κ-Carrageenan and β-lactoglobulin interactions visualized by atomic force microscopy

Mixes of κ-carrageenan and β-lactoglobulin were observed by atomic force microscopy (AFM). Diluted κ-carrageenan solutions showed helix formation and strand heights between 1 and 3 nm. Upon mixing β-lactoglobulin with carrageenan, AFM still revealed the presence of native protein with heights between 2 and 6 nm and the protein seemed to affect side-by-side aggregation of carrageenan. After heating, a network of β-lactoglobulin aggregates formed and the unheated protein was not affected by the presence of κ-carrageenan. At all the concentrations used in the study we observed phase separation. When studying the mixed gels under solid contact conditions, the surface topography of the samples clearly indicated phase separation and lack of interaction between β-lactoglobulin and κ-carrageenan. In general, the present research demonstrated how AFM imaging allowed direct visualization of mixed biopolymer systems such as β-lactoglobulin and κ-carrageenan. These observations were discussed in light of mechanisms of interactions previously suggested using other analytical techniques.