Comparison Of Bioreactor Systems Operated At High Bacterial Cell Density For The Production Of Lactic Acid: Batch – CSTR – CSTR Cascade – Tubular Reactor
Subject of the work was the microbial conversion of lactose, an abundant processing by-product in the dairy industry, to lactic acid. Lactic acid serves as a preservative in many areas in various product sectors, but its potential applications go far beyond this. In industrial applications lactic acid as a renewable material can be applied as a building block for novel technical materials such as biodegradable films for packaging purposes in replacement of materials based on fossile raw materials.
Purpose of the work was to compare different bioreactor systems with regard to achievable lactic acid concentrations and volumetric productivities. The goal was to quantitatively assess the standard batch stirred tank reactor (STR) in comparison to various continuous reaction systems. Emphasis was put on a continuous stirred tank reactor (CSTR), a CSTR cascade comprised of seven individual stages, and a tubular reactor (TR) (d = 50 mm, L = 600 m). The continuous systems were equipped with a dynamic microfiltration (MF) cell retention system using rotating membranes to prevent extreme deposit formation or centrifugal separation and cell recirculation to the reactor front.
In STR systems the reaction product is produced as a function of time and the lactic acid concentration reaches high levels at the end of the fermentation, which may take 8-14 h, depending on cell density inoculated. In a CSTR, operated such that the substrate lactose is fully converted in steady state mode, high lactic acid concentrations are also reached. In comparison to a STR, however, the CSTR is affected by the product inhibition anywhere in the reactor and anytime. Thus, the CSTR volumetric productivity was shown to be considerably lower despite the fact that the cell density reached in steady state was much higher than in the STR. Therefore, in a CSTR high levels of end product concentrations cannot be achieved simultaneously with high volumetric productivities.
The CSTR cascade and the TR systems are distinctively different from a CSTR in terms of their concentration profiles over time and reactor length. In comparison to the CSTR, the CSTR cascade and the TR are affected by high lactic acid concentrations only in the rear sections of the reactors. Therefore, volumetric productivities in these systems were drastically higher than in the CSTR.
The work also included the screening of lactic acid bacteria cultures, the optimization of the medium composition to achieve high end product concentrations and volumetric productivities. Special emphasis was put on the cell retention system ensuring high flux levels, small effects of deposit formation and long term stability of high flux levels. Thus, it was possible to recirculate the mactic acid culture with little aqueous phase and lactic acid contained therein. It was shown to be of great importance to realize a low recirculation ratio of the aqueous phase including lactic acid and very low carbohydrate concentrations in order to maintain the spatial effects, i.e. the reduced impact of product inhibition described above. In other words, the challenge for cell retention systems in such reactor systems with spatial distribution of concentrations is to achieve high cell concentration factors so that only small amounts of already fermented medium are recirculated to the reactor front. Dynamic membrane systems were found to be capable to achieve this, because they do not require high crossflow volume throughputs and they can cope with high viscosities of the cell concentrates produced for recirculation. Volumetric productivities were thus increased by a factor of 10+.