E63 Freezing - Ice Cream

Topic

Ice cream is a multiphase system consisting of ice crystals dispersed within a concentrated liquid phase of sugar, protein, and fat, with air bubbles incorporated into the mixture; its quality—specifically whether the texture is smooth or grainy—depends primarily on the size of the dispersed ice crystals. Smaller ice crystals result in a smoother texture.

Freezing-point depression explains why ice cream does not freeze completely at standard household freezer temperatures: dissolved sugars—such as sucrose, glucose, or fructose—lower the freezing point of the liquid phase in proportion to their molal concentration. Consequently, the frozen fraction of the ice cream increases gradually as the temperature drops, without ever freezing solid at typical serving temperatures. A higher molal concentration of dissolved sugars leads to greater freezing-point depression, resulting in a smaller frozen fraction at any given temperature.

Small ice crystals yield a smooth, creamy texture, whereas large crystals create a gritty, grainy mouthfeel; achieving small crystals requires both rapid cooling—driven by a large temperature difference between the ice cream and the freezing agent—and vigorous agitation during the freezing process to break up the crystals while they are still growing. The overrun of this ice cream—the fraction of air incorporated into it—also contributes to the smoothness of its texture. More intense agitation during freezing results in smaller ice crystals and, consequently, a smoother texture.

Plank's equation calculates the freezing time based on the ice cream's size, the conditions of the freezing medium, and the ice cream's thermal properties; a thinner mold and a greater temperature difference between the ice cream and the freezing medium reduce the required freezing time. Furthermore, the thermal conductivity of the frozen ice cream is higher than that of liquid water, precisely because of the ice content within the frozen product. A thinner mold results in a shorter freezing time, according to Plank's equation.

Thus, this model—incorporating the small ice crystals and freezing time mentioned earlier—identifies the glass transition temperature as the threshold below which the ice cream becomes amorphous and glassy. Above this glass transition temperature, yet still below the freezing point, ice crystals can undergo Ostwald ripening (a process described by another model in this set), wherein small crystals dissolve and large crystals grow at their expense, thereby degrading the ice cream's texture during storage. The fact that this ice cream is stored at a temperature closer to this freezing point—rather than closer to this glass transition temperature—causes Ostwald ripening to proceed more rapidly within it, and consequently leads to greater deterioration of the ice cream's texture during that same storage period.

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