17,24,28 In the flow curves presented in the Figure 1.7, the differences in shear stress values of each type of tomato product are clear. The flow behavior index of the power law model varies from about 0.2 for tomato paste and ketchup (pseudoplastic behavior) to almost 1 for tomato serum (nearly Newtonian behavior), at room temperature. Note that this model can be used as a general model for describing all the previously described fluid behaviors:Įach type of tomato product has its own compositional characteristics, which results in different rheological behavior. The most common mathematical model used to describe fluids with a yield stress is the Herschel–Bulkley model ( eqn (1.5)). On the other hand, if after the application of the yield stress the fluid behaves similarly to a pseudoplastic fluid, the fluid is called a Herschel–Bulkley fluid. When flowing, some fluids (including some juices) behave similarly to Newtonian fluids these are known as Bingham plastic fluids. 5 After the application of a shear stress higher than the yield stress, the fluid flows. The presence of yield stress is characteristic of multiphase materials such as fruit pulps and juices, which are formed by particles in suspension (cells, cellular wall, fibers) in an aqueous solution of sugars, proteins, soluble polysaccharides, and minerals (the serum phase). This minimum shear stress is known as yield stress ( σ 0). There are other types of fluids that need a minimum shear stress to start to flow.
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