Strategies for the continuous production of ligninolytic enzymes in fixed and fluidised bed bioreactors

Production of ligninolytic enzymes by white-rot fungi is a phenomenon typically associated to the secondary metabolism the main drawback of which is the limited yield of the enzyme obtained under growth-limiting conditions and the morphological behaviour of these filamentous microorganisms while growing. An efficient production system is needed for the promising applications of these enzymes in bioprocesses such as detoxification of effluents and biobleaching of kraft pulp. Several bioreactor configurations were considered which were operated in batch or semi-continuous modes. These operational conditions permitted high titres of enzymatic activity, however, the extent of production was limited. A different modus operandi is then to be proposed to overcome these limitations. The present work reviews the state of production of ligninolytic enzymes and proposes a new method to achieve continuous production of manganese peroxidase (MnP). The method comprises a feeding policy based on controlled feed rates of nutrients and the use of an adequate bioreactor configuration. They were both key factors to attain and sustain continuous production of MnP. The feeding of limited concentrations of nutrients kept secondary metabolism in steady-state conditions and limited the fungal growth. The configuration of two conventional bioreactors (fluidised bed and fixed bed bioreactors) was modified with the introduction of pulsation of gas. Pulsation proved to be a good alternative to enhance nutrients and oxygen transfer as well as to maintain fungal cultures as independent bioparticles both for pellets and foam blocks of mycelia. Other environmental factors such as a Mn2+ concentration of 5 mM and high oxygen tension enhanced MnP production. The hydraulics of the bioreactor also had effect on MnP production, the optimum corresponded to a plug how model with partial mixing and an operating HRT of 24 h. This policy facilitated long operation periods, obtaining higher productivity rates than the best reported to date. (C) 1998 Elsevier Science B.V. All rights reserved.