A method for treating amino acid based substrates comprising the step of applying to such amino acid based substrate at least one polyorganosiloxane having an average number of 2 to 1000 siloxy units selected from the siloxy groups of the formulas: wherein R is selected from R 1 and R F3 , wherein R 1 is selected from organic groups, R F3 is selected from R F4 and R F5 , wherein R F4 is selected from organic groups different from R 1 which contain at least one functional group F4 selected from - a di-, or trihydroxy-substitued aromatic group, R F5 is selected from organic groups different from R 1 which contain at least one functional group F5 selected from: - alkoxy silyl group, - amino group, - ammonium group, - phosphonium group, - epoxy group, - carbonate group, - urethane group, - isocyanate group, including blocked iscocyanate group, - urea group, - amido group, - aldehyde group, - acetale or half acetale group, - Schiff-Base or enamine group, - zwitterionic group, - carboxylic acid or carboxylate group, - sulfonic acid or sulfonate group, - sulfuric acid half ester or sulfate group, - phosphoric acid ester or phosphate group, - phosphonic acid ester or phosphonate group, - phosphorous acid ester or phosphite group, - xanthogenate/xanthogenate ester group, - thiosulfato group, - mercapto group, - saccharide group, and - polyether group with up to 60 carbon atoms, with the proviso that R comprises at least one group R F4 . R is selected from R 1 and R F3 , wherein R 1 is selected from organic groups, R F3 is selected from R F4 and R F5 , wherein R F4 is selected from organic groups different from R 1 which contain at least one functional group F4 selected from - a di-, or trihydroxy-substitued aromatic group, - a di-, or trihydroxy-substitued aromatic group, R F5 is selected from organic groups different from R 1 which contain at least one functional group F5 selected from: - alkoxy silyl group, - amino group, - ammonium group, - phosphonium group, - epoxy group, - carbonate group, - urethane group, - isocyanate group, including blocked iscocyanate group, - urea group, - amido group, - aldehyde group, - acetale or half acetale group, - Schiff-Base or enamine group, - zwitterionic group, - carboxylic acid or carboxylate group, - sulfonic acid or sulfonate group, - sulfuric acid half ester or sulfate group, - phosphoric acid ester or phosphate group, - phosphonic acid ester or phosphonate group, - phosphorous acid ester or phosphite group, - xanthogenate/xanthogenate ester group, - thiosulfato group, - mercapto group, - saccharide group, and - polyether group with up to 60 carbon atoms, - alkoxy silyl group, - amino group, - ammonium group, - phosphonium group, - epoxy group, - carbonate group, - urethane group, - isocyanate group, including blocked iscocyanate group, - urea group, - amido group, - aldehyde group, - acetale or half acetale group, - Schiff-Base or enamine group, - zwitterionic group, - carboxylic acid or carboxylate group, - sulfonic acid or sulfonate group, - sulfuric acid half ester or sulfate group, - phosphoric acid ester or phosphate group, - phosphonic acid ester or phosphonate group, - phosphorous acid ester or phosphite group, - xanthogenate/xanthogenate ester group, - thiosulfato group, - mercapto group, - saccharide group, and - polyether group with up to 60 carbon atoms, with the proviso that R comprises at least one group R F4 . <EOS>