Understanding temperature-sensitive caseinate emulsions: new information from diffusing wave spectroscopy

Changes in the dynamical properties of sodium caseinate emulsions (4 wt% protein, 30 vol.% vegetable oil) containing various concentrations of glucono-δ-lactone and calcium chloride have been monitored as function of temperature (25-65 °C) by diffusing wave spectroscopy (DWS). Depending on the pH and ionic calcium content, these systems exhibit a pronounced heat-induced thickening behaviour which was previously described as reversible heat-induced gelation. The changes in the DWS relaxation time and correlation function amplitude have been compared with changes in the small-deformation rheological parameters and confocal microscopy observations over the same temperature range. Analysis of the combined experimental data indicates the difficulty of defining the onset of gelation in this system. We suggest a tentative molecular mechanism for the behaviour observed.