The aim of this study was to elucidate the mechanism by which protein molecules become denatured in foam. It was found that damage to the protein is mainly due to surface denaturation at the gas-liquid interface. A fraction of the molecules adsorbed do not refold to their native state when they desorb. The degree of denaturation was found to correlate directly with the interfacial exposure, which, for mobile or partially mobile interfaces, is increased by drainage. Experiments with two different proteins showed that, under the conditions of the tests, around 10% of BSA molecules which had adsorbed at the surface remained denatured when they desorbed, For pepsin the figure was around 75%. Oxidation, which was previously thought to be a major cause of protein damage in foam, was found to be minimal. Neither do the high shear stresses in the liquid bulk encountered during bubble bursting cause denaturation, because energy is dissipated at a much greater length scale than that of the protein molecule.