Supervisors: Prof. Dr. M.A.J.S. van Boekel (WU) Dr. H.P. van Leeuwen (WU) Ir. H. van Valenberg (WU)
Project term: February 2006 - February 2010
Sponsor: Campina Innovation
Introduction
Some dairy products contain a fruit layer with a neutral gelled dairy layer on top. Such products are subject to diffusion phenomena of acids and salts, resulting in changes in ion activities and pH. The present problem originates from the observation that undesired gelation takes place at an interface between an acid fruit layer and a neutral milk gel. The mechanisms of gelation of dairy proteins are complex, but the role of salts has been clearly demonstrated to be important in determining the onset pH of aggregation and gelation. Studies of casein micelles show clearly that colloidal calcium phosphate is instrumental in stabilizing the micelle structure. It has been proposed that the solubilization of ions out of the micelle leads to the rearrangement and large scale aggregation that result in the final gelled structure. In addition, diffusion of acids and salts driven by chemical potential differences could affect ion activities and pH and further result in protein gelation. When reviewing recent literature, it becomes clear that control of the specific ion equilibria and ion transport in dairy systems provides a way to control the onset and structure of protein gels. However, a complete understanding of the role of salts in protein gelation is still not well understood.
Aim
The objective is to understand the role of ion transport phenomena, the resulting ion activities and pH changes, and their consequences for protein gelation. A combination of measurements of ion activities, ion transport and modelling of these phenomena, and physical characterization of protein aggregation will shed light on the complex ion equilibria and lead to improved control of gelation phenomena.
Research
The undesired gelation is due to protein aggregation that is mediated by pH, the presence of certain mono- and divalent ions, and ionic strength. It is well known that aggregation and gelation of denatured dairy proteins can be
induced by the presence of divalent cations, particularly Ca2+ ions. The proposed mechanism for this type of gelation is divalent bridging. Current work focuses on quantifying Ca2+ ion activity during acidification. Further research includes monitoring milk salt concentration changes under acidification, especially colloidal calcium phosphate and magnesium citrate concentration which are essential to stabilize milk protein structures. Finally, the ion mass balances in milk can be established at different pH with dedicated software (AESolve).
Future research
Diffusion of salts and acids is another reason of protein aggregation at the interface. Diffusion arises because of differences in chemical potential of the compounds of interest. The question is how fast this occurs and how it can be influenced and controlled. Equipment has been designed and built with which acid and salt transport can be measured to 0.5 millimeter precision. The resulting ion activities will be measured using ion selective micro-electrodes. These measurements will form the basis for mathematical models describing the transport phenomena using Maxwell-Stefan equations and the resulting changes in ion activities, ion equilibria and pH will be modeled as well.
References
1. Warin, F. Voirin, A. Paulsson, M. Dejmek, P. Formation of a protein aggregate layer at a milk/acidified gel interface. Int. Dairy J.1998, 8: 801-806. 2. Holt C, Dalgleish DG, Jenness R. Calculation of the ion equilibria in milk diffusate and comparison with experiment. Anal Biochem 1981, 113: 154-163. 3. Gaucheron F. The minerals of milk. Reprod. Nutr. Dev. 2005, 45:473-483
