Using ternary phase diagrams to characterize the solution behavior of beta-lactoglobulin

The solution behavior of [beta]-lactoglobulin ([beta]-Lg) at pH 7, 5 and 3 as a function of temperature, protein and solute concentration was studied. All possible morphologies for [beta]-Lg in water and ethanol or (NH4 )2SO4 were represented in ternary phase diagrams in the full range of each component. The effect of solute addition and temperature on phase behavior was found to be different according to pH. The characteristics of the intrinsic species involved in the formation of the different morphologies were studied in dilute solutions. It was found that dimers tended to dissociate into monomers as protein concentration decreased. Ethanol or (NH4) 2SO4 induced a conformational change on dissociated monomers that was accompanied by protein swelling. Changes in protein association were correlated with the rheological and thermal behavior of [beta]-Lg. The formation of gel and paste morphologies was compared using rheological analysis. Ethanol addition decreased while (NH4)2SO4 increased the thermal stability of [beta]-Lg and this behavior could explain several observed morphology changes. The microstructure of heat-induced gels as determined by scanning electron microscopy, also showed differences according to pH and solute addition. The solution behavior of [beta]-Lg and the previously studied proteins, ovalbumin, bovine serum albumin and gelatin were compared. From this work, it can be concluded that protein solution behavior is a complex function of a wide range of structural parameters.