A method of preparing a recombinant polypeptide of interest includes fermenting a host cell being transformed with a recombinant expression system capable of bringing about secretion of a polypeptide of interest into the periplasm of said host cell. The polypeptide of interest is extracted from the periplasm by applying a continuous osmotic shock to the host cells contained in a fermentation medium. An apparatus for osmotically shocking cells includes a first reservoir containing cells in a first solution and a second reservoir containing a second solution, the first solution having a higher osmolarity than the second solution. A method for osmotically shocking cells using the first and second solutions is also disclosed. Also disclosed is a method of isolating a recombinant polypeptide of interest from a cell.