Hemicellulose hydrolysis removal

These results could suggest that what has been traditionally been described as "biphasic" behavior may reflect a combination of chemical reaction and mass transfer effects, with each limiting xylan reaction and removal at different stages or modes of operation. This effect might be better described by a model that incorporates reaction of solids to form soluble species as a function of temperature and acid concentration coupled with a second mass transfer step that is affected by flow. On this basis, we plan to investigate whether the pore leaching model could be simplified and adapted in this way to better describe hemicellulose hydrolysis. 

Xylan remaining in solid residues was also analyzed. As seen in Fig.5, increasing velocity significantly increased xylan removal, especially in the first 8 min. For example, xylan removal for operation at 2.8,5.2, and 10.7 cm/ min was 60, 70, and 82% for hot water only pretreatment of corn stover at 200°C after 8 min. However, after 16 min, the differences in xylan removal were less for all velocities run, suggesting that fluid velocity has less impact on the overall degree of hemicellulose hydrolysis. 

The effectiveness of dilute acid hydrolysis as a pretreatment has been verified experimentally. Researchers at NREL have characterized the susceptibility of a variety of short rotation woody and herbaceous crops and agricultural residues upon dilute acid pretreatment [17,38-41]. What is more interesting is that the maximum digestibility usually coincides with complete hemicellulose removal. The dilute acid treatment of biomass aimed at hemicellulose hydrolysis has since become a widely accepted pretreatment method for enzymatic hydrolysis [16,42-44]. 

Figure 6. Effect ofxylan removal (a) and lignin removal (b) on enzymatic digestibility of cellulose produced by hemicellulose hydrolysis of com stover at the conditions noted.

Acid hydrolysis of cellulosic materials that include some hemicellulose, produces D-xylose, D-glucose, and cellobiose, as well as 11, 2-furalde-hyde (5), levulinic acid, formic acid, and acetic acid. In order to lessen the contamination due to hemicellulose, acid hydrolysis is generally performed in two steps dilute sulfuric acid (1%) at 80-120° followed by 5-20% sulfuric acid at 180°. The initial stage removes most of the pentogly-cans (pentosans). 

Other abundant carbohydrates, such as hemicelluloses and pectin, are usually highly branched and thus not very suitable for fiber and film production. Hemicelluloses and some pectins are also acetylated in the native state, which makes them more resistant to enzymatic hydrolysis (20,21) and changes their solubility properties (9-77,75). Branching does not, however, preclude their utilization in such potentially large markets as thickeners and adhesives. Xylans, for example, show such a strong adhesion to cellulose fibers that they are very difficult to remove completely by both acidic and alkaline pulping processes (22). 

Attempts to remove hemicellulose for production of dissolving pulps with very low hemicellulose contents have shown that complete enzymatic hydrolysis of hemicellulose within the pulp is difficult to achieve. The xylan content in delignified mechanical aspen pulp was reduced from approximately 20 to 10%, whereas in bleached hardwood sulphite pulp the xylan content was decreased from 4 to only 3.5% even at very high enzyme dosages (50). The complete removal of residual hemicellulose seems thus unattainable, apparently due to modification of the substrate or to structural barriers. 

Samples of the sugars being removed are cooled to 20°C. and tested for density by a sugar Brix spindle., Analysis of these solutions shows that the reducing sugar content of the solutions is 60 to 70% of the concentration shown by the Brix spindle. If the hydrolysis of hemicellulose is not complete, the ratio of reducing sugar to Brix will be low and will indicate that insufficient acid has been used. 

Processes need to be developed to expose LHC for effective penetration of chemicals that promote separation of LHC or which render cellulose accessible by swelling. These processes can have many objectives (1) to get at cellulose (2) to remove hemicellulose from the reaction site before cellulose hydrolysis (3) to remove lignin from the site and (4) to bring penetrating or hydrolysis agents into effective contact with cellulose. 

The combined action of xylanase and mannanase on sprucewood holocellulose increased the hydrolysis of hemicelluloses without any detectable attack of cellulose. At the end of the experiments—i.e., after 48 hr of xylanase incubation followed by 32 hr of combined xylanase-mannanase incubation—about half the hemicelluloses present in the starting material were selectively converted into low-molecular-weight sugars. The amount of mannan removed was two times higher than after 80 hr of incubation with mannanase only. Unexpectedly, the xylan dissolution was scarcely increased by the combined action of the two hemicellulases. 

The current observations confirm previous studies on beechwood and sprucewood holocellulose (7,10,19). The attack of the hemicellulose proceeds from the primary wall/Si as well as from the tertiary wall into S2 the pit chambers constitute preferred paths of enzyme diffusion into the walls. Also, substances of the middle lamella, especially in the cell corners, are removed by the xylanase and the mannanase treatments. Parallel to the removal of hemicelluloses, the fibrillar structure of the cellulose and its lamellar arrangement in transections of cell walls became obvious. In samples treated with cellulases, the cellulose fibrils were often completely hydrolyzed in the Si layer, occasionally accompanied by complete dissolution of cell-wall portions. This is also in conformity with the previous conclusion that the cellulases hydrolyze highly ordered zones of cellulose and remove hemicelluloses by hydrolysis or by detachment. 

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