Frontal microfiltration and grazing incidence small angle X-ray scattering (GISAXS) are used to investigate how beta-lactoglobulin (beta-Lg) influences the deposit structure of casein micelles (CM). Filtration experiments show that the mean specific deposit resistance decreases with increasing concentration of beta-Lg in the suspensions of CM. GISAXS is used to probe surface-near structures of deposits on a nano-meter scale. A core-shell model used to compute the scattering from CM shows agreement with measurements on films prepared by spin-coating on silicon wafers. The core of CM becomes more compact as a result of preferential binding of beta-Lg on the micellar surface. Scattering functions of deposits prepared by filtration on micro-sieves show power-law behavior characteristic for mass-fractal structure. This structure consists of dense packed CM with interconnected beta-Lg molecules on the surface. The resulting network of CM is porous and explains the reduced mean specific deposit resistance, as well as the increased permeate flow, in the presence of beta-Lg. (C) 2014 Elsevier B.V. All rights reserved.