In this work we use a low frequency ultrasonic technique to study the change of physical states in viscoelastiques media. The measurement is based on two sensors resonating in longitudinal mode and behaving as point sources in the analysed medium. This configuration of the ultrasonic sensors makes it possible to follow all the steps of milk gelation process. In this paper we firstly investigated the ultrasonic response influenced by the powder milk components during the reconstitution period. Then, we study the ultrasonic response through the measurement of the time of flight during milk gelation in relation with several technological factors known to influence changes in the process, such as temperature, pH and milk powder concentration. (C) 2003 Published by Elsevier Ltd.