Using the concept of a Newtonian fluid in which there is a fixed proportionality between shear stress and the applied shear rate and with a simple linear form of the flow curve, such liquids can be characterised by a single term, namely the constant of proportionality or the viscosity. More importantly, a single experiment such as the measurement of the shear stress at one surface at a single shear rate is sufficient to quantify the rheological characteristics of the fluid. However, few food liquids follow this simple relationship (water, unconcentrated milk, vegetable oils, some dilute solutions) and most foods may be classified as non-Newtonian and exhibit responses or flow curves such as those of (b), (c) and (d) in Fig. 5.1. Obviously, such fluids cannot be characterised by a measurement at a single shear rate as can the simple Newtonian fluid, and it is the ignoring of this requirement that produces the most common rheological measurement errors in the food industry. Furthermore, for many food liquids shear stress is not only determined by shear rate but is also time dependent, a factor which demands its own unique measurement system.