Fermentation processes growth kinetics

As shown in Fig. 6B, a two-phase pattern occurred for the substrate uptake. It can be observed that during the exponential growth phase, sucrose assimilation by the bacteria was small, corresponding to about 20% of the initial amount introduced into the medium. However, after a 40-h process corresponding to the end of the growth phase, there was a rise in the substrate uptake, suggesting that the carbon source was directed to biosurfactant production, for the conditions tested. It should be emphasized that the fermentative process, when the medium was supplemented with microsalts and EDTA (Fig. 6A), generated a different substrate kinetics in comparison with that obtained for the nonsupplemented medium (Fig. 6B). 

There are a large number of different types of fermentation processes that are used commercially, which are selected based on several different factors.19 21 Depending on the strain to be used, the fermentation could be aerobic or anaerobic, and the desired product could be either the biomass itself or a metabolite or polymer produced by the biomass. The kinetics of product formation, whether growth associated or nongrowth associated, also influences the process. Often procedures downstream of the fermentation unit operation have a major control of the overall process and determine how the fermentation is conducted. 

Although several types of models have been explored in the description of biological systems, the unstructured black box continuum models based on a linear equation for substrate consumption are still most frequently used for the description of fermentation processes. In these models the growth rate of the cells generally is assumed to be a function of the external concentration of the growth-limiting substrate Cs according to Monod kinetics ... 

The ability of Monod s empirical relation to fit kinetic data for biochemical reactions has its foundations in generalizations of two phenomena frequently observed for fermentation processes (1) nature places a cap on the quantity of microorganism that can be achieved during the exponential phase of growth in a bioreactor operating in a batch mode and (2) as the concentration of the limiting substrate approaches zero, the rate laws for biochemical reactions approach pseudo-first-order behavior with respect to that substrate. The cap indicated on the cell growth rate has been associated with the natural limit on the maximum rate at which replication of DNA can be achieved. 

In the laboratory, the cultivation is conducted in a liquid Hestrin-Schramm medium (glucose, yeast extract, peptone, citric acid and sodium phosphate) at pH 5 and 28°C. The cultures are grown in static containers or air flow (airlift bioreactors). The growing time depends on the desired thickness of the ensuing cellulose membrane. Several studies can be found on the growth kinetics of those microorganisms, based on fermentative processes [5]. 

Measurement of specific growth rate of the microorganism is necessary for the kinetic study of the fermentation process. 

Mechanism and kinetics in biochemical systems describe the cellular reactions that occur in living cells. Biochemical reactions involve two or three phases. For example, aerobic fermentation involves gas (air), liquid (water and dissolved nutrients), and solid (cells), as described in the Biocatalysis subsection above. Bioreactions convert feeds called substrates into more cells or biomass (cell growth), proteins, and metabolic products. Any of these can be the desired product in a commercial fermentation. For instance, methane is converted to biomass in a commercial process to supply fish meal to the fish farming industry. Ethanol, a metabolic product used in transportation fuels, is obtained by fermentation of corn-based or sugar-cane-based sugars. There is a substantial effort to develop genetically modified biocatalysts that produce a desired metabolite at high yield. 

Further study of the mass and heat transfer, the kinetics of substrate consumption, cell growth and product formation, estabhshment of a workable mathematical model for process scale up and optimization for sohd-state fermentation. 

Data about the physical characteristics of a bioreactor can provide only limited information. The complete characterization of a bioreactor requires additional studies involving biological test systems ( reference fermentations ). The fluid dynamics and rheological behavior of media are both directly and indirectly influenced by the presence of biological cells (Fiechter, 1978). Microbial processes whose growth or production kinetics are specifically dependent on changes in their medium or the reactor come into consideration as biological test systems (Karrer, 1978). Because of the central role of mass... 

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