Résumé
Coconut water is a well-known beverage under the tropics, but its popularity on the international market has recently been increasing. In fact, it has drawn the attention of manufacturers as a natural functional drink. In this context, and in order to reach the large volume expected by the international markets, one of the key processes is “coconut water concentration”. To optimize and control this unit operation, viscosity of the fluid is one of the most important physical/rheological parameter to be evaluated along with the soluble solid content (SSC in °Brix). Usually, the viscosity of a fluid is measured by rheometry. However, manipulations are time consuming and not suitable for online measurements of the visco-elastic flow properties. In this communication, we suggest two high frequency ultrasonic approaches dedicated to fluid viscosity estimation using both longitudinal or shear wave devices. At first, the investigation carried out using longitudinal waves will be presented. The relationship between longitudinal viscosity and SSC of the coconut water will be demonstrated, prior to the discussion on the relationship between longitudinal viscosity (novel in food rheology) and shear viscosity. Later on, the use of a shear ultrasonic reflectometry approach will be suggested. Thanks to an enhanced experimental device, the direct estimation of the shear viscosity with ultrasound will be compared to shear viscosity by rheometry. A discussion will be proposed for the measurement of viscosity and SSC during concentration using longitudinal or shear ultrasonic waves, on the one hand, for on-line process control on pipelines and, on the other hand, for off-line portable devices (suggestion of a new device coupled to refractometers). These solutions could be applied to any other viscous fluid in the food industry. (Texte intégral)