Flow and Functional Models for Rheological Properties of Fluid Foods

Flow and Functional Models for Rheological Properties of Fluid Foods 

A flow model may be considered to be a mathematical equation that can describe rheological data, such as shear rate versus shear stress, in a basic shear diagram, and that provides a convenient and concise manner of describing the data. Occasionally, such as for the viscosity versus temperature data during starch gelatinization, more than one equation may be necessary to describe the rheological data. In addition to mathematical convenience, it is important to quantify how magnitudes of model parameters are affected by state variables, such as temperature, and the effect of structure/composition (e.g., concentration of solids) of foods and establish widely applicable relationships that may be called functional models. 

Eilis modei for low shear rate data containing /jo (Brodkey, 1967) 

Flow models have been used also to derive expressions for velocity profiles and volumetric flow rates in tube and channel flows, and in the analysis of heat transfer phenomenon. Numerous flow models can be encountered in the rheology literature and some from the food rheology literature are listed in Table 2-1. Also, here those models that have found extensive use in the analysis of the flow behavior of fluid foods are discussed. Models that account for yield stress are known as viscoplastic models (Bird et al., 1982). For convenience, the flow models can be divided in to those for time-independent and for time-dependent flow behavior. 

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Knowledge of the rheological properties of food pastes, slurries and sauces, such as ketchup, mayonnaise and salad creams, is important both for quality assurance and for optimizing industrial flow and mixing processes. Unfortunately, many food slurries and pastes are opaque and do not lend themselves to flow studies with conventional techniques such as laser Doppler anemometry. Moreover, conventional rheological measurements are model-dependent in that it is necessary to fit the data by assuming a function relationship between the stress and strain (or strain rate) and to assume a set of boundary conditions (such as slip or stick) at the fluid-container... 

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