An Attempt to Identify and Model Transient Viscoelastic Flow in Foods2

Shear stress development data for commercial 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, .1, 1, 10, and 100 s −1 , shear stresses displayed different degrees of overshoots 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 steady viscous and elastic properties of food materials. This model assumes both the steady viscosity function, η, and the steady primary normal stress coefficient Ψ to have a prominant power‐law region with increasing shear rate. This assumption is shown to be justified for all foods studied between shear rates of .1 s −1 and 100 s −1 . When this assumption is justified a time constant Λ 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.