The influence of shear on the fermentation of Penicillium chrysogenum.

Intensive mechanical agitation in the stirred tank fermenters is known to improve mass and heat transfers. However, it can also detrimentally influence productivity and morphology of shear sensitive microorganisms. The influence of shear, meaning the mechanical forces resulting from the rotation of (multiple) turbine impellers, was investigated in batch fermentations of Penicillium chrysogenum Panlab P-1 using semidefined media. Experiments were carried out in three different scales of fermenter, 7 L, 150 L and 1500 L total volume, with the impeller tip speed ranging from 2.5 to 6.3 m/s. Throughout all fermentations, the dissolved oxygen concentration never fell below the critical values necessary for growth and penicillin production. Morphological measurements using image analysis showed that the main hyphal length, total hyphal length and hyphal growth unit increased during the fast growth period and then decreased to a relatively constant value dependent on the agitation intensity. The specific rate of penicillin production (qpen) and the average main hyphal length during the linear penicillin production phase decreased with increase in agitation speed. A higher degree of agitation promoted more rapid mycelial fragmentation and a higher branching frequency. The percentage of clumps was relatively high in all fermentations and was independent of the sample dilution. Comparison of the results from the three scales of fermenter shows that scale up cannot be based adequately on the impeller tip speed. A model based on the power input per unit mass (ϵ) was derived using experimental data from the three fermenter scales. It was found that for ϵ < 10 W/kg, qpen and the mean main hyphal length was practically independent of power input per unit mass, and for ϵ > 10 W/kg, qpen and the mean main hyphal length were proportional ϵ-0.3. Experimental data from previously published articles were analysed in the light of the model and were found to follow the same general trends as found in this work. However, the transition point from no dependency on ϵ to relatively weak dependency was strain dependent. The data obtained from this work were also found to be reasonably well correlated to the model based on the mycelial circulation through the zone of high energy dissipation (P/D3tc).