The basic and most important step of a biochemical fermentation process is the preparation of a culture medium (nutrient broth). The production of a particular substance in large amounts should be the final result of those processes. Here, the fungus grows on the surface of the culture medium which can produce our particular substance. Every type of fungus grows in a specific way in a different type of medium! This point could be problematical if we try to find an acceptable medium which is in the position to produce our desired product in huge amounts. The whole phase of development about the growth and extraction of Penicillin is a good example to explain how important the choice of the culture medium is.
Howard W. Florey has put a lot of effort into the surface-culture method during the very early time of the development of the production of Penicillin in Oxford. No-one to this point of time had any real idea how they could produce large amounts of this "substance" or which special combination of chemicals should be used for the culture medium to get the best yields and results. What followed was a long series of experiments with different chemicals and culture vessels to find out what would be the best medium and the most suitable vessel for the growth of the Penicillium notatum fungus to get high active Penicillin material. The mould can grow and produce the Penicillium spores on a variety of different types of culture media, but not every medium is suitable for the Penicillium fungus because it will produce the spores only under stress in a particular synthetic environment. After the incubation time of about 8 to 10 days the medium can be replaced with fresh medium. (refresh cycle)