Soft Sphere Behavior of Casein Micelles in Concentrated Milk Suspensions

OLIVARES M.L.; BERLI C.L.A.; ZORRILLA S.E.

New Orleans

Congreso; 2014 Annual Meeting and Food Expo; 2014

Institute of Food TEchnologist

Native casein micelles (CM) are colloidal particles formed via self-assembly of four casein proteins and calcium phosphate. Even if the structure and composition of the CM are now reasonably known, little has been done in understanding how these particles interact each other. Several authors showed that dispersions of CM can be closely described by the so-called hard sphere model. However, at higher packing fractions, the deformability of CM plays an important role in the viscous behavior. Therefore, the aim of the present work is to predict the rheological behavior of CM dispersions considered as soft sphere particles. For this analysis, a simple structure for CM composed of a spherical core with a brush of k-casein was considered. A rheological model that allows obtaining an explicit relationship between viscosity and shear stress for non-Newtonian behavior of concentrated soft sphere suspensions was used. Experimental viscosity curves were used to obtain the parameters of the viscosity model. The effective maximum packing fractions  and  corresponding to the shear stress limits  and , respectively were obtained. The ratio  grows almost linearly as the core volume fraction  increases and reaches the limit value of 1 when  0.5 - 0.6. This behavior agrees with the packing fraction of CM at the critical overlap concentration, 0.51. However, the ratio  as function of  shows a lower slope and it does not reach the limit value of 1. This behavior may be explained taking into account that CM behave as soft spheres and start to deform and align along the flow direction at high shear rates. In addition, the effective radius of CM may decrease as a consequence of hydrodynamic forces on the k-casein layer. This study has important implications for predicting the behavior of concentrated CM suspensions in industrial operations such as membrane filtration, evaporation, and drying.