Physical Properties of Chocolate and Couverture

Chocolate and couvertures are dispersions of small (10-25 im) particles of sugar, and, depending on the type of chocolate, cocoa powder solids and non-fat milk solids, in a continuous fat phase. A layer of emulsifier (usually lecithin) is strongly adsorbed to the surface of the sugar particles. Chocolates and couvertures for ice cream products generally have higher fat contents (40-60%) than their normal ambient counterparts (28-35%). This is because chocolate on ice cream products is eaten at lower temperatures than ambient chocolate. An ambient chocolate would be very hard at —18 °C, and would have a waxy texture when eaten in conjunction with ice cream, because the mouth would be too cold to melt the fat in the chocolate. 

The Casson equation (equation 5.1) is often used to describe the rheology of liquid chocolates and couvertures. The relation between the shear stress (a) and the shear rate (y) depends on two parameters the yield stress, Oy, i.e. the shear stress required to initiate flow, and the plastic viscosity, i.e. the viscosity after yield. 

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