Rheology of κ-carrageenan and β-lactoglobulin mixed gels

Gel formation and the melting of κ-carrageenan in the presence of β-lactoglobulin were investigated using dynamic rheological techniques as well as a sequence of experimental sweeps of time–temperature, frequency, and strain. The blends, initially prepared at 45°C, show homogeneous mixtures, which then lead to the formation of a gelled κ-carrageenan network containing inclusions of native β-lactoglobulin during the controlled cooling phase from 45 to 20°C. In its native state, the protein seems to weaken the polysaccharide network, particularly when present in high concentration. Upon subsequent heating to 90°C, mixed gels presented a biphasic profile: the first phase, characterized by a decrease in the storage modulus, involves a heat-induced meltdown of the κ-carrageenan network, whereas the second phase, exhibiting an increase in the storage modulus, corresponds to the build up of a protein network. The DSC analysis showed that each biopolymer undergoes specific conformational changes. These results are highly indicative of the lack of association between the biopolymers and suggest a phase separation and gelation in β-lactoglobulin and κ-carrageenan mixtures. The final cooling phase of the mixed gels, from 90 to 20°C, induces a consolidation of the protein network and a gelation of κ-carrageenan. This rheological behaviour suggests that gelation of κ-carrageenan in the presence of a gelled protein network would lead to the formation of a phase-separated bicontinuous network. The strain sweep results for mixed gels obtained at the end of the experiments support that hypothesis.