Effect of polysaccharide-protein interactions on the multi-scale structure of hybrid micellar casein-xanthan gum systems

This study reports on the effect of pH and polysaccharide concentration (protein:polysaccharide ratios of 95:5, 85:15 and 75:25) on the interactions established between xanthan gum (XG) and micellar casein (MC), investigating the impact on the type of structures formed at the micro- and nanoscale. At pH ≥ 6, the micellar casein formed stable suspensions where micelles with diameters of ca. 130–150 nm showed a compact inner structure stabilised by colloidal calcium phosphate (CCP) nanoclusters, while at pH ≤ 3, a more loosely packed interconnected network structure, stabilised by protein-protein interactions, was noted. The addition of XG, even at low MC:XG ratios (95:5), resulted in increased stability of the system, preventing the formation of micelle aggregates at the microscale, and reducing the pH range at which casein precipitated close to the isoelectric point. For all the XG loadings, coacervates were formed at pH = 2–3 due to polysaccharide-protein electrostatic interactions, leading to separation into a solid gel-like phase composed of polysaccharide-protein fibrillar structures and a liquid phase rich in protein. At the nanoscale, XG was mostly coating the micelles and filling in the inner water channels when added at MC:XG ratios ≤85:15, while the higher polysaccharide content at the ratio 75:25 allowed to form an interconnected structure. Therefore, by adjusting pH and XG concentration, it is possible to improve casein stability, reduce agglomeration and manipulate the size of protein-polysaccharide complexes formed. This will be of great relevance for the development of novel food formulations with improved stability and modulated sensory and nutritional properties.

This work was financially supported by the projects RTI2018-094408-J-I00 and RTI-2018-094268-B-C22, funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. Synchrotron experiments were performed at NCD beamline at ALBA Synchrotron with the collaboration of ALBA staff (proposal 2022025569). The publication was written as a result of the author's internship in Instituto de Investigación en Ciencias de la Alimentación, co-financed by the European Union under the European Social Fund (Operational Program Knowledge Education Development), carried out in the project Development Program at the University of Warmia and Mazury in Olsztyn (POWR.03.05. 00-00-Z310/17).

Peer reviewed