Cellulosic materials enzymatic hydrolysis

The Nature of the Substances with Which the Cellulose Is Associated and the Nature of That Association. As discussed earlier in this review, cellulose in cotton and wood is associated with a variety of other materials. These substances frequently have an influence on the susceptibility of the cellulose to enzymatic hydrolysis. The influence of several different types of material will be discussed in turn. 

Increasingly, biochemical transformations are used to modify renewable resources into useful materials (see Microbial transformations). Fermentation (qv) to ethanol is the oldest of such conversions. Another example is the ceU-free enzyme catalyzed isomerization of glucose to fmctose for use as sweeteners (qv). The enzymatic hydrolysis of cellulose is a biochemical competitor for the acid catalyzed reaction. 

The advances made in enzymatic hydrolysis of cellulosic materials (14) are also of interest. This technology involves only moderate temperature processes in simple equipment which promises to be of significantly lower capital cost than the pressure equipment associated with conventional acid wood hydrolysis processes. All of these considerations combined to lead us to study processes for ethanol production from wood, especially in an effort to obtain data for material and energy balances, and possibly for the economics. 

A few animals (especially ruminants and termites) are able to metabolize cellulose, but even these animals depend on appropriate microorganisms in their intestinal tracts to hydrolyze the -1,4 links other animals, including man, cannot utilize cellulose as food because they lack the necessary hydrolytic enzymes. However, such enzymes are distributed widely in nature. In fact, deterioration of cellulose materials —textiles, paper, and wood —by enzymatic degradation (such as by dry rot) is an economic problem that is not yet adequately solved. Efforts to turn this to advantage through enzymatic hydrolysis of cellulose to glucose for practical food production have not been very successful (see Section 25-12). 

While this reaction is substantially exothermic (6), it provides an intriguing approach to the production of fuels from renewable resources, as the required acids (including acetic acid, butyric acid, and a variety of other simple aliphatic carboxylic acids) can be produced in abundant yields by the enzymatic fermentation of simple sugars which are, in turn, available from the microbiological hydrolysis of cellulosic biomass materials ( ] ) These considerations have led us to suggest the concept of a "tandem" photoelectrolysis system, in which a solar photoelectrolysis device for the production of fuels via the photo-Kolbe reaction might derive its acid-rich aqueous feedstock from a biomass conversion plant for the hydrolysis and fermentation of crop wastes or other cellulosic materials (4). 

Influence of Fine Grinding on the Hydrolysis of Cellulosic Materials—Acid Vs. Enzymatic... 

Marsden, W. L. and P. P. Gray, "Enzymatic Hydrolysis of Cellulose in Lignocellulosic Materials," CRC Critical Review Biotech. 3 (1986) 235-276. 

Cowling, E. B., and Brown, W. (1969). Structural features of cellulosic materials in relation to enzymatic hydrolysis. Adv. Chem. Ser. 95,152-187. 

The following gives a brief compilation of procedures to determine hexenuronic acids in cellulosic pulp samples. The common methods are based on hydrolysis of HexA moieties from pulp, either enzymatically or chemically, with subsequent quantification of the hydrolysis products either directly or after chemical conversion into UV active compounds. A comparison of these three methods is given by Tenkanen et al. [136]. For comparison rather than exact determination of HexA, e.g., during bleaching stages, the diffuse reflection UV VIS method can be applied [137]. A photoacoustic FTIR procedure based on chemometric analysis has been described as well [138]. In Table 3, the available methods to analyze HexA moieties in cellulosic material are summarized. 

The production of fuel ethanol from renewable lignocellulosic material ("bioethanol") has the potential to reduce world dependence on petroleum and to decrease net emissions of carbon dioxide. The lignin-hemicellulose network of biomass retards cellulose biodegradationby cellulolytic enzymes. To remove the protecting shield of lignin-hemicellulose and make the cellulose more readily available for enzymatic hydrolysis, biomass must be pretreated (1). 

Corn stover, like lignocellulosic materials in general, is resistant to enzymatic hydrolysis, because of both the tight network in the lignocellulose complex and the crystalline structure of the native cellulose. These difficulties can be overcome by employing a suitable pretreatment (7). 

A major problem in the commercialization of this potential is the inherent resistance of lignocellulosic materials toward conversion to fermentable sugars (4). To improve the efficiency of enzymatic hydrolysis, a pretreatment step is necessary to make the cellulose fraction accessible to cellulase enzymes. Delignification, removal of hemicellulose, and decreasing the crystallinity of cellulose produce more accessible surface area for cellulase enzymes to react with cellulose (5). 

Carbohydrates would be the predominant raw materials for future biorefineries. The major polysaccharides found in nature are cellulose, hemicellulose and starch (see Chapter 1). These molecules would be mainly utilised after they are broken down to their respective monomers via enzymatic hydrolysis, thermochemical degradation or a combination of these two. Cellulose and hemicellulose, together with lignin, constitute the main structural components of biomass. Starch is the major constituent of cereal crops. This section would focus on the potential utilisation of carbohydrates and lignocellulosic biomass for chemical production. 

When cellulose is used as a raw material, the activity of cellulase (the enzyme catalyzing cellulose hydrolysis) is inhibited by glucose and short cellulose chains. One way to overcome this inhibition is to combine enzymatic hydrolysis with glucose fermentation to ethanol, as the accumulation of ethanol in fermenter does not inhibit cellulase. 

---