Rheology is the study of deformation and flow of foods under well-defined conditions. These conditions could be defined in terms of their rate of deformation or in terms of the magnitude of the stress or the strain applied. Foods of differing internal structure and bonding react in different manners to these applied conditions. In the simplest case the shear stress developed in the fluid is directly proportional to the rate of deformation or the rate of strain. In such cases, the liquid is said to be Newtonian and obeys the relationship
where τ is the shear stress and View the MathML source is the shear rate. Such a relationship is shown by line (a) of Fig. 5.1. In SI units, τ will normally be in pascals (Pa), View the MathML source in reciprocal seconds (s− 1) and μ in pascal seconds (Pa s). The constant of proportionality μ between the shear stress and the shear rate is termed the viscosity of the fluid, and from the 1663 definition of a fluid by Pascal can be viewed as a measure of its internal friction (i.e. ability to resist motion when a shearing stress is applied)