A description of micellar casein/kappa-carrageenan mixed systems by means of calorimetry and rheology.

The properties of kappa-carrageenan (in the K+ form; 0.5%, w/w) in the presence of micellar casein in 0.25 M NaCl or 0.01 M KCl + 0.05 M NaCl have been investigated at temperatures ranging from 20 degreesC to 50 degreesC by combining microDSC and rheology. MicroDSC allowed one to monitor the conformational transition of the kappa-carrageenan. This occurred at 29-30 degreesC upon cooling and 43-44 degreesC upon heating in both ionic conditions. Mixing kappa-carrageenan with micellar casein at 50 degreesC, the carrageenan being in the coil conformation, led to a phase separation phenomenon. Demixing was ascribed to a depletion-flocculation phenomenon. Rheological measurements at 20 degreesC upon cooling showed that, whatever the conditions, the system had gelled. Its viscoelastic properties depended strongly upon the medium conditions and the casein content. These results could be interpreted on the basis of microscopic observations by Phase Contrast Microscopy (PCM) and Confocal Laser Scanning Microscopy (CLSM) that have been reported recently. The systems were clearly biphasic, the casein being concentrated in one phase while the kappa-carrageenan was located in the other. Moreover, in the casein-enriched phase, the micelles of casein appeared aggregated and tended to form a continuous network when the casein concentration was high enough (>3%). Phase separation phenomena appear to be a major event taking place when kappa-carrageenan is gelled in the presence of micellar casein. The way the two phases are organised with respect to each other determines the resulting rheological properties. Particularly, at a high enough casein concentration, two interpenetrated continuous networks are formed which both seem to contribute to the viscoelastic properties of the system.