Analysis of the interaction between chitosan with different molecular weights and casein based on optical interferometry

Chitosan can be used as a stabilizer in dairy products to help suspend and stabilize casein micelles, but the molecular weight (MW) effect of chitosan has not been much explored. This work aimed to construct interaction models with casein using chitosan with MW of 30 kDa, 150 kDa, 250 kDa and 300 kDa to reveal the effect of MW on the structure and stability of chitosan-casein complexes. These models were based on the ordered porous layer interferometry (OPLI) system that can generate time-resolved signals to monitor the chitosan-casein interaction process dynamically in real time. The zeta potential, size distribution, binding mechanism and binding constants of chitosan-casein complexes with different MW were also analyzed. It was found that the chitosan and casein at pH 5.5 with the strongest interaction. The zeta potential and size analysis of chitosan-casein complexes showed opposite trends with MW before and after the saturation concentration, and MW(150 kDa) chitosan produced the largest size change in the bridging flocculation stage. The order of binding constants for chitosan-casein in-teractions is MW(300 kDa) < MW(250 kDa) < MW(30 kDa) < MW(150 kDa). In addition to the attraction between chitosan and casein by electrostatic driving forces, the dependence of the hydrophobic interaction between chitosan molecules on MW all influenced the interaction with casein. The dynamic monitoring and accuracy of the OPLI technique open up more possibilities to obtain the interactions of chitosan/casein and other bio-polymers in different condition variables.