Yield stress is defined as the shear stress required to initiate the flow of the chocolate and hence gives information about potential enrobing properties, whereas the plastic viscosity relates to the shear stress required to maintain a constant flow. The Casson method gives information about the yield stress and the plastic viscosity. The flow properties are usually described by the Casson flow curve and for this, it is necessary to make measurements at different rotational speeds so that shear stresses at different shear rates can be determined. The measurement of chocolate viscosity is highly specialized and requires a specific type of viscometer for the measurement to be performed accurately. The viscosity plays a crucial role as it affects texture, that is, how it flows in the mouth. However, the flow behavior is complex due to the fact that a number of ingredients, namely sugar, cocoa butter, cocoa particles, and milk products, need to be finely dispersed. Chocolates have different flow properties depending on application and products are made for enrobing and for making blocks.

The flow behavior of chocolates is important both during processing as well as for organoleptic reasons. Another type of vicious behavior exhibited by fluid foods and polymer systems is thixotropy which is again shear-thinning of the material with increasing rates of shear but is also dependent on the duration of shear.īy far the most researched food in terms of viscosity is chocolate. This is referred to as time-independent (steady-state) flow and materials showing this type of behavior are called pseudoplastic. The viscosity of some fluids is dependent on the rate used to shear the material, a high rate of shear making the fluid thinner compared with the fluid that was sheared more slowly. If this is not observed then the liquid is non-Newtonian.

An easy way to demonstrate Newtonian behavior is to double the shear stress during a viscosity test and this should result in doubling of the shear rate. However, it must be pointed out that long-chain polymers at low concentrations can also show Newtonian behavior. Newtonian behavior is displayed by simple liquids consisting of small molecules that do not interact or form any connected structure.

Spindle viscosity testing