Study by ultrasound of the impact of technological parameters changes in the milk gelation process Georges Nassar a, * , Bertrand Nongaillard a , Yolande No€el b,1 a IEMN, Department O.A.E., UMR CNRS 8520, Universit e de Valenciennes, BP 311, 59313 Valenciennes Cedex 09, France b INRA-SRTAL, 39800 Poligny, France Received 19 March 2003; accepted 24 July 2003 Abstract 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. Ó 2003 Published by Elsevier Ltd. Keywords: Milk; Gelation; Ultrasonic 1. Introduction The research works and the design of new methods studying milk coagulation have several goals, such as the knowledge of the coagulation phenomena, the evaluation of the technological clotting aptitude of milk, or in-line control. Milk gelation is a major step in the cheese making process. It produces a critical change in the state of milk from a liquid to a viscoelastic semi-solid under the ac- tion of enzymes. Even though the major parts of this step are known, the mechanisms governing the changes are not fully clear (Walstra & Vliet, 1986). Milk coagu- lation, especially the enzymatic one, has been extensively studied by several authors, such as Payens (1976, 1979), Marshall, Hatfield, and Green (1982), Green, Hobbs, Morant, and Hill (1978), Dalgleish (1979, 1982, Chapter 5), Carlson, Hill, and Olson (1985, 1986, 1987), Van Hooydonk and Van Den Berg (1988), Zoon (1988), No€el, Durier, Lehembre, and Kobilinsky (1991), Ben- guigui, Emery, Durand, and Busnel (1994) and O’Callaghan, O’Donnell, and Payne (2000). In this paper we present the investigation of the ability of a new ultrasonic low frequency (less than 200 kHz) transmission technique using sensors with highly sharp- ened ends (Nassar, Nongaillard, & No€el, 2001). The study aimed at contributing to the characterization of the physical properties of weak media such as milk gels. After a description of the measurement system and the experimental protocol, we analyzed the results of the changes of an ultrasonic wave time of flight (ultrasonic compressional wave velocity) in a changing medium considered as a coupling medium. First, we study the evolution of the ultrasonic response as a function of hydration protein phenomenon during milk reconstitu- tion phase. Then, we study the ultrasonic response during milk coagulation through the measurement of the time of flight of an ultrasonic pulse versus different technological factors known to influence changes in milk coagulation, such as temperature, pH and milk powder concentration. 2. Materials and methods 2.1. Samples The measurements were made on reconstituted milk from a single skim milk powder batch in order to reduce measurement variations caused by different milk origins. Journal of Food Engineering 63 (2004) 229–236 www.elsevier.com/locate/jfoodeng * Corresponding author. Fax: +33-3-27-51-11-89. E-mail address: gnassar@univ-valenciennes.fr (G. Nassar). 1 Fax: +33-3-84-37-37-81. 0260-8774/$ - see front matter Ó 2003 Published by Elsevier Ltd. doi:10.1016/j.jfoodeng.2003.07.004