Comparison of surface and foaming properties of soy lipophilic protein with those of glycinin and β-conglycinin

Soy lipophilic protein (LP) is considered to form a major fraction of soy protein isolate, in addition to β-conglycinin (7 S) and glycinin (11 S). LP consists of phospholipid-protein complex and phospholipid-free protein molecules. Surface and foaming properties of LP were compared to those of 7 S and 11 S. These soy protein samples were prepared under mild conditions in order to avoid thermal denaturation, which enabled us to study the effects of heat treatment on surface and foaming properties of the proteins. Without heat treatment, the surface and foaming properties of LP were superior to those of other soy protein fractions, suggesting the phospholipid-protein complex in LP can produce fine bubbles with high drainage stability. Despite low surface and foaming properties of non-heated 7 S and 11 S, heat treatment improved the surface and foaming properties of 7 S and 11 S dramatically, which could be relevant to heat-induced changes in particle size, zeta-potential and surface hydrophobicity of 7 S and 11 S molecules. On the other hand, the foaming ability of LP was declined, but the foam stability was increased to maintain the foam volume up to the final stage of observation, by heating. In the foam stabilized by heated LP, medium sized bubbles as well as fine bubbles were produced. The improved foam stability may be attributed to the combination effect of the delayed drainage from the lamellar phase of fine bubbles and the increased resistance to the bursting of medium sized bubble.