A method of mixing including providing a flexible container (4) which comprises at least two components, assembling the components from outside the flexible container to form a centrally disposed magnetic driven shaft (6) with at least one impeller (10), whereby the integrity of the flexible container is maintained, filling the flexible container having the centrally disposed magnetic driven shaft with one or more ingredients of the contents to be mixed; engaging a magnetic element (8) of the centrally disposed magnetic driven shaft with an external magnetic drive element; analyzing the contents of the flexible container using at least one sensor (24) or a sampling line (26) on the flexible container; and draining the contents of the flexible container through a drain port (18) at the bottom of the flexible container.