Not just gelled desserts: the dynamics of complex liquids

The texture of food is an important part of enjoying foods. In order to develop enjoyable food, it is important to understand the properties that determine how consumers experience biting and swallowing. To assist with this understanding, better methods for testing are required to capture the motion inside liquid materials, especially in the case of foods that are complex liquids, like gelled desserts.

Testing devices have been improved using different geometries in the testing chamber, and more recently, better results have been achieved using information from rheological testing coupled with results from other tests, such as inner visualisation techniques and ultrasonic imaging. But traditional methods have been unable to produce information about time-dependent properties.

Researchers have now introduced an updated method that can measure linear viscoelasticity and phase lag simultaneously in an opaque liquid, capturing information about complex rheological properties. In a study published in Physics of Fluids, from AIP Publishing, Taiki Yoshida, Yuji Tasaka and Peter Fischer present the details of the ultrasonic spinning rheometry method they developed. It substitutes velocity profiles of food into the equation of motion to capture information about complex rheological properties.

The researchers used a popular Japanese dessert called Fruiche, which includes fruit pulp and whole milk that transforms into a gelled form with an egg carton-shaped structure. The complexity of this liquid includes properties that are hard to measure with traditional rheometry methods because of the effect of shear history, shear banding, shear localisation, wall slip and elastic instability.

“Evaluation of food rheology with time dependence is a challenging target,” Yoshida said. “Based on the equation of motion, the ultrasonic spinning rheometry method can evaluate instantaneous rheological properties from the measured velocity profiles, so it can present true rheological properties and their time dependence from the perspective of physics of fluids.”

The updated method also has applications in chemical engineering for understanding polymerisation and dispersion densities, as well as in complex fluids such as clay, with applications in civil engineering and cosmetics. The researchers plan to further advance the method to include more points at which information can be gathered about the invisible properties of complex liquids. They also plan to further develop the industrial aspects of the technique, including inline rheometry for test samples flowing in a pipe.

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