Characterization of the foaming properties of lysozymes and alpha-lactalbumins: a structural evaluation

The foaming properties of three lysozymes (human, turkey-egg and hen-egg) and two alpha-lactalbumins (bovine and hum an) have been assessed using a micro-conductivity method. Despite the very close structural similarities between these five proteins, major differences in foaming characteristics were found. For the three lysozymes, the order of increasing foam expansion and stability was turkey < hen < human. Although the two alpha-lactalbumins displayed similar foaming properties, these contrasted sharply with those of the lysozymes. For example. in the case of the lactalbumins measurable foams were obtained at concentrations as low as 0.05 mg/ml, whereas the best lysozyme required a concentration of 1 mg/ml. The foam expansion and foam stability of alpha-lactalbumin were largely independent of protein concentration, but the reverse was true for lysozyme. The effect of pH and protein con formation on the foaming of bovine alpha-lactalbumin and hen-egg lysozyme was. also studied. An explanation for the differences in foaming behaviour between the two protein families is proposed, based on our present understanding of the mechanism of protein foaming, and the structure of these proteins. The interpretation is extended to the "molecular level". Inspection of the aligned sequences of the homologous lysozymes and alpha-lactalbumins highlighted 25 positions in the consensus sequence that are postulated tentatively as playing a centra l role in the foaming properties of these proteins.