INFLUENCE OF THE PRECULTURE CONDITIONS ON THE PELLET SIZE DISTRIBUTION OF PENICILLIUM-CHRYSOGENUM CULTIVATIONS

The influence of shear forces on pellet formation and the productivity of Penicillium chrysogenum was investigated. By moderate increase of the shear forces in the precultures, a narrow pellet size distribution with a mean pellet diameter of less than 300 mum was obtained. At higher shear forces, the cells were damaged and the cell concentration decreased. Small pellets in the preculture yielded small pellets in the main culture. By reducing the mean pellet diameter from 1 mm to 0.6 mm in the main culture, the final product concentration doubled. The advantageous effect of the reduction of pellet size on the final product concentration was partly offset by the increase in cell concentration, which led to a decrease in the dissolved oxygen partial pressure in the cultivation medium because of the higher medium viscosity. As a result, the oxygen-limited region in the centre of the pellets increased. Because of lack of oxygen, the cells, starting from the centre of the pellets, gradually decomposed and died. Consequently, the larger pellets were broken down, and the mean pellet size decreased during the production phase. At higher cell concentrations, the death rate was higher than the growth rate, and the cell concentration therefore decreased during the production phase.