Precipitation of Casein from Acidic Solutions by Divalent Anions

Abstract

Whole casein, αs-, β-, and κ-caseins in acidic solutions are precipitated by low concentrations (about 0.005m) of sodium sulfate. The presence of sulfate in casein solutions broadens the isoelectric precipitation zone on the acid side with no change on the alkaline side. Other divalent anions such as pyrophosphate also precipitate the caseins; chloride, phosphate, and citrate (pH 3.2) do not precipitate caseins in this concentration range. The precipitation is markedly pH-dependent; the lower the pH the greater the concentration of sodium sulfate required for precipitation. The order of precipitation (mm of sodium sulfate giving 50% precipitation at pH 3.2) for the caseins (concentration: 0.4%) is αs, (3.7), κ (4.4), whole (5.6), β (8.3). The precipitation curves are S-shaped except that for κ-casein; precipitation of this casein tends to be incomplete, even with considerable increase in the concentration of sulfate, particularly at lower pH values. Sodium sulfate does not precipitate β-lactoglobulin. The polyanions such as heparin and polyphosphates precipitate both caseins and β-lactoglobulin.