Effects of carrageenan type on the behaviour of carrageenan/milk mixtures

The apparent hydrodynamic diameter of casein micelles was determined as a function of hydrocolloid concentration at 60°C and during cooling to 20°C. The systems studied contained skim milk 100-fold diluted in permeate in the presence of either lambda-, iota-, kappa-carrageenan or guar gum. These measurements show that all three forms of carrageenan adsorb onto casein micelles, whereas guar gum does not. Lambda-carrageenan (which is always in the coil conformation) adsorbs at all the temperatures studied, whereas the iota and kappa forms adsorb only at temperatures below the onset of the helix–coil transition. The results agree with the idea that adsorption of carrageenan occurs only above a certain minimum charge density. Phase diagrams were established at 60°C for the three carrageenans and the rheological behaviour was followed on cooling and at 25°C. The results suggest that the lambda-carrageenan bridges the casein micelles at low carrageenan concentrations, leading to the sedimentation of carrageenan/milk mixtures at 60°C and structure formation on cooling. At 60°C, iota- and kappa-carrageenan induce depletion flocculation of casein micelles above a critical carrageenan concentration. On cooling, systems containing iota-carrageenan clearly form a network at the helix–coil transition temperature. The network is formed by mixed carrageenan/casein micelle crosslinks. In the presence of excess carrageenan, this network is reinforced by carrageenan/carrageenan crosslinks. For the kappa-carrageenan/casein micelle mixtures the existence of mixed carrageenan/casein micelle crosslinks is less obvious but seems likely.