H-1-NUCLEAR MAGNETIC-RESONANCE STUDIES ON THE CONFORMATIONAL-CHANGES RELATED TO THE FOAMING PROPERTIES OF BETA-LACTOGLOBULIN

beta-Lactoglobulin (beta-LG) is a whey protein with foaming ability th at can be used as a food ingredient. The structural changes that occur during foaming cannot be easily assessed. In combination with deuteri um exchange, H-1-NMR allows new insight into the structural features o f beta-LG during foaming. beta-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 t he structure had remained intact. Protein in solutions that had produc ed stable foams showed extensive exchange. Protons of both Trp(19) and Met(107) exchanged with deuterium, indicating that most of the interi or had been exposed to solvent. Most of the beta-LG; structure was rec overed after collapse of the foam. In the early steps of foaming, appa rently only random coil or other exposed regions are involved in foam stabilization. More vigorous sheer stress forces induce further unfold ing of the beta-strands. This unfolding is suggested to be a dynamic, reversible equilibrium between an open and closed conformation of beta -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 discus sed.