In the last few years technologies breakthrough has compelled us for an alternative feedstock due to consid­erable shortage in agricultural land. In this sense, ad­vances in metabolic pathway engineering/genetic engineering have led to the development of microbes skilled enough to convert biomass into ethanol (Das Neves et al., 2007). Generally, such development de­pends on expansion of the substrate range and inclusion of other biomass sources like arabinose or xylose in strains that cannot ferment sugars other than glucose. Examples of such microorganisms include genetically modified Escherichia coli, Saccharomyces sp., and Zymomo — nas mobilis, etc. (Davis et al., 2006).

In cellulosic ethanol industry, aside from Pichia stipitis, natural xylose fermenting yeast, more efforts are being taken in obtaining recombinant bacterial and yeast strains that are able to ferment pentose sugars, such as arabinose and xylose. Figure 1.5 is one among the best examples depicting recombination process in microbes, where the tail end in E. coli and Klebsiella oxytoca or the front end of

S. cerevisiae and Z. mobilis can be recombined for improved production of ethanol (Hagerdal et al., 2006).

Moreover, genetic engineering of plants is another promising area, which most likely plays a key role in bio­fuel industry. The latest hybrid varieties have helped us considerably in improving starch yield from energy crops. For example, 25 kg of corn contains about 15 kg of starch. In the near future, that same 25 kg may contain as much as 17 kg of starch through hybrid corn. This would result in a gain of nearly $2 million in annual in­come by processing the same amount of corn in a 120 million liter per year ethanol production (DOE, 2007).

Bioreactors in Ethanol Production

A major commitment in cost-effective lignocellulosic bioethanol production is to employ reactor systems yielding the maximal cellulose conversion with the min­imal enzyme. As a result, one of the most vital parame­ters for the fabrication and operation of bioreactors for lignocellulosic conversion is the efficient use of the en­zymes to gain high specific rates of cellulose conversion (yield of glucose attained/amount of enzymes). The maximization of the product concentration, i. e. the amount of glucose obtained per liquid volume, is also a significant parameter as well as the optimization of the volumetric productivity.

When hydrolysis is carried out with biomass comprised of high cellulose levels, the product concentration will drive up. For this reason, few researchers are attempting the enzymatic biomass conversion with high biomass loads (Jorgensen et al., 2007). The most imperative

difficulty in high biomass loads is related to the viscosity of reaction mixture, which also influences the rheology of the mixture. In particular, mixing and mass transfer limita­tions and presumably increased inhibition by intermedi­ates come into play. A variety of fed-batch strategies have been adopted with the scope of supplying the substrate without reaching excessive viscosities and unproductive enzyme binding to the substrate (Rudolf et al., 2005).

General criteria in bioreactor design and in the choice of the operating conditions could be use of bioreactors or reaction regimes that allow a rapid decrease in the glucose concentration; running of the reactions at low to medium substrate concentrations in order to maintain higher conversion rates and thus obtain higher volu­metric output of the reactor (Andric et al., 2010).

The combination of the bioreactor with a separation unit has obtained prospective results with product inhibited or equilibrium limited enzyme-mediated con­versions, because it potentially removes the products as they are accumulated (Gan et al., 2002). In this regard, membrane bioreactors could be a feasible process configuration. Unlike the Solid State Fermentation (SSF) approach in which the glucose consumption is car­ried out by the microbes simultaneously accessible in the hydrolyzate, the use of membrane bioreactors would finish the same function without any compromise in the reaction parameters. A membrane bioreactor (Figure 1.6) is a multitasking reactor that combines the reaction with a separation, namely, in this case the product was taken away by membrane separation, as one integrated unit (in situ removal) or alternatively in two or more separate units. The membrane bioreactors used for this separa­tion processes are mainly ultra — and nanofiltration types (Pinelo et al., 2009). However, the use of this technology is restricted by the accumulation of unreacted lignocel — lulosics in large level and/or continuous processing (Andric et al., 2010). Already in the past, few scientists enhanced the efficiency of the continuous stirred tank

bioreactor by incorporating membrane separation tech­nologies during the reactor design.

Recently, an advanced reactor system was intended that removes the reducing sugars during the enzymatic hydrolysis of cellulose through a system consisting in a tubular reactor, in which the substrate was retained with a porous filter at the bottom and buffer entered at the top through a distributor (Yang et al., 2006). This hol­low fiber ultrafiltration module with polysulfone mem­brane enabled the permeation and the separation of the sugars. To keep the volume constant in the tubular reactor, the entire buffer was recycled back from the ultrafiltration membrane and the makeup buffer was continuously sup­plied from the reservoir. In some applications an addi­tional microfiltration unit has exceptionally been used to retain the unconverted lignin-rich solid fraction due to the presence of firmly bound enzymes or has been employed to remove the unconverted substrate from the reactor. These setups result in slightly complex process layouts for the hydrolysis (Knutsen and Davis, 2004).

It is obvious that the optimization of the reactor designs will allow overcoming both the rheological and inhibition limit of the bioconversion and maximizing the enzymatic conversion. Therefore, the reactor design becomes more relevant for large-scale processing of cellulosic biomass.