NMR microscopy and the non-linear rheology of food materials

The non-linear viscosities of a number of different food products were examined using NMR microscopy. These materials included xanthan gum–water solutions, cream, semi-soft butter, egg white, cornflour–water mixtures and two different varieties of tomato sauce. The non-linear rheology is manifest in the velocity profiles measured across pipe, cylindrical Couette cell and cone-and-plate geometries. The properties observed included apparent slip, shear thinning, shear thickening, shear banding and yield stress behaviour. The results show that accurate, non-invasive velocity profiling is essential if the results of mechanical rheometric measurements are to be interpreted correctly. They also illustrate the power of NMR both to demonstrate the existence of and, potentially, to investigate the particulate or molecular origins of complex rheological properties in food materials.