Infodoc : Sciences, Techniques et Culture

The research described in this thesis is about ion behaviour in milk systems as a function of changing conditions (pH, ionic strength, addition of electrolytes and non-electrolytes). The focus was on (i) quantification of the ion equilibria in milk systems based on experimental conditions, (ii) development of quantitative models describing the ion equilibria. The so-called Donnan Membrane Technique (DMT) was successfully applied to determine the concentrations of free Ca2+, Mg2+, Na+ and K+ ions in milk-like systems. Next, models were developed to describe ion equilibria in simulated milk ultrafiltrate solution (SMUF) as a first step to build models for milk systems. Freshly prepared SMUF appeared not to be in thermodynamic equilibrium regarding the supersaturation of calcium phosphat[...]

The research described in this thesis is about ion behaviour in milk systems as a function of changing conditions (pH, ionic strength, addition of electrolytes and non-electrolytes). The focus was on (i) quantification of the ion equilibria in milk systems based on experimental conditions, (ii) development of quantitative models describing the ion equilibria. The so-called Donnan Membrane Technique (DMT) was successfully applied to determine the concentrations of free Ca2+, Mg2+, Na+ and K+ ions in milk-like systems. Next, models were developed to describe ion equilibria in simulated milk ultrafiltrate solution (SMUF) as a first step to build models for milk systems. Freshly prepared SMUF appeared not to be in thermodynamic equilibrium regarding the supersaturation of calcium phosphate. Therefore, the equilibrium ion speciation (EIS) model and the dynamic ion speciation (DIS) model were developed for SMUF at equilibrium and at initial state, respectively. Both EIS and DIS models gave satisfactory predictions in ion equilibria in SMUF enriched with different electrolytes. Also, addition of non-electrolytes such as various disaccharides appeared to remarkably increase the Ca2+ and H+ ion activities in SMUF and milk. The observed results could be explained by a theory called Mean Spherical Approximation (MSA), which suggests that excluded volume effects due to addition of sugars account for the increase in Ca2+ and H+ ion activity coefficients and thus on Ca2+ and H+ ion activities. Based on the EIS and DIS models, a model named Milk Ion Speciation (MIS) was built to describe ion equilibria in milk. The MIS model was able to predict the ion composition in milk and milk enriched with different electrolytes. In addition, the methodologies of Ca2+ activity measurement were reviewed. We found that the calcium ion selective electrode (Ca-ISE) underestimated the Ca2+ activity at ionic strength lower than 0.1 mol/kg water compared to DMT. It appeared that the ionic composition of conventional calibration standards significantly affects the Ca ion activity measurement. A series of new calibration standards, having similar composition to milk serum, was developed and this remarkably improved the Ca2+ activity measurement in milk-like systems. The developed ion speciation models were carefully evaluated. Particularly, the current limitations of the models such as the nature of CCP and the effect of adding polyphosphate were intensively discussed. Moreover, the models were tested to calculate ion composition in cheese. It turns out that the models offer great potential to correlate ion properties with the functional properties of dairy products.