Ultrasound tests in a stirred vessel to evaluate the ability of dairy powders to reconstitute

This study demonstrated the feasibility of applying an ultrasound test to evaluate the ability of a powder to interact with a solvent and thus to predict the reconstitution behavior of powders. The ultrasound test consisted of using a probe immersed in a stirring tank to monitor the ultrasound attenuation parameter when powder particles were dispersed in a solvent and to calculate a relaxation time. Analysis of the ultrasound signal and images obtained in the course of dispersion of a native phosphocaseinate dairy powder in various solvents under stirring allowed us to establish that the time required by the ultrasound attenuation parameter to relax is strongly associated with the time required by the solvent to penetrate primary particles. It was also shown that this step is of crucial importance in the dissolution mechanism as it precedes the fragmentation phase, identified as the rate-limiting step affecting the dispersion of the primary particles constituting the agglomerate. The reliability of the ultrasound evaluation to classify powders according to their reconstitution ability was then shown by comparing the ultrasound relaxation times obtained for various milk powders (pure native phosphocaseinate ([NPC]), pure whey protein isolate (WPI), mixture of (NPC/lactose) and mixtures of (NPC/WPI) stored under various conditions) with the rehydration times required to transform the powders into solutions (rehydration test). A static light scattering technique was used to determine particle size changes during rehydration tests and to calculate rehydration times. Classification of dairy powders according to their relaxation times (ultrasound test) and rehydration times (rehydration test) was in good agreement. The acoustic test was demonstrated to be effective in tracking solvent interactions, making it possible to assess the ability of a solvent to bind with the primary particle, and thus revealing it to be a powerful tool to evaluate the ability of a powder (and not only dairy powders) to disperse in a solvent.