Growth and product formation

As you might have already gathered, the majority of industrial fermentations are batch processes. In closed batch systems, the growth medium is inoculated with cells and growth and product formation is allowed to proceed until the required amount of conversion has taken place. After harvesting the culture the vessel is cleaned, sterilised and filled with fresh medium prior to inoculation. For some processes, addition of all the feedstock prior to inoculation, as is done in closed batch fermentations, is undesirable and it is preferable to incrementally add the carbon source as the fermentation proceeds. Such a process is known as fed-batch culture and the approach is often used to extend the lifetime of batch cultures and thus product yields fed-batch cultures are considered further in Section 2.7.4. 

Continuous culture is not considered suitable for citric add production the requirement for a multi-tank system to separate growth and product formation would make the process uneconomic. 

The continuous regime of poly(3HB) production has some important advantages. High and constant productivities are obtainable because high rates of growth and product formation can be maintained over long periods. Continuous cultivation methods are also convenient for the use of toxic substrates, and constant product quality should be easily obtained. 

The third major Hmitation of bioprocesses is the quite low product concentration compared with chemical processes, resulting in high downstream processing costs. This is mainly caused by product inhibition of cell growth and biosynthesis. Physiological improvements in cell growth and product formation only have a limited impact on this aspect. Chemical or directed mutagenesis may provide better chances for improvement. Unfortunately, the molecular mechanisms of product inhibition are not well understood. 

Most of the variables of these processes were monitored, material and energy balances were determined, pathway analysis was performed, regulation of growth and product formation was investigated, recovery and purification of the products were established and the systems were modelled. 

The indirect methods for measuring cell mass are based on the overall stoichiometry for growth and product formation, which may be written in the general form ... 

The oscillatory behavior of fermentors can be investigated utilizing such a model. Here the synthesis of a cellular component that is essential for both growth and product formation depends nonlinearly on the ethanol concentration. Hence the inhibition by ethanol does not directly influence the specific growth rate of the culture, but it affects it indirectly instead. 

Owing to the low concentration of soluble oxygen in aqueous media (8 mM or 32 mg L-1 at 25 °C) oxygen is often the limiting component in the fermentation. Therefore, the amount of oxygen required to be transferred into a fermentation culture for the desired cell growth and product formation has to be determined. 

From kinetics studies of unicellular organisms, a set of mathematical expressions have been established to represent the most frequent phenomena in bioprocesses. These phenomena involve a limitation or inhibition of growth and product formation, caused by the presence of substrates, products, or byproducts in culture media. Many of these expressions do not derive from known kinetic mechanisms. In fact, they are simply mathematical expressions with fitted parameters that are able to reproduce experimentally observed kinetic profiles. These equations have been derived and used in many unstructured microbial or cell models. 

Le Duy A, Zajic JE (1973), A geometrical approach for differentiation of an experimental function at a point applied to growth and product formation, Biotechnol. Bioeng. 15 805-810. 

As indicated for other modes, it is necessary to maintain the inoculum concentration in the range of 0.1 X 106 to 0.4 X 106 cells mL"1, to minimize the adaptation lag phase at the beginning of a cultivation process. The profiles for cell growth and product formation will depend on the feeding strategy adopted, on the cell line characteristics, and on the performance of the cell retention device. 

These phenomena principally govern the performance of a bioreactor. The first and second of these phenomena are independent of scale. Neither a typical thermodynamic property (e.g., the solubility of oxygen in a broth) nor microkinetic properties (e.g., the growth and product formation by the microorganism) are dependent on the scale of the bioreactors. However, the actual oxygen concentrations and the kinetic behavior of microorganisms in a bioreactor are dependent on... 

Fig. 1. Batch growth and antibody production of Nicotiana tabacum (BY-1) cells in terms of pg 1 1 protein, wet cell concentrations, and dry cell concentrations. Dashed vertical lines represent the boundaries between lag, exponential, and stationary growth phases. The 4-day offset between peak concentrations for biomass and protein product indicates that growth and product formation are not directly linked...

The key components for process monitoring were selected according to these data and some additional information. The in situ monitoring of the DO as well as the oxygen and carbon dioxide concentrations in the off gas allowed the evaluation of the oxygen transfer rate (OTR), the C02 production rate (CPR) and the respiratory quotient (RQ). The control of the pH-value and the DO was the prerequisite for the maintenance of the optimal growth and product formation conditions. 

Mammalian cell culture processes must be tightly controlled to attain acceptable cell density and maximize product titer. Slight deviations in pH, temperature, nutrient, or catabolite concentrations can cause irreparable damage to the cells. This section covers the effects of pH, shear stress, catabolite, and carbon dioxide accumulation on cell growth and product formation, and discusses the importance of controlling glucose and glutamine concentrations... 

Albers, E., Larsson, C., Liden, G., Niklasson, C., Gustafsson, L. (1996) Influence of the nitrogen source on Saccharomyces cerevisiae anaerobic growth and product formation. Applied and Environmental Microbiology, 62, 3187-3195. 

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