In Chapter 7, Section 7.1.4.3, the co-fermentation was presented as a way for a more complete utilization of all the sugars present in the hydrolyzates of lignocellulosic biomass. This process can be considered an example of reaction-reaction integra­tion since two biochemical processes (fermentation of glucose and fermentation of xylose) are combined and simultaneously accomplished in the same single vessel. For this, mixed cultures can be used as shown in Table 7.3. In addition, an enzyme transforming the xylose into another compound more assimilable by conventional yeasts can be added to the culture medium in order to allow the utilization not only of glucose, but also of xylose. The other approach consists of using recombinant microorganisms able to assimilate these two sugars as presented in Chapter 6, Table 6.3. From the viewpoint of process systems engineering, the modeling of this process plays a crucial role for simulation procedures intended to assess different technological configurations of biomass-to-ethanol conversion in the framework of process synthesis. The aspects concerning this topic were discussed in Chapter 7, Section 7.2.2. In general, through co-fermentation, it is possible to implement simultaneous fermentation processes with higher compactness and lower pro­duction costs since a unit for pentose fermentation is not required (Figure 9.3).

Nevertheless, the process itself corresponds to an SHF scheme because the cel­lulose has to be hydrolyzed in a previous bioreactor using cellulases.