Cell mass production, optimization

Lim, H.C., Y.J. Tayeb, J.M. Modak, and P. Bonte, Computational Algorithms for Optimal Feed Rates for a Class of Fed-batch Fermentations Numerical Results for Penicillin and Cell Mass Production, Biotechnol. Bioeng., 28,1408-1420 (1986). 

Optimization with Regard to Cell Mass Productivity... 

As can be seen from Table 8.7 productivity (expressed in g h b is highest for precursor addition. The production of L-phenylalanine from phenylpyruvic add also has the shortest reaction time to obtain hi conversions. The pH commonly used is around 75, quite normal for biological processes. Only the enzyme phenylalanine ammonia lyase shows an optimiim pH of lO.The process temperature varies between 30 and 40°C with an average of 35°C. No extreme temperatures have been reported due to the fact that denaturation occurs at hi temperatures. The optimal concentration for cells frequently used is 10-20 g 1". However, conversion of ACA is done with hi cell mass concentrations in recent studies possibly to compensate for substrate inhibition and thus to maintain hi product concentration. The processes using PPA and ACA need an amino add as amino donor, usually L-aspartic add is used. 

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. 

The economics of enzyme production are dominated in most cases by recovery costs. For this reason, the objective functions for optimization are to maximize the specific activity (units of enzyme activity per weight of cell mass) and volumetric productivity (units per liter-h). Maximization of specific activity ensures both efficient conversion of raw materials to the desired product as well as facilitating enzyme recovery maximization of total volumetric productivity helps to minimize recovery costs and maximize economic return on capital investment. To achieve these objectives in process development, one utilizes both genetic manipulations and environmental management to achieve a high enzyme specific activity and a high cell density. 

A maximum peak power density at optimal conditions of 210 mW cm and stable long-term operation at 80 mW cm was achieved at different ambient conditions. The total performance of the micro fuel cells is in the same range of current and power density compared to the best conventional planar PEM fuel cells. At the same time this technology offers a high degree of miniaturisation and the capability for mass production which is a clear success of our micro-patterning approach. 

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