E34 Foams

Topic

A foam is a dispersion of gas bubbles within a liquid or a solid; in the kitchen, it appears in meringues, soufflés, mousses, beer, cappuccinos, and whipped batters. The stability of a foam depends on the interplay of three distinct degradation mechanisms acting upon it: drainage, coalescence, and Ostwald ripening. If these degradation mechanisms proceed more intensely, the foam collapses more rapidly.

Drainage occurs when gravity causes the liquid within the films separating adjacent gas bubbles to flow downward. The speed of this drainage increases sharply with the thickness of the films and decreases as the viscosity of the constituent liquid rises; consequently, a foam with thick films and low-viscosity liquid drains quickly. Egg proteins in a meringue or fat in whipped cream increase the viscosity of the liquid in these films, thereby stabilizing the foam against drainage. Conversely, thinner films or lower liquid viscosity accelerate the drainage process.

Ostwald ripening involves the diffusion of gas from a small bubble (characterized by high Laplace pressure) to a larger bubble (with lower Laplace pressure) within the same foam. Over time, small bubbles disappear while large ones grow at their expense, resulting in a foam that is coarser and has a rougher texture than it did initially. Gas dissolved in beer foam continuously undergoes this same Ostwald ripening as long as the foam remains in the glass. The greater Laplace pressure difference between a small bubble and a large bubble in the foam causes this Ostwald ripening to proceed more rapidly between them.

The Gibbs-Marangoni modulus of the foam interface measures the interface's ability to resist deformation: if the interface stretches locally at any point—causing surface tension to drop at that location—the resulting surface tension gradient between the stretched point and the rest of the interface generates a flow that restores the foam film's thickness at that spot; this is the Marangoni effect. A surfactant in the foam with a higher Gibbs-Marangoni modulus causes the Marangoni effect to restore the interface more forcefully, resulting in a more stable foam.

This is why, within the framework of this model—which incorporates the three degradation mechanisms and the Marangoni effect—whipping cream at a low temperature is crucial for the stability of the resulting foam: the cream's fat, being partially solid at low temperatures, provides higher viscosity and a higher elastic modulus than at higher temperatures, thereby stabilizing the foam for hours against drainage and coalescence. Swiss meringue—made by heating egg whites and sugar together before whipping—achieves greater stability than cold-whipped French meringue because the heat denatures the egg white proteins, forming a more rigid network at the foam interface that more effectively resists the three degradation mechanisms outlined in this model.

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