The precipitation of proteins by carboxymethyl cellulose

Carboxymethyl cellulose (CMC) formed insoluble complexes with beta-lactoglobulin, bovine serum albumin and Na caseinate. Maximum precipitation of the beta-lactoglobulin-CMC complex occurred at pH 3-2, whereas maximum precipitation of the bovine serum albumin. The percentage of protein precipitated by CMC decreased with increasing ionic strength of the solution, the rate of decrease being least for bovine serum albumin. At a given ionic strength, more protein was precipitated by CMC of high degree of substitution than by CMC of low degree of substitution. The change in pH (delta pH) occurring on mixing CMC and unbuffered protein solutions, each initially at the same pH, was measured. delta pH was negative for beta-lactoglobulin-CMC mixtures over the pH range 7--2 (minimum at pH 5-5). For bovine serum albumin--Cmc and Na caseinate--CMC mixtures, delta pH was positive between pH 7 and 3-2 (maximum at pH 4-5), zero at pH 3-2 and negative between pH 3-2 and 2-0 (minimum at pH 2-8).