ABSTRACT Shear stress development data for commecial ketchup, mustard, mayonnaise, apple butter, butter, margarine and canned frosting were obtained using the cone and plate geometry of the Rheometrics Mechanical Spectrometer. At sudden imposition of four shear rates, 0.1, 1, 10, and 100 s-1, shear stresses displayed different degrees of overshotts with margarine exhibiting a maximum overshoot of 320% that of the steady-state value at a shear rate of 100 s-1 The actual extent of overshoot depended on the particular food and the particular shear rate applied.
An attempt was made to explain observed transient shear stresses with the Bird-Leider equation, a four parameter empirical model which incorporates both stead viscous and elastic properties of food materials. This model both the steady viscosity function, and the steady primary normal stress coefficient to have a prominant poower-law region with increasing shear rate. This assumption is shown to be justified for all foods studied between shear rates of .1100 s -1, and 100100 s-1, . When this assumption is justified at a time connstant can be constructed. This time constant is found to be an approximate indicator of relative stress overshoot for the foods studied. It is found that the Bird-Leider equation provides a moderately good prediction of peak shear stresses and peak times and only a crude prediction of shear stress decay. Nevertheless, the model is a definite improvement over time independent models such as the power-law model. |
