Fermentation is commonly used for a variety of pharmaceutical production processes, such as the production of recombinant therapeutic proteins in yeast and the biotransformation of chemical drug intermediates to reduce the feedstock to the correct derivative. In some cases fermentation is used to create indirect gene products, including antibiotics and vitamins, while elsewhere it is used for creating direct gene products – typically hormones, monoclonal antibodies, proteins and antigens.
As well as being employed for production, fermentation also plays an important role in drug discovery, research and development. Because of the nature of the products of fermentation and the potentially devastating impact of impurities, EMEA (European Agency for the Evaluation of Medicinal Products) approval is required under EC Regulation EEC 2309/93 (which lays down the Community procedures for coordinating the evaluation of safety, quality and efficacy of medicinal products for human and veterinary use). Strict monitoring and control of the fermentation process is therefore mandatory as well as necessary to indicate the possible presence of impurities, perhaps as a result of a new strain of micro-organism or a minor change in the process. While off-line analysis can be used to meet the regulatory requirements, on-line monitoring provides faster results and a further advantage in that it can be incorporated within the process control system. With real-time data available, the process can be optimized, leading to increased yields, shorter processing times and lower operating costs through, for example, reduced oxygen consumption.
On-line monitoring and real-time process control can also enhance process reproducibility and lead to more consistent product quality. As aerobic fermentation progresses, oxygen is consumed and carbon dioxide is released by the cultures present, but the rates vary enormously, depending on the respiratory metabolism. In those processes where oxygen is injected directly in addition to air injection (which is still required for mixing the fermentation broth and stripping it of carbon dioxide and other undesirable reaction by-products), the requirement for oxygen can be monitored and the supply adjusted to suit.
Early in the fermentation process the demand for oxygen is low but, after an initial time lag, culture growth becomes exponential and the oxygen addition should be boosted accordingly. Once growth enters a stationary phase in the fermentation cycle, the oxygen injection can be reduced in line with the requirement. Monitoring the level of carbon dioxide in the headspace above the fermentation broth gives an indication of the biomass growth, which can be translated into an oxygen demand by means of a mathematical correlation model. It has been reported1 that direct injection of oxygen can increase yields by as much as 65 per cent while, at the same time, reducing operating costs, thereby offering pharmaceutical manufacturers the potential to save millions of dollars.
To optimize the process, however, a fast response is needed from the carbon dioxide analyzer, and parallel monitoring of the oxygen in the headspace serves to indicate if the injected oxygen is bubbling through the fermentation broth undissolved, which could be the case if supply exceeds demand. Furthermore, monitoring both carbon dioxide and oxygen and comparing the measurements against a process model can give an almost immediate indication of any deviation from the norm. This might be as a result of a minor problem with the process or a new impurity. Either way, the early warning is beneficial in terms of product quality, plant efficiency and regulatory compliance.
References:1How to make oxygen economical for fermentation, Dr Alan TY Cheng, Praxair Inc, paper delivered to the 1998 Pharmaceutical Ingredients Worldwide (CPhI) Conference.