PH-DEPENDENT MOLECULAR-WEIGHT CHANGES OF KAPPA-CASEIN GLYCOMACROPEPTIDE AND ITS PREPARATION BY ULTRAFILTRATION

k-Casein glycomacropeptide (GMP) is a biologically active peptide rele ased from k-casein by chymosin. Although the theoretical molecular wei ght (MW) of GMP was about 7 kDa, the apparent MW of GMP on gel filtrat ion was changed depending on pH. The distribution of the apparent MW r anged from 20 to 50 kDa at pH 7, but from 10 to 30 kDa at pH 3.5. G M P was rejected at pH 6.5 with ultrafiltration (UF) membranes with MW c ut off of 50 or 20 kDa, but was mostly permeated through the same UF m embranes at pH 3.5. We developed a simple procedure using UF for isola ting GMP from whey protein concentrate (WPC), taking the pH-dependent change in the apparent MW into account. The WPC solution was subjected to UF and diafiltration under acidic conditions using UF membrane wit h MW cut off of 50 kDa. After the permeate was neutralized, the second UF with the same UF membrane was carried out to retain GMP in the ret entate. A lyophilized GMP powder was recovered with 63 % yield and 81 % purity.