Coagulation Under High Hydrostatic Pressure: Polymer Latices as a Model for Casein Dispersions

Abstract

The acidification of skim milk by glucono-δ-lactone (GDL) under high hydrostatic pressure leads to aggregate structures different from gels formed by acidification at ambient pressure. For a better explanation of the mechanism of acid coagulation carboxylated polystyrene latices were studied as a model system. A dispersion of carboxylated polystyrene particles was acidified by hydrolysis of GDL at 20 °C at pressures of 1 bar and 2 kbar, respectively. Colloidal stability, pH and aggregate structure as a function of time and initial concentration of GDL were observed.Under high pressure coagulates were formed at low initial concentrations of GDL and short durations of pressure treatment. After a short time of pressurization small floccules were formed. Longer durations of pressure treatment led to aggregate structures similar to a cobweb depending on initial concentration of GDL. It was observed that during acidification at ambient pressure the mobility and the streaming potential of the dispersions decreased whereas the charge of the particles remained negative during the observed reaction time. Coagulation at ambient pressure needed higher concentrations of GDL and longer reaction times. All aggregates had a strong tendency to form closer structures under very low shear stress. A mechanism of colloidal stability of the latex particles based on the structure of water surrounding the carboxylic groups at the surface of the particles is proposed.