Precipitation of Caprine and Bovine Caseins from Acidic Solutions by Sodium Polyphosphate: Influence of pH and Urea. Utilization for Separation of αs- and κ-caseins

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Abstract
The caseins, both caprine (goat) and bovine, are precipitated by relatively low concentrations (approximately 4 mg/10 ml for a 0.5% casein solution at pH 2.0) of sodium polyphosphate. The casein precipitation is pH dependent (1.5 mg/10 ml precipitates 50% at pH 2.0, whereas only 0.5 mg is required at pH 3.5), but this pH dependence is much less than that shown by sulfate precipitation of the caseins. [kappa] -Casein in the presence of 2.0 M urea is not precipitated by polyphosphate; this concentration of urea has a negligible effect on the precipitation of as-casein. This behavior provides a means for separating [alpha]s- and [kappa]-caseins. Separation is facilitated by reducing the [kappa] -casein to the monomeric form with mercaptoethanol. The caprine and bovine -caseins cannot be distinguished in the stabilization of [alpha]s-casein against precipitation with Ca ions.