Fed-batch operation

In fed-batch culture, a fresh medium which contains a substrate but no cells is fed to the fermentor, without product removal. This type of operation is practiced in order to avoid excessive cell growth rates with too-high oxygen demands and catabolite repression with high glucose concentration, or for other reasons. Fed-batch operations are widely adopted in the culture of baker s yeast, for example. 

In fed-batch culture, the fresh medium is fed to the fermentor either continuously or intermittently. The feed rate is controlled by monitoring, for example, the dissolved oxygen concentration, the glucose concentration, and other parameters. Naturally, the volume of the liquid in the fermentor V (m3) increases with time. The feed rate F (m3 h ), though not necessarily constant, when divided by V is defined as the dilution rate D (h ). Thus, 

Usually, D decreases with time, but not necessarily at a constant rate. 

The increase in the total cell mass per unit time is given as 

if D is made equal to the cell concentration would not vary with time. For a given substrate concentration in the fermentor Cs, is given by the Monod equation (i.e., Equation 4.6). Alternatively, we can adjust the substrate concentration Cs for a given value of pL. Practical operation usually starts as batch culture and, when an appropriate cell concentration is reached, the operation is switched to a fed-batch culture. 

Usually D decreases with time, but not necessarily at a constant rate. The increase in the total cell mass per unit time is given as 

The total substrate balance for the whole fermentor is given as  

Fed-batch culture is not a steady-state process, as the liquid volume in the fermentor increases with time and withdrawal of products is not continuous. However, the feed rate and the concentrations of cells and substrate in the broth in a fermentor can be made steady. 

The characteristics of the fed-batch culture, shown by Equations 12.21 and 12.23, make it possible to keep the concentrations of the substrate and/or the cell at the desired values. For example, after a batch culture, a feed medium that contains the substrate at a high concentration can be fed, either continuously or intermittently, to the fermentor under a fed-batch operation. The values of the dilution rate and the substrate concentration in the feed medium can be determined using Equation 12.23. Thus, by using the fed-batch operation, the yield and/or productivity can be greatly improved in a variety of areas of biotechnology by controlling the concentrations of substrate and cell. Some examples of where the fed-batch operation can be effectively used are as follows. 

---

Because of the differences in primary and secondaiy metabolism, a reactor may have a dual-stage fed-batch system. In other words, fed-batch operation optimizes growth with little or no product formation. When sufficient biomass has accumulated, a different fed-batch protocol comes into play. 

The most limiting factor for enzymatic PAC production is the inactivation of PDC by the toxic substrate benzaldehyde. The rate of PDC deactivation follows a first order dependency on benzaldehyde concentration and reaction time [8]. Various strategies have been developed to minimize PDC exposure to benzaldehyde including fed-batch operation, immobilization of PDC for continuous operation and more recently an enzymatic aqueous/octanol two-phase process [5,9,10] in which benzaldehyde is continuously fed from the octanol to the enzyme in the aqueous phase. The present study aims at optimal feeding of benzaldehyde in an aqueous batch system. 

Batch or Fed-Batch Operation This mode of operation is typical of biologicals and juice processing where high solicfs and low fluxes require multiple passes, and batch operation is characteristic of the manufacturing process. Formulas in Table 20-19 can be used to calculate the required volume reduction factor X, diafiltration volumes... 

The system area A needed for batch or fed-batch operation can be calculated by using the formulas in Table 20-19 for production rates V(/t based on feed volume Vq and average fluxes during process steps. The final retentate batch volume Vr = Vq/X or permeate batch volume Vp = Vo(l — l/X + N/X) can be used to restate the production rate on other bases. Although experience can be used to estimate solute passage and process fluxes, they should be determined e3q)erimentally for each application. 

When comparing fed-batch operated cultures it can be seen that the main difference between poly(3HB) and poly(3HAMCL) production that affect the process parameters is the lower cellular poly(3HAMCL) content, compared to the cellular poly(3HB) content. Not surprisingly, it has been reported that a low poly(3HAMCL) content decreases the productivity and yield and increases the costs for downstream processing and waste disposal [102]. 

It can be shown by simulation that a quasi-steady state can be reached for a fed-batch fermenter, where dX /dt=0 and //= l /V (Dunn and Mor, 1975). Since V increases, // must therefore decrease, and thus the reactor moves through a series of changing steady states for which //= I), during which Sj and ft decrease, and X remains constant. A detailed analysis of fed batch operation has been made by Keller and Dunn (1978). 

Start as a batch reactor and then switch to fed batch operation. Explain why this procedure is preferable to starting initially as a fed batch ... 

Fig. 3 Results from a fed batch operation. Here both the biomass and the biomass concentration are plotted.

Kargi, E., Pamukoglu, M.Y. 2004a. Adsorbent supplemented biological treatment of pre-treated landfill leachate by fed-batch operation. Biores Technol 94 285-291. 

Bioreactors are operated in discontinuous mode, with a sequential or continuous feed of the substrate (fed-batch operation) or in continuous mode. The choice of the operating mode depends mainly on the reaction characteristics ... 

Any reaction exhibiting substrate inhibition should not be carried out in batch since it results in a longer residence time the high concentration of the substrate at the beginning lowers the reaction rate. A continuously operated stirred tank is preferred. At laboratory scale, fed-batch operation enables a low substrate concentration in the reactor and a higher reaction rate. 

In the fed-batch operation of fermentors (which is also commonly practiced), the feed is added either continuously or intermittently to the fermentor, without any product withdrawal, the aim being to avoid any excessive fluctuations of oxygen demand, substrate consumption, and other variable operating conditions. 

High Cell Density Culture The fed-batch operation that maintains the substrate concentration at a suitable value for a high cell-growth rate can achieve a high cell concentration (50-100gl ). 

Avoiding Substrate Inhibition, Enzymatic Inhibition, and Catabolite Repression When using the fed-batch operation, the concentration of a substrate can be kept at a low value so that substrate inhibition, enzymatic inhibition, or catabolite repression is practically avoided, which results in high productivity [14]. 

The fed-batch operation is effective in controlling its concentration by regulating the feeding rate of the required nutrient ] 15. ... 

Bioreactors that use enzymes but not microbial cells could be regarded as fermentors in the broadest sense. Although their modes of operation are similar to those of microbial fermentors, fed-batch operation is seldom practiced for enzyme reactors. The basic equations for batch and continuous reactors for... 

There are several barriers to the successful control of bioprocesses due to particular circumstances that are related to their characteristics the complexities of microbial metabolisms, the nonlinearity of microbial reactions, the frequent use of batch and fed-batch operations, and the limited availability of sterihzable online sensors for important process variables such as cell and product concentrations. Furthermore, it is difficult to construct mathematical models that can predict the entire range of batch or fed-batch operations that many fermentation processes require. 

Liquid-Flow Rate Liquid flow rate is measured when a medium is fed into a biore-actor in continuous and fed-batch operation. The flow rate of cooling water is also monitored in industrial bioprocessing plants. 

The RQ control, where the sugar-feeding rate is controlled so as to maintain the RQ value at approximately 1.0, is the distinct control method in aerobic fed-batch cultivation such as with baker s yeast production. By keeping the RQ at 1.0 during the fed-batch operation, cell production with a high yield will be achieved. 

One important issue of the fed-batch operation is the variable volume of material in the reactor and its effect on heat transfer area. If jacket cooling is used, the heat transfer area covered by the liquid in the reactor will be proportional to the volume of the liquid at any point in time. However, if the reaction liquid is circulated through an external heat exchanger, the full heat transfer area is available throughout the batch cycle. 

For fed-batch operation, however, there is a significant difference between the reaction A — B and the reaction A + B —> C. One of the reactants can be charged to the reactor. Then the other can be fed at a rate consistent with the heat transfer capacity of the system. 

A second stream can also be used in RBatch by attaching it to the blue arrow on the side of the reactor called Continuous Feed (optional) (see Fig. 4.21). As the name implies, this stream is used if fed-batch operation is desired. This stream is a real flowing stream, whose flow-versus-time profile can be specified, as we will illustrate later. 

No major difference in performance of the strains was found in the batch cultivations, but the performance was quite different during fed-batch operation. This shows the importance of using not just batch cultivations for comparing strains. Presumably, the toxicity levels in the batch case were so high that both strains were completely inhibited. However, at intermediate levels, as obtained in fed-batch cultures, the strain performances were quite different. 

My third example of chemical engineering challenges in biotechnology is a problem in monitoring and control. In virtually all practical fermentations where the medium contains solids and/or where the cell concentrations are greater than 10 g/liter, it is impossible to monitor directly the cell concentration and hence cell activity. It would be most helpful to know the instantaneous cell concentration and activity in order to control the substrate feed rate, particularly in a fed-batch operation. 

Because of the need to avoid mutations and maintain the superior qualities of the genetically developed strain, batch or fed-batch operations are used in most applications. Continuous culture operations, however, provide a time-invariant environment that facilitates greatly the study of a biological process in research laboratories. Moreover, some industrial operations employ continuous reactors, such as the single-cell protein facility of ICI in Billingham, England (total reactor volume of about 2,300 m3), all waste treatment processes, and others. It should be noted that it is relatively common to follow a batch process with a period of fed-batch or continuous operation. Also, in most cases batch cultivation is the optimal start-up procedure for continuous or fed-batch cultivation (Yamane et al, 1977). 

Gordillo, M. A., Sanz, A., Sanchez, A., Valero, F., Montesinos, J. L., Lafuente, J., and Sola, C. 1998b. Enhancement of Candida rugosa lipase production by using different control fed-batch operational strategies. Biotech. Bioeng, 60,156-168. 

In the fed-batch fermentation, nutrients can be added making it an open system for substrates, but still a closed system for biomass and biomass-derived products. As shown in Figure 30.4B, in this type of process, the volume is not constant. The flow rate of nutrients can vary during the course of the fermentation. Fed-batch operation is very commonly used in industrial processes for the production of baker s yeast, enzymes, amino acids, and many other metabolites. 

---