In the manufacture of penicillin, fermentation is the preferred route. The fermentation substrate consists mainly of the sucrose found in corn steep liquor and lactose together with minerals and phenylacetic acid. The preferred mold is Penicillium chrysogenum and the first stage of the process is the transfer of the mold to the fermenter from the test tube where it has been stored..

            The whole process is aerobic (it requires air). The fermentation is carried out over 4 or 5 days at 25-27C. This gives a yield of about 0.5% penicillin. The mycelium, the mold cells plus insoluble metabolites, is filtered off in a coated drum filter. The filtrate is adjusted the pH 2 with sulphuric acid. At this pH, the penicillin exists as an undissociated acid; consequently, it is soluble in organic solvents. After extraction the solution is decolourised and impurities are removed by activated carbon in a second drum filter. The penicillin is then precipitated by addition of a solution of potassium acetate. This converts the penicillin into a negative ion, which makes it insoluble in the organic solvent. Inn the final steps the penicillin is crystallized and dried.