cake of the same size. It is the cake's effective thermal diffusivity—defined by that same model—that ultimately determines the time required for heat to reach the center during baking. A more porous cake, with lower effective thermal conductivity, consequently has lower effective thermal diffusivity, meaning it takes longer for heat to reach the center.

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