EFFECT OF SODIUM-CHLORIDE ON THE SOLUBILITY OF CASEINS

The functionality of caseins in food systems depends on their solubility. The solubilities of whole casein, alphas1-casein, beta-casein, and dephosphorylated whole casein were measured as the pH was adjusted from 7 to 2 with HCl. When NaCl was added before PH adjustment, solubilities of whole casein at pH 2.5 decreased by 25, 40, 85, and 98% at NaCl concentrations of .01, .05, .1 and .2 M, respectively. Likewise, solubilities of dephosphorylated whole casein decreased by 50%, alpha(s)1-casein by 30%, and beta-casein by 90% in . 1 M NaCl. When .01 and .05 M NaCl were added to whole casein solutions after pH adjustment, effects on solubility were slight; however, . 1 and .2 M NaCl decreased solubility by 30 and 95%, respectively. When NaCl was added to casein solutions, pH decreased if the original solution was above the isoionic point, indicating Na ion binding; however, at pH between 5 and 3, the pH of the solution increased when NaCl was added. The pH shift upon addition of NaCl to dephosphorylated casein was less than with native caseins, suggesting that Na ion binding by phosphate and acid groups occurred in casein. Although NaCl is commonly used in food processing, these unusual solubility effects should be considered when NaCl is used with casein.