The first penicillin, penicillin G, is produced using a submerged fermentation process in 40,000 - 200,000 litre fermentors. Penicillin production is a highly aerobic process and efficient aeration is necessary. Penicillin is a typical secondary metabolite. Very little penicillin is produced during the growth phase, but once the carbon source has been exhausted, the penicillin production phase begins. The production phase can be extended for several days by adding various culture medium components.
Certain culture media produce high penicillin yields while others are effective for growth but lead to little production. A major ingredient of most penicillin production media is corn steep liquor. This ingredient serves as the nitrogen source and source of other growth factors. The carbon source is generally lactose which is not used as rapidly as glucose and does not cause catabolite repression. However, glucose can also be used as a carbon source. When it is used, it must be added slowly to avoid catabolite repression. The acyl side chain of penicillin G is the phenylacyl moiety. Yields of penicillin G can be increased by adding phenylacetic acid or phenoxyacetic acid continuously as a precursor. Production of a penicillin with another acyl side chain can be increased by adding its appropriate precursor.
Penicillin is excreted into the medium and less than 1 percent remains in with the fungal filaments. The fungal mycelium is removed by filtration and the pH of the medium is lowered. The antibiotic is extracted from the filtered broth with amyl or butyl acetate. After concentration into the solvent, the antibiotic is back-extracted into an alkaline aqueous medium, concentrated further and crystallized. Highly purified penicillin can be obtained this way.