Frederike Carstensen

A membrane stirrer for product recovery and substrate feeding

During fermentation processes, in situ product recovery (ISPR) using submerged membranes allows a continuous operation mode with effective product removal. Continuous recovery reduces product inhibition and organisms in the reactor are not exposed to changing reaction conditions. For an effective in situ product removal, submerged membrane systems should have a sufficient large membrane area and an anti‐fouling concept integrated in a compact device for the limited space in a lab‐scale bioreactor. We present a new membrane stirrer with integrated filtration membranes on the impeller blades as well as an integrated gassing concept in an all‐in‐one device. The stirrer is fabricated by rapid prototyping and is equipped with a commercial micromesh membrane. Filtration performance is tested using a yeast cell suspension with different stirring speeds and aeration fluxes. We reduce membrane fouling by backflushing through the membrane with the product stream. Biotechnol. Bioeng. 2015;112: 331–338.

In Situ Product Recovery of Single-Chain Antibodies in a Membrane Bioreactor

The demand for biopharmaceuticals, such as monoclonal antibodies, has risen significantly over the last years. To be competitive, continuous production processes that yield consistent product quality and an economic advantage are desirable. In this study, an in situ product recovery process is described, involving use of submerged membranes to recover single‐chain antibodies from a continuous fermentation of Hansenula polymorpha yeast cells. Reverse‐flow diafiltration (RFD) was applied to prevent cake‐layer formation. Optimal flux ranges for this process could be identified by a systematic flux step method. The RFD process was optimized, preventing mixing of permeate and unreacted substrate: the space–time yield of antibodies using RFD could be tripled. Increase of the fouling related transmembrane pressure was below 45 Pa min−1 for all applied dilution rates, indicating that the filtration process was stable. The membrane as well as the feeding mode of RFD did not influence cell viability nor product concentration. A wide range of dilution rates was successfully tested, demonstrating that this process is suitable for industrial applications. Biotechnol. Bioeng. 2014;111: 1566–1576.

In situ cell retention of a CHO culture by a reverse-flow diafiltration membrane bioreactor

Heterogeneities occur in various bioreactor designs including cell retention devices. Whereas in external devices changing environmental conditions cannot be prevented, cells are retained in their optimal environment in internal devices. Conventional reverse‐flow diafiltration utilizes an internal membrane device, but pulsed feeding causes temporal heterogeneities. In this study, the influence of conventional reverse‐flow diafiltration on the yeast Hansenula polymorpha is investigated. Alternating 180 s of feeding with 360 s of non‐feeding at a dilution rate of 0.2 h−1 results in an oscillating DOT signal with an amplitude of 60%. Thereby, induced short‐term oxygen limitations result in the formation of ethanol and a reduced product concentration of 25%. This effect is enforced at increased dilution rate. To overcome this cyclic problem, sequential operation of three membranes is introduced. Thus, quasi‐continuous feeding is achieved reducing the oscillation of the DOT signal to an amplitude of 20% and 40% for a dilution rate of 0.2 h−1 and 0.5 h−1, respectively. Fermentation conditions characterized by complete absence of oxygen limitation and without formation of overflow metabolites could be obtained for dilution rates from 0.1 h−1 – 0.5 h−1. Thus, sequential operation of three membranes minimizes oscillations in the DOT signal providing a nearly homogenous culture over time.