A method of providing polypeptide preparations having a reduced content of undesired enzymatic side activities, the method comprising subjecting a medium containing a desired polypeptide such as an enzyme, a pharmaceutically active or immunologically active polypeptide to a pH of less than 2 for a period of time that is required to inactivate the side activities whilst retaining the activity of the desired polypeptide. The method is useful for providing milk clotting enzyme products including rennets or coagulants based on chymosin or pepsin or microbial aspartic proteases e.g. derived from bacterial species and species of filamentous fungi.