Microparticulated whey protein (MWP) concentrate was kindly supplied by Arla Foods Ingredients (Viby J, Denmark) and used without further treatment. All chemicals used, including hydrochloric acid,sodium hydroxide, sodium phosphate monobasic, sodium phosphate dibasic, and Fluorescein isothiocynate (FITC) isomer I, were purchased from Sigma-Aldrich (St Louis, MO, USA) and were of analytical grade.Double distilled water was used as the solvent to prepare buffer solutions and colloidal suspensions.