¹H-Nuclear Magnetic Resonance Studies on the Conformational Changes Related to the Foaming Properties of ß-Lactoglobulin

¹ Department of Food Science and Technology, University of California, Davis 95616

ß-Lactoglobulin (ß-LG) is a whey protein with foaming ability that can be used as a food ingredient. The structural changes that occur during foaming cannot be easily assessed. In combination with deuterium exchange, ¹H-NMR allows new insight into the structural features of ß-LG during foaming. ß-Lactoglobulin was dissolved in D₂O and forced to foam. During foaming, the amide protons of exposed residues were exchanged for deuterium atoms, which do not appear in the ¹H-NMR spectrum. Protein in solutions that had produced unstable foams showed no exchange beyond that found in unfoamed controls, indicating that the structure had remained intact. Protein in solutions that had produced stable foams showed extensive exchange. Protons of both Trp¹⁹ and Met¹⁰⁷ exchanged with deuterium, indicating that most of the interior had been exposed to solvent. Most of the ß-LG structure was recovered after collapse of the foam. In the early steps of foaming, apparently only random coil or other exposed regions are involved in foam stabilization. More vigorous shear stress forces induce further unfolding of the ß-strands. This unfolding is suggested to be a dynamic, reversible equilibrium between an open and closed conformation of ß-strands that allows not only for interaction of the protein with the air phase but also for some secondary structure to be retained and the original structure to be recovered. Freeze-dry foaming is also discussed.

Key Words: foaming • whey proteins • ßbeta;-lactoglobulin structure • nuclear magnetic resonance

Submitted on December 5, 1997
Accepted on July 6, 1998