Hemicellulose fermentation

Here, we draw upon recently published and unpublished works, to provide an aggregated perspective on the status and prospects for CBP using thermophilic bacteria. We focus on two microbes that have received particular attention in this context the cellulose-fermenting Clostridium thermocellum and the hemicellulose-fermenting Thermoanaerobacterium saccharolyticum. We focus... 

Processes based on mierobial fermentation are currently regarded as alternatives having the most potential in eonverting hemicelluloses into biofuels and chemicals such as ethanol, butanol, hydrogen, and succinic acid due to the advantages of low cost and environmental friendliness. Recent studies on biofuels and chemicals from hemicellulosic fermentation are shown in Table 7.2. 

Table 7.2 Biofuels and chemicals from hemicellulosic fermentation. ...

C. The pentose sugar of straw, cotton-seed hulls and various hemicelluloses, and of some glycosides, including the primeverosides. It is not fermentable and behaves chemically as other sugars. 

Wood chips can also be utilized as such to produce bioethanol. The cellulose and hemicellulose material is hydrolyzed in the presence of acids (H2SO4, HCl, or HCOOH) or enzymes to yield glucose and other monosaccharides [16]. Lignin is separated by filtration as a solid residue and the monosaccharides are fermented to ethanol, which, in turn, is separated from water and catalyst by distillation. Ethanol can be used not only as energy source but also as a platform component to make various chemicals, such as ethene and polyethene. Today green acetaldehyde and acetic acid from wood-derived bioethanol is manufactured by SEKAB Ab, at the Ornskoldsvik Biorefinery of the Future industrial park. 

Simultaneous saccharification and co-fermentation (SSCF) one-stage enzymatic hydrolysis of cellulose and fermentation of pentoses and hexoses all in one process step. The upstream hydrolysis of the hemicellulose takes place in a separate process step. 

Various solvents are being investigated to dissolve lignocellulosic materials. Some approaches focus on the selective depolymerization and extraction of lignin and hemicellulose as pre-treatment to produce clean cellulose fibers for subsequent fermentation or for pulping. Other approaches attempt to dissolve the whole lignocellulose with or without depolymerization. The liquefaction processes that are carried out at high temperature (>300 °C), and produce a complex oil mixture, are discussed above with the pyrolysis processes. 

A solution of ethanol-water with diluted H2SO4 selectively dissolves the lignin and hemicellulose at 180-200 °C [55]. The resulting purified cellulose can subsequently be fermented to ethanol at high yield. Copersucar and Dedinin are developing a process for bagasse-ethanol that is based on such a pre-treatment of the bagasse [56]. 

To break up cellulose/hemicellulose, it is treated physically (milling), with heat, and hydrolyzed (sulfuric acid + enzymes). Also in this case, improved (bio)cata-lytic hydrolysis processes for cellulose/hemicellulose are needed. The sugar can then serve as feedstock for standard fermentation plants. 

For monitoring the extent of polysaccharide hydrolysis, l.c. methods that sepeu ate and analyze the non-fermentable oligosaccharides (d.p. 3-30) derived from cellulose, hemicellulose, and pectins are useful, and have already been described (see Section III,l,c). For determination of the monosaccharide composition of completely hydrolyzed, plant polysaccharides, l.c. is especially useful and has been applied to the compositional analysis of hydrolyzed plant fiber,wood pulps,plant cell-walls,and cotton fibers.In these representative examples, the major sugars of interest, namely, glucose, xylose, galactose, arabinose, and mannose, have traditionally been difficult to resolve by l.c. The separa-... 

Pretreatment of Substrate. Several different lignocelluloses were pretreated with NaOH. This pretreatment partially solubilizes the hemicelluloses and lignin and swells the cellulose so that the organism can utilize it for its growth and for production of a cellulase system in SSF. The treated lignocelluloses were not washed. The NaOH treatment is done with a minimum amount of water so that, after the addition of nutrient solution and inoculum, the moisture content is less than 80% wt/wt and there is no free water in the medium. More water was added to make suspensions of different lignocellulosic substrates of the desired concentration (1% or 5%) for liquid-state (submerged) fermentation (LSF). 

Substrate and Pretreatment. Sweet corn (hybrid Lingodor) of W.H. Perron Laval, Quebec was grown in well prepared soil in a plot of 3 x 2 meters. Corn stalks were ground to 20 mesh to be used as a substrate. It was pretreated with 1.5% sodium hydroxide (NaOH) wt/vol with substraterwater ratio of 1 10 at 121 C for 60 minutes. The substrate was not washed after the pretreatment, and all the solubilized polymers (hemicelluloses and lignin) were retained along with the insoluble polymer (cellulose) in the fermentation medium. The composition of corn stalk is presented in Table 1. 

Analysis of Lignin of Corn Stalks Before Fermentation. During the pretreatment of corn stalks with NaOH, the lignin and hemicelluloses were solubilized. The solubles obtained after filtering through... 

Alcohol recovery from the fermentation brews was less than complete in most cases, which may be attributable to less than ideal conditions. The best yields, 60 to 97% of theory, were obtained with sugars obtained by hydrolysis of cellulosic residues of the autohydrolysis-extraction process. Unextracted pulps, or the hemicellulose solutions, gave poor ethanol formation, which suggests inhibition. In the calculation of material and energy balances which follows, we have assumed 95% yields of ethanol from wood sugars, which is readily achieved in industrial practice and which we believe to be achievable with our wood sugars as well. 

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