Hydrolysis of beta-lactoglobulin (beta-Lg), genetic variant A, using a serine protease specific for glutamic and aspartic acid residues from Bacillus licheniformis (BLP), resulted in improved foam overrun and foam stability. Limited hydrolysis (19-26% hydrolysed beta-Lg) led to a more rapid increase in the viscoelastic properties of air/water interfacial films and a concomitant increase in foam overrun compared with intact beta-Lg, presumably due to increased exposure of hydrophobic areas. The increased exposure did not, however, cause formation of an interfacial layer with increased viscoelastic properties. More extended hydrolysis (86% hydrolysed beta-Lg) resulted in a higher initial overrun than the unhydrolysed sample and the best foam stability. The interfacial elasticity and viscosity, though, was the lowest observed. Thus, high maximum values of these interfacial properties are not necessary prerequisites for formation of a voluminous and stable foam.