Influence of enzymatic cross-linking on the apparent viscosity and molecular characteristics of casein micelles at neutral and acidic pH

Individual caseins represent almost 80% of the protein in milk, they are organized as protein assemblies called casein micelle (CM). The micelle is a heat-resistant structure, but it becomes dissociated in acidic environments. Microbial transglutaminase (mTGase) promotes the stability of the CM under acidic environments by catalyzing the formation of covalent cross-links. However, no quantitative data are available concerning the structure of the CM at pH 3.0 as a function of the progress of the enzymatically induced cross-linking reaction, measured through a polymerization degree (DP). This study aims to provide information on the cross-linked CM structure at acidic pH (3.0) and neutral pH (7.0) according to their DP and correlate these changes with the viscosity of the dispersions. The size, the molar mass, the apparent density, and the molecular shape of the casein micelle (CM-0%) and cross-linked CM at three cross-linking degrees (TG-41%, TG-51%, TG-56%) were determined using Asymmetrical-Flow-Field-Flow-Fractionation coupled with light scattering techniques. At pH 7.0, the cross-linked CM depicts an increase in the apparent density and a spherical shape due to intra-molecular cross-linking. At pH 3.0, the loss of the colloidal calcium phosphate (CCP) leads to a change in the shape and the density, decreasing the viscosity of the CM dispersions. A master curve describing the evolution of apparent viscosity as a function of DP suggests that DP is the key parameter to tune the viscosity of acidified casein dispersions and predicts structural changes. [Display omitted] •TG-CM is not dissociated at pH 3 and it keeps a spherical shape.•TG-CM retains higher molar mass values after acidification in comparison to C-CM.•It exists a quasi-linear relation between the casein cross-linking and apparent density.