Towards the integration of fermentation and crystallisation - A study on the production of L-phenylalanine

Industrial biotechnology can be defined as the utilization of living organisms or their enzymes for the industrial production of food, beverages, commodity chemicals and pharmaceuticals, among others. A biotechnological production process, so-called bioprocess, mostly comprises product formation (e.g. by fermentation) and product recovery and purification. In order to be commercially interesting, industrial fermentation processes should result in high product concentrations. High product concentrations during fermentation translate into high productivity and are also linked to simpler and more efficient recovery and purification processes. Although high product concentrations can be achieved by several tools provided by modern biotechnology, factors such as product toxicity to the micro-organism, product degradation or control mechanisms like feedback repression continue to be limiting. In these cases, it may be beneficial to remove the product from the vicinity of the micro-organism as soon as it is being formed. This approach, known as in situ product removal or in situ product recovery (ISPR), has been described in literature with techniques such as adsorption, extraction and membrane technology. Recently, product recovery by crystallisation has received att ention for products that are commercialised in crystal form. In such cases, product recovery by crystallisation might not only lead to higher fermentation productivities (by reducing product toxicity and/or degradation) but also to simpler downstream operations (by recovering the product already in crystal form), without requiring the use of auxiliary materials. The feasibility of such a concept should be evaluated at early stages of process development. At early stages, however, the available information and experimental data are usually limited. Generic methods or rules of thumb, derived from the study of model systems, should contribute in this evaluation. In this thesis, the production of the amino acid L-phenylalanine (Phe) by Escherichia coli is used as a model system. This thesis shows that the concept of product recovery by crystallisation during fermentation to improve the production of Phe is viable. The approach used in this thesis allowed exploring the possibilities and limitations of the process by means of simple experiments with aqueous model solutions and black-box models. Although this approach can be used for evaluating the potential of product recovery by crystallisation during fermentation in other processes, it fails in taking into account the particularities of the system. These particularities can strongly impact the feasibility of the integrated process