Zonal Ultracentrifugation of β-Lactoglobulin and κ-Casein Complexes Induced by Heat

A zonal ultracentrifuge was used to isolate the heat-induced conlplexes of 8lactoglobulin and K-casein. The complexes formed a t 80 C in sodium cacodylate buffer, p H 6.65, ionic strength 0.08, varied in size and composition, but moved as a single band in starch-gel electrophoresis. The maximum ratio in the complex was estimated to be 3 P-lactoglobulin : l K-casein. When a 1 : 1 mixture was heated at 110 C the major component banded in 6.9% sucrose, had a sedimentation coefficient of 2.35, and moved as a single band in starch gel. At 140 C, K-casein was degraded extensively, and no complex could be detected. The conmlex formed at 90 C from a 1 : l mixture in a synthetic serum containing calcium had a sedimentation coeficient of 39. For other mixtures in the presence of calcium the unreacted proteins could not be quantitatively separated from the complex. Zonal ultracentrifugation of heated skimmilk and colloidal calcium phosphate free milk failed to show a discrete complex peak.