Fermentation of Cellulosic Materials

FIGURE 8.3 Composition of residual matter obtained after the fermentation of 200 g-dry-weight/L rice straw, hydrothermally pretreated (Matano et al., 2012), in the presence of 10 FPU/g-biomass cellulase with wild-type (a) and ceUulase-displaying Saccharomyces cerevisiae strain, NBRC1440/B-EC3 (b). For a color version, see the color plate section.) 

3 Application of Cell-Surface Engineering to Thermotolerant Yeast 

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The process of fermentation of cellulose materials differs from that of starch or sugar materials (1) the hydrolysate of the lignocellulosic material is toxic to the fermentative microorganisms (2) there is more xylose in the hydrolysate. So, the utilization of pentose is one of the most important decisive factors for the process economics (Zaldivar et al. 2001). 

Two broad areas of application for xylanolytic enzymes have been identified (1). The first involves the use of xylanases with other hydrolytic enzymes in the bioconversion of wastes such as those from the forest and agricultural industries, and in the clarification and liquification of juices, vegetables and fruits. For these purposes, the enzyme preparations need only to be filtered and concentrated as essentially no further purification is required. Several specific examples of applications involving crude xylanase preparations include bioconversion of cellulosic materials for subsequent fermentation (2) hydrolysis of pulp waste liquors and wood extractives to monomeric sugars for subsequent production of single cell protein (3-5). Xylose produced by the action of xylanases can be used for subsequent production of higher value compounds such as ethanol (6), xylulose (7) and xyIonic acid (8-9). 

The de novo synthesis of fatty acids in the mammary gland utilizes mainly acetate and some (3-hydroxybutyrate. These precursors arise from the microbial fermentation of cellulose and related materials in the rumen. Once in the mammary gland, acetate is activated to acetyl-CoA. The mechanism of fatty acid synthesis essentially involves the carboxylation of acetyl-CoA to malonyl-CoA, which is then used in a step-wise chain elongation process. This leads to a series of short-chain and medium-chain length fatty acids, which differ by two CH2 groups (e.g., 4 0, 6 0, 8 0, etc.) (Hawke and Taylor, 1995). These are straight-chain, even-numbered carbon fatty acids. However, if a precursor such as propionate, valerate or isobutyrate, rather than acetate, is used, branched-chain or odd-numbered carbon fatty acids are synthesised (Jenkins, 1993 see Chapter 2). 

The most attractive features of solid-state fermentation include low capital investment and low operational costs. These features are favorable in the hydrolysis of cellulosic materials because the cost of cellulase is a key factor for commercial production. Currently, cellulase production by submerged culture has made great progress and been commercialized, while solid-state fermentation is only operated on a small scale in a few countries, such as China, and further efforts in research and development are still needed in order to improve the process and the equipment. 

Pulverization can reduce the size as well as the crystallinity of cellulosic materials and increase the surface area and bulk density. It is also possible to separate part of the hgnin from carbohydrates which makes it easier for microorganisms to digest cellulose. Various equipment, such as a compression mill, a bead mill, an extruder, a roll mill and disc refiners, etc., can be used for pulverization. Unfortunately these methods tend to be very expensive and too energy intensive. For sohd-state fermentation, if the particles are too fine, the oxygen mass transfer will become a big problem therefore, hghtly crushed or just ground raw material will suffice. 

Alkali treatment of cellulosic materials can remove hgnin as well as part of the hemicellulose and expose cellulose for fungi to digest. In addition, the alkah treatment method can swell and disrupt the structure of cellulose. Commonly used alkalis includes NaOH, Ca(OH)2 or CaO and aqueous as well as gaseous NH3. For solid-state fermentation, after alkah treatment, the cellulosic materials do not need to be washed, because all the solubilized hgnin and hemicellulose can be retained in the raw material [15]. After the addition of other nutrients, the pH has to be adjusted to a value of approx. 6.5 with H2SO4. 

Costs might be lowered by combining steps and by starting with cheaper raw materials, such as wastes. Simultaneous saccharification and fermentation of cellulose (a polymer of glucose) has been accomplished by combining a... 

Celluclast. [Novo Nordisk] Cellulase enzyme for breakdown of cellulosic material for prod, of fermentable sugar and for municipal cellulosic waste treatment. 

Biological enzymatic conversion of glucose by fermentation also has been extensively studied for the production of many products one of the most important is bioethanol. Many bacteria and fungi could produce enzymes for the hydrolysis of cellulosic material. These microorganisms can be aerobic or anaerobic, mesophillic or thermophillic. ... 

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. 

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