Ultrasound can determine rheological properties of fluid foods.

Sensory tests and descriptors are useful when it comes to understanding product rheology, and sophisticated instruments designed for making rheological measurements are becoming common and easier to use--and they provide reliable data. Instrumental techniques that measure mouthfeel are based on the science of rheology, which measures the deformation and flow of materials.

The challenge confronting food designers who want to quantitate mouthfeel characteristics using an instrumental technique involves successfully measuring forces, distances and other data, and relating them to what is relevant to consumers when they taste, chew and swallow. The flow behavior of fluid foods is important when considering the processing and storage stability of these products.

Characterizing fluid flow behavior is often accomplished using conventional rotational instruments that are based on the fundamentals of fluid mechanics. These instruments measure the response of a fluid to an applied stress or strain rate. While these measurement techniques provide vital information on the fluid's properties, they typically are performed off-line and are not performed under conditions that simulate the processing environment.

To address the need for making rapid in-line measurements, scientists at the University of California have developed an ultrasonic-based tomographic technique. This approach is able to characterize fluid rheological properties. In this process, velocity measurements are achieved by emitting multiple ultrasound pulses through a pipe system and recording the echoes, which are scattered from the particles in the fluid.

Researchers calculate the position data from the time of flight of the ultrasonic wave. The velocity data are calculated using the Doppler shift frequency effect, in which a sound wave scattered from a moving particle is frequency-shifted by an amount proportional to the velocity of that scattering particle.

Combining the velocity measurements with a simultaneous pressure-drop measurement enables investigators to determine the rheological properties. These properties include the shear viscosity, apparent wall slip and apparent yield stress values. The results agree well with conventional off-line techniques and with other tomographic-based methods.

Further information. Michael McCarthy, Department of Food Science and Technology, University of California, 1 Shields Ave., Davis, CA 95616; phone: [phone]; fax: [phone]; email: [e-mail].