DOT PH DOT- s 24 48 72 96 120 Time(h) Schematic presentation of a penicillin production process.Initially,when sugar is available in excess,pH decreases because of production of organic acids.When the sugar concentration is exhausted after about 24 h most of the pH increase is attributed to the consumption of some of the organic acids of the medium,which gives a pH increase.When pH reaches a suitable value in the range 6-7 a continuous feed of sugar is started,which stabilises pH.Further pH control is achieved by titration.The onset of energy starvation initiates the penicillin production,which is first quite constant but eventually declines due to reduced specific production rate and increased penicillin degradation.DOT is a function of the sugar feed rate and the KL a,which is strongly dependent on the viscosity of the broth