E72 Foams - Meringue

Topic

This meringue is a foam stabilized by egg white proteins—primarily ovalbumin and ovotransferrin. Whisking the egg white incorporates air, and these proteins adsorb onto the air-water interface, partially unfolding to form a viscoelastic protein film. Whisking more vigorously results in the incorporation of more air.

The adsorption of these proteins onto the air-water interface is a kinetic process: as whisking creates new air-water interfaces by breaking up existing bubbles, the proteins must migrate from the liquid egg white to adsorb onto each newly created interface. Excessive whisking speed creates new interfaces faster than the proteins can cover them, thereby reducing the meringue's stability. Conversely, an optimal protein concentration—without dilution by added water—ensures that the amount of protein adsorbed at each air-water interface approaches the maximum possible value.

According to the Princen model, the meringue's elastic modulus increases with a higher air fraction, higher air-water interfacial tension, and a smaller bubble radius. Consequently, a well-whisked meringue with a high air fraction and small bubbles achieves an elastic modulus corresponding to the firm texture characteristic of "stiff peaks." Thus, smaller bubbles result in a higher elastic modulus for the meringue, consistent with the Princen model. The stability of this meringue improves drastically upon the addition of sugar: the sugar increases the viscosity of the meringue's liquid phase—thereby reducing the drainage characteristic of this type of foam—and stabilizes the protein interface by interacting with the hydrophilic groups of the proteins already adsorbed onto it. Consequently, Italian meringue (prepared with hot syrup) and Swiss meringue (heated before whipping) achieve a longer shelf life than cold-whipped meringue; the heat partially denatures the egg white proteins, thereby establishing a more rigid interfacial network at the air-water interface. Using more sugar increases the viscosity of the liquid phase, which in turn slows the rate of drainage.

Nevertheless, given the protein adsorption and sugar stabilization described earlier, gravitational drainage remains the primary mechanism of deterioration for this meringue: liquid from the continuous phase flows through the Plateau borders—the junctions where three bubbles meet—and drains toward the base of the meringue over time. A slow drainage rate, typical of a well-made meringue, keeps the foam stable for hours, whereas visible liquid accumulation at the base indicates that excessive gravitational drainage has already occurred.

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