Scale-up agitation criteria for Trichoderma reesei fermentation

Scale-up of aerobic fungal fermentation processes still remains a challenging issue for the biotechnology industry. This difficulty arises due to the complex interactions between operating conditions (agitation, aeration, etc.), the physicochemical state of the broth (viscosity, the dissolved oxygen concentration, etc.) and the biology of fungi (growth, production, morphology, etc.). Because of their size, filamentous fungi are affected by fluid dynamic stresses but quantification of this complex parameter is a difficult task. In general, indirect criteria are used for the effect of fluid dynamic stresses on scale-up (tip speed, power draw or the energy dissipation/circulation function (EDCF)). In order to better understand the impact of such criteria on the fermentation of the fungus Trichoderma reesei, a wide range of agitation conditions has been explored. The morphology of T. reesei fungus, its specific growth rate and the rheological properties of the broth have all been measured both at bench scale (∼2.5 L) and for the first time, at a typical commercial scale. These three aspects of the fermentation at both scales were then compared with respect to tip speed, specific power and EDCF. This work has shown that tip speed as a correlator of any of these parameters is totally ineffective whilst the EDCF is clearly the best for extrapolating laboratory data to the commercial scale.