AFM study of casein micelles cross-linked by genipin: effects of acid pH and citrate

Casein micelles (CMs) are supramolecular structures highly dependent on their mineral content, especially colloidal calcium phosphate. When colloidal calcium phosphate is removed, the micellar structures dissociate and release minerals and casein molecules in the continuous phase. The aim of this work was to study the effects of solubilization of the colloidal calcium phosphate induced by acidification or addition of sodium citrate (100 mmol.L−1) on the structure of cross-linked CMs using atomic force microscopy (AFM). AFM was used to determine the dimensional characteristics, height, diameter, and contact angle of control and cross-linked CMs. To this end, micellar suspensions were cross-linked by genipin and then acidified or demineralized. Colloidal calcium was solubilized, and the results showed that the cross-linked CMs retained more calcium than control CMs. The AFM analysis revealed that control CMs were deformed after immobilization and shrunk upon acidification. As control CMs dissociated in the presence of citrate, dimensional characteristics were not determined by AFM. The crosslinked CMs were also deformed on immobilization but to a lesser extent than control ones. The dimensional characteristics of cross-linked CMs were not modified by acidification. Conversely, demineralization by citrate induced structural rearrangement of the micellar structures. The higher structural resistance of cross-linked CMs was related to intramicellar cross-links between basic amino acids (lysyl and arginyl residues) present in casein molecules. Although AFM was performed in the air, it allowed us to know how the ionic environment modulates the intrinsic nanomechanical properties of CMs.