This work aims for determining the impact of different environmental conditions, such as pH, ionic strength (salt concentration) and the chemistry of the membrane surface (hydrophilic/hydrophobic character) on the structure of permeating proteins after ultrafiltration. In the permeation experiments reported in this paper, different solutions of a model protein - beta-lactoglobulin - at pH 3, 5 and 8 and salt concentrations of 1, 10 and 100 mM were processed with membranes of different molecular weight cut-off (10 and 30 kDa) and materials (regenerated cellulose-RC, and polyethersulfone-PES). The analysis of the structural alterations induced in the permeating molecules of P-lactoglobulin after ultrafiltration was performed using a methodology that combines the use of complementary fluorescence techniques: steady-state fluorescence and steady-state fluorescence anisotropy. From the results obtained it was concluded that permeation performed under pH values below the isoelectric point of beta-lactoglobulin, at high salt concentrations and using membranes with hydrophobic character allows for minimization of the impact of the membrane processing in the structure of beta-lactoglobulin. (C) 2007 Elsevier B.V. All rights reserved.