The main purpose of this study was to determine the extent to which the process of foaming changes the structure of ob beta-lactoglobulin (beta-Lg). Beta-Lg unfolded while existing as a foam, but changes in conformation were reversible upon collapse of the foam. A plot of the overrun (a measure of fair incorporation in the foam) for beta-Lg foams (pH 7) versus temperature (3-45 degree C) was a sigmoidal curve that resembled a two-state denaturation curve. Foaming properties were influenced by temperature and pH, which was attributed to differences in protein structure, in diffusion rate and in the number of molecules in solution for beta-Lg at pH 9 as compared to pH 7. There was a strong correlation (r > 0.9) between the secondary structure of beta-Lg in solution and the observed foaming properties. A direct link between the structure of beta-Lg in solution and its foaming properties was established.