Degradation of hemicelluloses

The two intermediates of commercial furan resins are furfural and furfuryl alcohol. Furfural occurs in the free state in many plants but is obtained commercially by degradation of hemicellulose constituents present in these plants. There are a number of cheap sources of furfural, and theoretical yields of over 20% (on a dry basis) may be obtained from both com cobs and oat husks. In practice yields of slightly more than half these theoretical figures may be obtained. In the USA furfural is produced in large quantities by digestion of com cobs with steam and sulphuric acid. The furfural is removed by steam distillation. 

A probable pathway for the degradation of hemicelluloses via free-radical intermediates has been proposed by Fengel and Wegener (1989) and is shown in Figure 5.1. Hemicellulose polymers are depolymerized to form oligosaccharides and monosaccharides, which are dehydrated to form furfural (pentoses) and hydroxymethyl furfural (hexoses). 

After steam pretreatment the yield of solid material ranged from 37 to 85.4 g/100 g and decreased with increased acid concentration at the same temperature (see Table 1). The reduction in solid material was owing to the solubilization and/or degradation of hemicellulose and extractives. However, under more severe conditions, a part of the cellulose was also solubilized. 

The breakdown of furan aldehydes leads to the formation of formic and levulinic acid. Moreover, acetic acid is formed during the degradation of hemicellulose. Partial breakdown of lignin can generate a variety of phenolic compounds (23), which also inhibit S. cerevisiae (14,15). In contrast to furan aldehydes and aliphatic acids, the toxic effect of specific phenolic compounds is highly variable (15). Different raw materials and different approaches to prepare lignocellulose hydrolysates will result in different concentrations of the fermentation inhibitors (16,17). 

The complete degradation of hemicellulose becomes more complex than that of cellulase, since substituent-hydrolyzing activities are also necessary. With heteroxylans, apart from endo-l,4- 3-xylanase, which catalyzes the hydrolysis of internal 3-l,4-xylan links and P-xylosidase, which catalyzes the hydrolysis of xylooligossacharides, mainly xylobiose into xylose, other enzymes must act to accomplish complete hydrolysis, such as acetyl xylan esterase, a-glucuronidase, and a-L-arabinofuranosidase (1). 

Starch-free BSG was subjected to reaction with water (autohydrolysis) in a 2-L stainless steel Parr reactor model 4532 (Moline, IL), to cause the hydrolytic degradation of hemicelluloses, operating under optimized conditions (liquid-to-solid ratio of 8 1 [w/w], standard heating temperature profile up to 190°C, isothermal reaction at 190°C for 2.5 min) (2). After the reactor was cooled down, the oligosaccharide-containing liquor (OCL) was separated from the residual solid by filtration (Whatman no. 1 filter paper). 

In wood pyrolysis, it is known that several parameters influence the yield of pyrolytic oil and its composition. Among these parameters, wood composition, heating rate, pressure, moisture content, presence of catalyst, particle size and combined effects of these variables are known to be important. The thermal degradation of wood starts with free water evaporation. This endothermic process takes place at 120 to 150 C, followed by several exothermic reactions at 200 to 250°C, 280 to 320 C, and around 400 C, corresponding to the thermal degradation of hemicelluloses, cellulose, and lignin respectively. In addition to the extractives, the biomass pyrolytic liquid product represents a proportional combination of pyrolysates from cellulose, hemicelluloses. 

The degradation of hemicelluloses also proceeds via endo- and exohydrolases. The substrate specificity depends on the monosaccharide building blocks and on the type of binding, e. g., endo-1,4-P-D-xylanase, endo-l,5-a-L-arabinase. These enzymes occur in plants and microorganisms, frequently together with cellulases. 

When SO2 was used, it was observed that low temperature and long hydrolysis times had the greatest influence on product yield. Weight loss was dictated primarily by temperature. The lowest concentration of sulphur dioxide (2 w/w%), lowest temperatures and longest hydrolysis time resulted in the highest yield (8.3 w/w /o). Higher concentrations of SO2 resulted in extensive degradation of hemicelluloses. 

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