Bioreactor process development

Single-Cell Protein. Systems involving single-cell proteins are often very large throughput, continuous processing operations such as the Pmteen process developed by ICI. These are ideal for air-lift bioreactors of which the pressure cycle fermenter is a special case (50). 

BPD is a contract research facility, which started operations in 1996. From the storage of frozen materials, the customized laboratory services to the consulting in process development and scaling up, BPD has been accumulating useful experiences in the biotechnological scenario. Very flexible set of fermenters are used to satisfy customer needs, through very small and fast adaptation and modifications. Bioreactors productivity varies from 100 mg to 1000 mg scale. 

In this paper the fundamental aspects of process development for the production of core and virus-like particles with baculovirus infected insect cells are reviewed. The issues addressed include particle formation and monomer composition, chemical and physical conditions for optimal cell growth, baculovirus replication and product expression, multiplicity of infection strategy, and scale-up of the process. Study of the differences in the metabolic requirements of infected and non-infected cells is necessary for high cell density processes. In the bioreactor, the specific oxygen uptake rate (OURsp) plays a central role in process scale-up, leading to the specification of the bioreactor operational parameters. Shear stress can also be an important variable for bioreactor operation due to its influence on cell growth and product expression. 

The Bio-FGD process converts sulfur dioxide to sulfur via wet reduction (10). The sulfur dioxide gas and an aqueous solution of sodium hydroxide are contacted in an absorber. The sodium hydroxide reacts with the sulfur dioxide to form sodium sulfite. A sulfate-reducing bacteria converts the sodium sulfite to hydrogen sulfide in an anaerobic biological reactor. In a second bioreactor, the hydrogen sulfide is converted to elemental sulfur by Thiobacilh. The sulfur from the aerobic second reactor is separated from the solution and processed as a sulfur cake or liquid. The process, developed by Paques BV and Hoogovens Technical Services Energy and Environment BV, can achieve 98% sulfur recovery. This process is similar to the Thiopaq Bioscrubber process for hydrogen sulfide removal offered by Paques. 

Livingston, A.G., Arcangeli, J.P., Boam, T., Zhang, S., Marangon, M. and Freitas, L.M. (1998) Extractive membrane bioreactors for detoxification of chemical industry wastes process development Journal of Membrane Science, 151, 29. 

Another important aspect of process development is pH control. The basal medium is formulated to contain a buffer compatible with cell growth the current industry standard is bicarbonate. Bicarbonate is in equilibrium with CO2 such that bioreactor pH can be lowered by addition of CO2 and raised by addition of a base (such as NaOH). Particularly at large scale, CO2 accumulation has been shown to be detrimental to bioreactor performance, and CO2 levels are lowered by stripping this dissolved gas with sparged nitrogen or air. Because cell growth is dependent on pH, optimization of this parameter allows for maximal cell mass accumulation and increased production of the product of interest. 

Initial process development elforts usually begin with evaluation of a standard process that is well characterized. During this evaluation, various indicators of bioreactor performance, such as cell mass and productivity, are monitored and then analyzed in order to design further process development studies. Bioreactor parameters optimized often include inoculum cell density, impeller speed, medium pH, nutrient levels, temperature, and so forth. 

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