First Principles

Before proceeding to the detailed study of the application of rheology to cheese, it is pertinent to consider a few basic principles. These will appear obvious to the reader educated in a physical science, but may be less obvious to those from other walks of life. One is taught that matter exists in one of three distinct states: gas, liquid, and solid. This context is mainly concerned with only the latter two. Both are characterized by the fact that any sample has a well-defined volume. Solids have a definite shape which can be altered only by the application of an external agency. Liquids, on the other hand, have no characteristic shape: they take up the shape of the vessel containing them.

The property, then, which distinguishes a solid is its rigidity and this term has been formally adopted by physicists as a measure of the effort required to change the sample's shape. The precise definition will be given later. If the material is a true solid, it will follow that this rigidity is invariant for the material. Furthermore, time does not enter into this, so it is understood that the change is instantaneous and once the effort is removed, the sample will recover its original shape spontaneously. This is, in plain language, the physicist's concept of an elastic solid.

In contrast with the solid, the characteristic by which a layman distinguishes a liquid is its fluidity. This, again, has a precise definition, but in practice it is more convenient to use the inverse concept, which is known as the vis-

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