One of the strategies employed to improve the ethanolic fermentation is the uti­lization of immobilized cells. Cell immobilization consists of the attachment of cells into a support or a location in a defined space in order to utilize, in a con­trolled way, their capacity to accomplish biological transformation. Thus, the cells do not leave the bioreactor, and continuous fermentation processes can be imple­mented. In this case, the substrates contained in the feed stream are transformed into products in the biocatalyst (cells + support) bed. These products abandon the system in the cell-free effluent stream. This leads to an easier product recovery as well as avoiding the risk of cell washout. A better control of the fermentation pro­cess is achieved compared to suspended cell cultivation for which microbial cells are continuously removed from the fermenter. On the other hand, the biocatalyst can be readily recovered if the process is carried out in batch regime. All of these advantages make reactors with immobilized cells exhibit higher productivities allowing the utilization of smaller bioreactors (lower capital costs).

However, employing immobilized cells implies that they do not reproduce dur­ing reactor operation. The growth means that cell layers are accumulated on the support surface until the moment they start to deattach from the solid phase lead­ing to the system destabilization. To avoid this, the necessary conditions for the cells not to grow (nonviable cells) are ensured. In addition, if aeration is needed, a constant air supply has to be available, which can be difficult when a fixed bed

FIGURE 7.5 Most employed configurations for bioreactors with immobilized cells: (a) fixed-bed reactor, (b) fluidized-bed reactor.

of biocatalysts is used (Figure 7.5a). For this reason, an auxiliary tank is used to supply air to part of the effluent stream, which is then recirculated to the reactor to ensure the aerobic conditions of the culture broth within the bed. One alternative configuration is the fluidized-bed reactor where the liquid feed stream flows up inside the reactor containing mobile biocatalyst particles. In this way, the bed is expanded, as shown in Figure 7.5b. If aerobic conditions are required, the air can be directly injected into the bioreactor. Despite these advantages, processes using immobilized cells are not widespread in industrial microbiology today due to the complexity of the systems involved.

In the case of ethanolic fermentation, the implementation of continuous cul­tivation with immobilized cells can make possible processes with higher yields, greater productivities, and increased cell concentrations at the same time (Claassen et al., 1999), as presented in Table 7.2. Nevertheless, ethanol concentrations in the effluent tend to be lower than in other variants of continuous processes (see Table 7.1). Microbial cells for ethanol production are immobilized by entrapping within them porous, solid supports, such as calcium or sodium alginate, carra­geenan or polyacrylamide. In addition, they can be adsorbed on the surface of materials, such as wood chips, bricks, synthetic polymers, or other materials with a large surface area (Gong et al., 1999). It is remarkable that support particles have influence on cellular metabolism, as has been shown in the case of solid-state fermentation, biofilm reactors, and immobilized cell reactors. Prakasham et al.

Some Continuous Processes for Bioethanol Production from Sugarcane and Related Media Using immobilized Cells

a 120 g/L of reducing sugars. b 200 g/L + nutrients.

c High test molasses supplemented with ammonium sulfate. d Higher values correspond to acid treated and clarified molasses. e Measured in g EtOH/(1011 cells. h). f Multistage fluidized-bed reactor.

(1999) claim that the simple addition of a small fraction of solids in submerged cultures facilitates cell anchorage. This kind of adhesion enhances the metabolic activity and is an easier and more economical method than the immobilization of cells. In batch flasks cultures, these authors showed that materials, such as river sand, delignified sawdust, chitin, and chitosan, make possible the adhesion of S. cerevisiae cells leading to higher ethanol production in comparison to free cells. Thus, the application of these techniques of “passive immobilization” to continu­ous cultures should be experimentally tested. Nowadays, most of the configura­tions using immobilized cells are, so far, used in commercial operations. Hence, preliminary design and simulation of this type of process could become a very useful tool for defining new research lines at pilot and semi-industrial levels con­sidering the overall bioethanol production process (Sanchez and Cardona, 2008).