Effect of concentration, temperature and plasticizer content on rheological properties of sodium caseinate and sodium caseinate/sorbitol solutions and glass transition of their films

The rheological properties of sodium caseinate film-forming solutions were studied in terms of concentration, temperature and plasticizer content. The shear stress–shear rate plot showed typical Newtonian behaviour in all of the studied systems. The effect of temperature on the viscosity was described by the Arrhenius equation, where the activation energy values obtained varied from 7.5 to 12.1 kJ mol−1 as the concentration changed from 10.5 to 13.0% (w/w), respectively. The viscosity increased with the protein concentration and followed a power law equation. The presence of plasticizer in the sodium caseinate film-forming solutions decreased the viscosity and the partial specific volume due to the protein dehydration and also induced a more ordered structure, as observed by circular dichroism spectroscopy. In consonance with the solution behaviour, the glass transition temperature, for solid films of sodium caseinate, decreased with the sorbitol content. Apparently, the hydrogen bonding formation between the sorbitol and the amino acid side chain decreased the protein-protein interaction, leading to a higher mobility of the protein chain.