Fuente: WIPO "biotechnology" PCT No. PCT/GB97/00669 Sec. 371 Date Sep. 11, 1998 Sec. 102(e) Date Sep. 11, 1998 PCT Filed Mar. 12, 1997 PCT Pub. No. WO97/33973 PCT Pub. Date Sep. 18, 1997A process of culturing a microorganism in a culture medium in which process the addition of feed medium is controlled by using the production of a by-product as a measure of the culture conditions, characterized in that the by-product is an electrically charged metabolite produced by the microorganism, and in that the production of the metabolite is monitored by measuring the conductance of the culture medium. The metabolite may be acetate and the microorganism may be yeast which is genetically engineered to produce a desired polypeptide.