Effect of pH on the ternary solution behavior of beta-lactoglobulin.

Ternary phase diagrams (TPDs) were constructed for aqueous beta-lactoglobulin solutions containing ethanol and (NH4)2SO4 at pHs of 7, 5, and 3 for temperatures between 20 and 70 degrees C. The addition of (NH4)2SO4 generally led to the production of a reversible precipitate, a transformation that was not strongly influenced by temperature or pH. In contrast, at pH 7 and 20 degrees C, ethanol concentrations >12% led to the formation of a molten-globule structure, which gelled at protein concentrations >10%. Destabilization of beta-lactoglobulin structure occurred at lower ethanol concentrations as temperature was increased, until at 70 degrees C, all solutions that were previously liquid at room temperature had transformed into a gel. At pH 5.0, near beta-lactoglobulin's isoelectric point, demixing dominated, leading to the creation of either irreversible precipitates or a paste-like microgel. Elevated temperatures caused the previously liquid morphology to transform into either a reversible aggregate or microgel. Solution behavior at pH 3 had characteristics of what was observed at pHs 7 and 5. At moderated protein and ethanol concentrations, a paste-like microgel was observed, whereas at higher ethanol concentrations, beta-lactoglobulin formed a gel. This work demonstrates how small changes in protein structure at the molecular level can have a dramatic effect on macroscopic morphology.