Enzyme recycling, cellulose

Processes for the bioproduction of ethanol from cellulosic materials have been studied extensively. Some of the process steps are specialized and beyond the scope of this chapter. However, there are many recent review articles dealing with some specific subjects. Basically, the processes consist of a number of steps. They are availability and collection of raw feedstock [20], size reduction, pretreatment, fractionation of biomass components, enzyme production [21, 22], saccharification, enzyme recycle [23, 24], pentose fermentation, improvement of pentose-fermenting biocatalyst, overcoming of product inhibition, overcoming inhibition by substrate-derived inhibitors, ethanol recovery [25], steam generation and recycling [26], waste treatment, and by-product utilization. 

The cost of enzyme preparations has been decreasing in recent years however, it continues to affect considerably the price of ethanol obtained from cellulosic raw materials. Increased enzymatic hydrolysis efficiency is one way to reduce the enz)me cost in bioethanol production. Another method is enzyme recycle and reuse. Immobilization of biocatalysts allows for their economic reuse and development of continuous bioprocess. Although immobilization poses problems of substrate accessibility and binding for most endo- and exocellulases, P-glucosidase exhibits characteristics amenable to immobilization, such as activity on soluble substrates and the lack of a carbohydrate-binding module. Among the possible approaches, immobilization of (J-glucosidase is one prospective solution to the problem. 

Gregg, D. J., A. Boussaid and J. N. Saddler (1998). Techno-economic evaluations of a generic wood-to-ethanol process Effect of increased cellulose yields and enzyme recycle. Bioresource Technology 63(1) 7-12. 

As mentioned in the biological—biochemical section, another approach to improve alcoholic fermentation combines saccharification and fermentation, ie, simultaneous saccharification and fermentation (SSF). Enzyme-catalyzed cellulose hydrolysis and fermentation to alcohol takes place in the same vessel in the presence of enzyme and yeast (50). Reduced fermenter pressures and enzyme and yeast recycling result in 70 to 80% ethanol yields. These process modifications, coupled with more energy-efficient distillation and heat exchanger improvements, are projected to make fermentation ethanol from low value biomass competitive with industrial ethanol (51). 

Both pilot-plant and plant-scale processes for cellulose hydrolysis or digestion by acids, enzymes, and microorganisms have been built (I, 7,15,27). Acid and enzyme processes usually have as their objective the production of a sugar syrup, while the microbial process usually results in microbial protein for animal feed. Figure 10 is illustrative of a microbial process (29) that has been developed to convert the unused cellulosic material in manure to recycle feed. Similar processes have been developed... 

Various authors have shown that non-ionic surfactants have a beneficial effect on the hydrolysis of cellulosic and lignocellulosic substrates, whereas anionic and cationic surfactants alone interfere negatively (Castanon and Wilke, 1981 Helle et al, 1993 Park et al, 1992 Ooshima et al., 1986 Traore and Buschle-Diller, 1999 Ueda el al., 1994 Eriksson el al., 2002). Increases in the amount of reducing soluble sugars and substrate conversion were reported. The effect depends on the substrate and is not observed for soluble substrates, such as carboxymethylcellulose or cellobiose. Nonionic surfactants increased the initial rate of hydrolysis of Sigmacell 100, and when they were added later in the process they were less effective (Helle et al, 1993). They same authors found also that the addition of cellulose increases the critical micelle concentration of the surfactant, which indicates that the surfactant adsorbs to the substrate. Surfactants are more effective at lower enzyme loads and reduce the amount of adsorbed protein (Castanon and Wilke, 1981 Ooshima et al, 1986 Helle et al, 1993 Eriksson et al., 2002) which can be used to increase desorption of cellulase from the cellulosic substrate (Otter et al., 1989). Anyhow, the use of surfactants to enhance desorption of cellulases from textile substrates in order to recover and recycle cellulases was not successful (Azevedo et al., 2002b). 

Blaedel and Jenkins (1976) evaluated two LDH—NAD combinations for a reagentless lactate sensor (Fig. 55). NAD was either coimmobilized with LDH to cellulose or an NAD+-agarose complex was constrained together with LDH to a region near the electrode surface. By bringing the bound NAD into intimate physical contact with the electrode, the immobilized cofactor was recycled electrochemically and reused by the enzyme. This eliminated the need to supply NAD as a reagent. 

Enzymatic hydrolysis of polysaccharides (cellulose, starch) or oligosaccharides (maltose, saccharose, lactose) for the synthesis of food products is another class of processes MBR have been applied to. Paolucci-Jeanjean et al [4.56] have recently reported, for example, the production of low molecular weight hydrolysates from the reaction of cassava starch over a-amylase. In this case the UF membrane separates the enzyme and substrate from the reaction products for recycle. Good productivity without noticeable enzyme losses was obtained. Houng et al [4.57] had similar good success with maltose hydrolysis using the same type of MBR,... 

Enzyme activity loss because of non-productive adsorption on lignin surface was identified as one of the important factors to decrease enzyme effectiveness, and the effect of surfactants and non-catalytic protein on the enzymatic hydrolysis has been extensively studied to increase the enzymatic hydrolysis of cellulose into fermentable sugars [7, 9 19]. The reported study showed that the non-ionic surfactant poly(oxyethylene)2o-sorbitan-monooleate (Tween 80) enhanced the enzymatic hydrolysis rate and extent of newspaper cellulose by 33 and 14%, respectively [20]. It was also found that 30% more FPU cellulase activity remained in solution, and about three times more recoverable FPU activity could be recycled with the presence of Tween 80. Tween 80 enhanced enzymatic hydrolysis yields for steam-exploded poplar wood by 20% in the simultaneous saccharification and fermentation (SSF) process [21]. Helle et al. [22] reported that hydrolysis yield increased by as much as a factor of 7, whereas enzyme adsorption on cellulose decreased because of the addition of Tween 80. With the presence of poly(oxyethylene)2o-sorbitan-monolaurate (Tween 20) and Tween 80, the conversions of cellulose and xylan in lime-pretreated com stover were increased by 42 and 40%, respectively [23]. Wu and Ju [24] showed that the addition of Tween 20 or Tween 80 to waste newsprint could increase cellulose conversion by about 50% with the saving of cellulase loading of 80%. With the addition of non-ionic, anionic, and cationic surfactants to the hydrolysis of cellulose (Avicel, tissue paper, and reclaimed paper), Ooshima et al. [25] subsequently found that Tween 20 was the most effective for the enhancement of cellulose conversion, and anionic surfactants did not have any effect on cellulose hydrolysis. With the addition of Tween 20 in the SSF process for... 

In cellulosic ethanol production processes, a pretreatment procedure is needed to disrupt the recalcitrant structure of the lignocellulosic materials so that the cellulose can be more efficiently hydrolyzed by cellulase enzymes [2], These pretreatments include physical, biological, and chemical ways, such as uncatalyzed steam explosion, liquid hot water, dilute acid, flow-through acid pretreatment, lime, ammonium fiber/freeze explosion, and ammonium recycle percolation [3, 4], Most of these methods involve a high temperature requirement, which is usually achieved through convection- or conduction-based heating. 

The fourth step in the direct ethanol process is considered to be key to the economic viability of the bioconversion of cellulose to chemicals. The pretreated cellulose slurry is simultaneously converted to glucose and the glucose to ethyl alcohol in the same vessel in a continuous or semi-continuous mode. The enzyme sample is the whole culture from the enzyme production vessel. The feedstock is a slurry of 7.5% to 15% cellulose. The yeast is either added as a cake or recycled as a cream. 

Sur ctants have been used in deinking process of recycled paper to promote ink detachment from fibers and help the dispersion of detached ink in process water (17). Their typical dosages are 0.25-1,5 wt% based on dry paper weight. Surfactant, also, improves cellulose hydrolysis when it is added with enzyme because it helps cellulase to desorb easily from the cellulose surface after... 

Another merit of in vitro biosystems is easy and low-cost product separation from aqueous reaction solutions, especially for products that are usually located intracellularly, such as cytoplasmic proteins and sugar phosphates. The recycling of biocatalysts is also important as it can save process costs and separate biocatalysts from the product easily. Enzyme immobilization has been widely used to recycle immobilized enzymes from the liquid reaction system. For example, easily recyclable cellulose-containing magnetic nanoparticles have been developed for immobilizing enzymes. These nanoparticles are new solid supports for immobilizing enzymes, which can be selectively recycled or removed from other biocatalysts, substrates, and products by a magnetic force. 

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