When a technological flowsheet involving an SHF process is employed, the detoxified hemicellulose hydrolyzate can be unified with the cellulose hydrolyz — ate coming from the enzymatic reactor. The resulting stream contains mostly glucose, but also xylose and other hexoses released during biomass pretreat­ment. The simplest scheme includes the cultivation of S. cerevisiae that converts the glucose present in the medium into ethanol remaining in the xylose and the other hexoses. This implies a reduction in the amount of ethanol that could be obtained if the xylose and remaining sugars were utilized. To increase the amount of sugars converted into ethanol, yeast assimilating the xylose besides glucose can be used, but in this case the biomass utilization rates are lower relative to microorganisms that assimilate only hexoses. This is explained by the diauxic growth of this type of yeast (see Chapter 6). To offset this effect, sequential fermentations are employed in which S. cerevisiae utilizes the hexo — ses during the first days of cultivation and later xylose-utilizing yeast is added in order to complete the conversion to ethanol (Chandrakant and Bisaria, 1998), but achieved ethanol yields are not very high. Therefore, the most suitable con­figuration corresponds to the scheme of Figure 7.6 where both fermentations are performed independently.

One of the main challenges in pentose fermentation lies in the fact that the productivities of pentose-utilizing microorganisms are less than those of hexose — fermenting ones. Comparisons are conclusive: ethanol productivity for S. cer- evisiae can attain values of 170 g/(L/h) in the case of continuous systems with cell recycling, whereas the productivity for C. shehatae at high cell concentra­tions reaches values of only 4.4 g/(L/h) (Olsson and Hahn-Hagerdal, 1996). On the other hand, there are a few cases where the immobilization of these yeasts increases the ethanol productivity (Chandrakant and Bisaria, 1998), unlike the case of hexose-fermenting yeasts or Z. mobilis. In spite of these drawbacks, pen­tose-utilizing microorganisms are important for the design of processes involving separate fermentation of hexoses and pentoses during the processing of resulting streams from biomass pretreatment. As occurs with S. cerevisiae or Z. mobilis, most pentose-fermenting yeasts are mesophiles (Sanchez and Cardona, 2008). On the other hand, although thermotolerant yeasts such as Kluyveromyces marxi — anus have demonstrated their capability to ferment glucose at 45°C (Singh et al., 1998), there are no data about the assimilation of xylose by this yeast, according to Ryabova et al. (2003). These authors described the separate fermentation of glucose and xylose by native strains of methylotrophic yeast Hansenula polymor — pha at 37°C in flask cultures during 60 h achieving ethanol concentrations of 13.2 g/L and 2.98 g/L from glucose and xylose, respectively.

Lynd et al. (2001) report that for Thermoanaerobacterium thermosaccharolyt — icum cultivated in xylose-based media during batch and continuous cultures, the ethanol concentrations obtained are low (in the order of 25 g/L). These authors studied the influence of different factors limiting the substrate utilization for con­tinuous cultures at progressively higher feed xylose concentrations. Their results indicate that the salt accumulation due to the utilization of bases for pH-control of fermentation limits the growth of this bacterium at elevated values of xylose con­tent in the feed. These outcomes can explain the differences between the tolerance to added ethanol and the maximum concentration of produced ethanol for these microorganisms. Xylose-fermenting termophilic bacteria are prospective organ­isms to be co-cultured with cellulose hydrolyzing bacteria such as Clostridium thermocellum in order to directly convert pretreated lignocellulosic biomass into ethanol, a process called consolidated bioprocessing (CBP). Another promising microorganism capable of fermenting a great variety of sugars (including hexoses and pentoses) is the fungus Mucor indicus reaching ethanol yields of 0.46 g/g glucose when it is cultivated under anaerobic conditions. In addition, this fungus assimilates the inhibitors present in dilute-acid hydrolyzates (Sues et al., 2005). Other pentose-fermenting zygomycetes were also evaluated (Millati et al., 2005). The use of the fungus Chalara parvispora for ethanol production from pentose — containing materials has been patented (Holmgren and Sellstedt, 2006). Ogier et al. (1999) have compiled information about the main fermentative indexes for the pentose-assimilating yeasts Candida shehatae, Pichia stipitis, and Pachysolen tannophilus, and for the xylose-assimilating thermophilic bacteria T. thermosac — charolyticum, T. ethanolicus, and Bacillus stearothermophilus.