First and Second Cycle Casein in Milk

Abstract

THE component of the casein complex which, according to Waugh and Von Hippel1, is responsible for the colloid chemical stability of the casein micelles in milk is k-casein. The same authors have also given its preparation, as follows. The casein in skim milk is precipitated by addition of calcium, and by subsequent removal of calcium from the precipitate a solution of the so-called ‘first cycle casein’ is obtained. This first cycle casein contains β-casein and the α/k-complex. Reprecipitating first cycle casein with 0.25 M calcium chloride at 37° C. results in the splitting of the α/k-complex and hence in the chemical instability of the casein micelles in the colloid. The precipitate was named ‘second cycle casein’, and it no longer contains the protective k-casein in stabilizing quantities. The latter can be isolated from the remaining supernatant of second cycle casein. In its physico-chemical properties, such as electrophoretic mobility, diffusion and ultra-centrifugation characteristics, k-casein was shown to resemble α-casein closely.