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Gelation Mechanism of Milk as Influenced by Temperature and pH; Studied by the Use of Transglutaminase Cross-Linked Casein Micelles

A. J. Vasbinder^*, H. S. Rollema^*, A. Bot and C. G. de Kruif^*,

^* NIZO Food Research; P.O. Box 20, 6710 BA, Ede, The Netherlands
Unilever Research Laboratorium; P.O. Box 114, 3130 AC, Vlaardingen, The Netherlands
Van ’t Hoff Laboratory, Debye Research Institute, University of Utrecht, Padualaan 8, 3584 CH, Utrecht, The Netherlands

Corresponding author: C. G. de Kruif; e-mail:
kees.de.kruif{at}nizo.nl.

Casein micelles in milk are colloidal particles consisting of four different caseins and calcium phosphate, each of which can be exchanged with the serum phase. The distribution of caseins and calcium between the serum and micellar phase is pH and temperature dependent. Furthermore, upon acidification casein micelles lose their colloidal stability and start to aggregate and gel.

In this paper, we studied two methods of acid-induced gelation, i.e., 1) acidification of milk at temperatures of 20 to 50°C and 2) decreasing the pH at 20°C to just above the gelation pH and subsequently inducing gelation by increasing the temperature. These two routes are called T-pH and pH-T, respectively. The gelation kinetics and the properties of the final gels obtained are affected by the gelation route applied. The pH-T milks gel at higher pH and lower temperature and the gels formed are stronger and show less susceptibility to syneresis. By using intramicellar cross-linked casein micelles, in which release of serum caseins is prevented, we demonstrated that unheated milk serum caseins play a key role in gelation kinetics and characteristics of the final gels formed. This mechanism is presented in a model and is relevant for optimizing and controlling industrial processes in the dairy industry, such as pasteurization of acidified milk products.

Key Words: casein micelle • gelation kinetics • serum casein • transglutaminase

Abbreviation key: CL = cross-linked, DWS = diffusing wave spectroscopy, GDL = glucono--lactone, pH-T route = increasing temperature at a constant pH, T-pH route = decreasing pH at a constant temperature, WPF = whey protein free

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