Chocolate, viscosity measurement

Mechanical, as opposed to viscosity, measurements on fats are comparatively rare. Often this is due to the brittle nature of the materials, for example chocolate. However, one approach is to observe the major transitions using the Triton materials pocket or constrained layer damped beam method. At present, however, absolute values of moduli cannot be obtained. Nevertheless, transitions between polymorphs in cocoa butter were measurable. 

For a food whose flow behavior follows the Casson model, a straight line results when the square root of shear rate, (y), is plotted against the square root of shear stress, (cr) , with slope Kc and intercept Kqc (Figure 2-2). The Casson yield stress is calculated as the square of the intercept, ctoc = (Kocf and the Casson plastic viscosity as the square of the slope, r]ca = The data in Figure 2-2 are of Steiner (1958) on a chocolate sample. The International Office of Cocoa and Chocolate has adopted the Casson model as the official method for interpretation of flow data on chocolates. However, it was suggested that the vane yield stress would be a more reliable measure of the yield stress of chocolate and cocoa products (Servais et al., 2004). 

Figure 23 (a) Effect of adding soy lecithin [initial viscosity of plain chocolate (cocoa butter content, 34.8%), >200 s] on the viscosity of chocolate, (b) Fat and lecithin contents of chocolate of identical viscosity (42 s). (c) Effect of adding soy lecithin to suspensions of ( ) cocoa butter and (A) ground sugar in cocoa butter on viscosity. Viscosity was measured at 50°C on Bournville Redwood-type viscometer. 

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