Abstract: This disclosure relates to a method which involves the steps of: (a) providing an aqueous solution comprising a protein and a polyalkoxy fatty acyl surfactant of general formula I wherein R1—C(?O) is a fatty acyl group, R2 is H or a substituted or unsubstituted hydrocarbyl group, X1 is S, O or NH, X2 is S, O or NH, n is 0 or an integer of 1-5, R3 is a polymeric group comprising polymerized units of general formula II and III (b) contacting the aqueous solution with a separation membrane, and (c) subjecting the aqueous solution to a diafiltration step and/or to an ultrafiltration step to produce a retentate product which is an aqueous solution comprising the protein, whereby the compound of formula I reduces aggregation of the protein in method steps (a)-(c) and whereby the compound of formula I passes through the separation membrane in step (c).