axis at a nonzero value, then the material is said to be plastic, and if the rheogram is a linear function of shear rate, the material is said to show Bingham plastic behavior. Such behavior is unusual, although not unknown, in foods. Usually, the effect of shear is to break down the structure that may be present in a fluid at rest, so their apparent viscosity will decrease with increasing shear rate. This is especially evident in some gels that will flow like a normal fluid once the gel structure has been broken. Consumers commonly shake containers of products such as ketchup or Thousand Island salad dressing because the shearing makes such products more fluid.

A few products (some starch slurries, for example) show dilatant flow behavior, wherein the apparent viscosity increases with increasing shear rate. One explanation of non-Newtonian flow behavior is that long molecules or asymmetric particles may tend to align themselves so their long axes are parallel to the shear field and this orientation leads to a change in resistance to flow.

Some products are slow to recover from the effects of shearing, and under some circumstances some products may not recover at all. The time lag in recovering an initial structure after it has been broken by shear is the reason it is possible to pour ketchup following shaking the bottle. The shearing breaks the initial structure and it stays broken for a short time after the shearing has stopped. (If a bottle is completely filled, it may be difficult to apply much shear to the product and shaking may not initiate flow.) Such time dependence in recovering the initial structure is called thixotropy. If there is no recovery, the process is called rheodestruction. Dilatant fluids may also relax slowly to their unsheared (in this case, lower viscosity) initial state and that time dependence is called rheopexy. Shearing may be applied to a product at a fixed level for a variable amount of time, at levels that vary linearly as a function of time, or at any shear rate and time combination. While the process of applying shear to a product is somewhat arbitrary, for rigorous rheological analysis of the data, it is essential that the shearing history be exactly known; furthermore, certain controlled applications of shear to the product will give data that are easier to analyze, so in practice a specific, not arbitrary, shearing process is followed. Idealized rheograms for thixotropic and rheodestructive dispersions are shown in Figure 3 for processes in which a constant shear rate is applied for a fixed time and then stopped. Idealized rheograms for thixotropic and rheopectic fluids are shown in Figure 4 for shearing processes in which the shear rate is increased and then decreased linearly with time while recording the resultant shear stress. Instruments are commercially available for recording flow curves in this manner, using shear rate as the independent variable, or for recording shear rate as a function of shear stress. The choice of instrument type depends on the objective of the measurement.

Principles of Flow Measurement

The schematic diagram of a parallel plate viscometer shown in Figure 5 will be used to illustrate how rheological data are obtained. If a fluid is placed in the gap y between the parallel plates (each of area A) and a force F is applied c

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