Enhanced in situ dynamic method for measuring KLa in fermentation media

The overall oxygen mass transfer coefficient (KLa) is often used as scale-up factor of fermentation systems. In fermenter scale-up, it is desired to achieve the same KLa values at the larger scale than the one that was obtained at a smaller scale during the development stage. It is therefore important to be able to measure KLa in situ during fermentation and to also determine the action to be taken to maintain its value at its design set point. These objectives can be obtained by measuring KLa using the dynamic method and enhancing the KLa information by immediately conducting a series of changes in agitation speed and/or aeration rate to determine the influence of these variables on KLa. This enhanced dynamic method is demonstrated with two filamentous microorganisms: Trichoderma reesei for the production of cellulase and Aspergillus niger for the production of citric acid. Two different types of bioreactor were used: a reciprocating plate bioreactor and a stirred (Rushton) bioreactor. It is shown that the proposed method can provide a simple way to measure the local variation of KLa and to adjust its value to its set point during the course of fermentation.