Conformation of β-lactoglobulin studied by FTIR:: Effect of pH, temperature, and adsorption to the oil-water interface

The structure of mixed A and B genetic variants of beta-lactoglobulin (beta-Ig) in its native and denatured states has been studied by FTIR. The denaturation was achieved through either heating at various temperatures at pH 6 and 7 or by adsorption to the surfaces of oil droplets in oil-in-water emulsions. The time dependence of the structural changes of the adsorbed protein was also studied. It was found that beta-Ig was more resistant to heat denaturation at pH 6 than at pH 7. Conversely, the adsorbed protein appeared to be more deformable at pH 6 than at pH 7. Although heating at lower temperatures (70 and 80 degrees C) for a longer time causes extensive denaturation, as does heating at higher temperature (90 degrees C) for a short time, the structure of the denatured protein was different in the two cases. When denatured at lower temperatures, more beta-structure remained as indicated by the presence of bands around 1632 cm(-1) in the IR spectrum, whereas evidence for extensive intermolecular beta-structure was found on the sample heated at higher temperature, indicated by the strong adsorption band at 1684 cm(-1). This explains that although gelation of the protein can be achieved in both cases, the gel structure may be different. Denaturation of beta-Ig during heat treatment and upon adsorption appears to occur via similar intermediate structures, which begin with the loss of beta-sheet structure, but whereas heat denaturation generates more intermolecular beta-sheet and a small amount of unordered structure, adsorption of the protein to the oil-water interface induces a larger amount of unordered structure and a small amount of intermolecular beta-sheet. The denaturation of beta-Ig on the interface is a much slower process compared to heat denaturation; even though some changes are detectable shortly after the adsorption of the protein, more extensive denaturation occurs during storage of the emulsions for 72 h. (C) 1997 Academic Press.