Precipitation of Casein from Acidic Solutions by Divalent Anions

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Abstract
Whole casein, [alpha]s-, [beta]- and k-caseins in acidic solutions are precipitated by low concentrations (about 0.005 M) of Na2SO4. 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 concentration of Na2SO4 required for precipitation is greater the lower the pH. The order of precipitation (mM of Na2SO4 giving 50% precipitation at pH 3.2) for the caseins (concentration: 0.4%) is a s (3.7), k(4.4), whole (5.6) and [beta] (8.3). The precipitation curves are S-shaped except that for k-casein; precipitation of this casein tends to be incomplete, even with considerable increase in the concentration of sulfate, particularly at lower pH values. [beta]-Lactoglobulin is not precipitated by Na2SO4. The polyanions such as heparin and polyphosphates precipitate both casein and [beta]-lactoglobulins.

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