The invention concerns a simple ion-exchanging industrial process which consists in treating a liquid lactic raw material containing glycomacropeptide in the presence of a weak anionic resin to obtain an improved protein product useful in food processing and said glycomacropeptide which is selectively adsorbed on the resin, then eluted in said resin. Before being treated with resin, the liquid raw material is decationized so that its pH has a value between 1 and 4.5.