alpha(s)-Casein-PE6400 mixtures: Surface properties, miscibility and self-assembly

Surface properties, miscibility and self-assembly of mixtures of a food-grade as-casein and the triblock copolymer PE6400 (PEO13-PPO30-PEO13) were examined. The properties at the surface were determined by surface pressure measurements for a 1:1 molar mixture. Comparison of the measured with the calculated isotherms show attractive interactions at surface pressures above 9 mN/m. The miscibility gaps of solutions containing 0.004-0.2 mmol/l alpha(s)-casein and 0.02-0.1 mol/l polymer were examined. It was found that a one-phase region exists at distinct mixing ratios and temperatures. Comparison of the cloud points of mixtures of alpha(s)-casein and PE6400 with pure alpha(s)-casein showed that the presence of the triblock copolymer enhanced the solubility of the protein. The zeta-potential of the alpha(s)-casein solution decreased by addition of PE6400 to zero. Our results thus suggest that alpha(s)-casein and PE6400 are miscible. The results of the cloud point and zeta-potential measurements were explained by formation of a mixed aggregate where the PPO chains are anchored inside the hydrophobic part of the alpha(s)-casein while the PEO chains cover the charged hydrophilic part of the alpha(s)-casein thereby leading to an increase of the cloud point and a decrease in zeta-potential. This is in agreement with the attractive interactions between alpha(s)-casein and PE6400 as observed via surface pressure measurements at the surface. (C) 2014 Elsevier B.V. All rights reserved.