Xylan digestibility

The effect of reaction time on the performance of ARP pretreatment with the HL hybrid poplar is shown in Table 3. Lignin removal of hybrid poplar increases with reaction time. Xylan removal, however, was only slightly affected by ARP reaction time. After a certain point, the xylan content remained constant, indicating that there is a portion of xylan that is resilient and difficult to remove. The 72-h glucan and xylan digestibilities correlate more closely with xylan removal than lignin removal. 

As is the case with the ARP, the glucan/xylan digestibilities with xylanase addition are higher for LL hybrid poplar than HL hybrid poplar after the SAA treatment too. 

Xylanase supplementation in enzymatic hydrolysis is effective for the substrates with high xylan content. Xylanase supplementation not only increases the xylan digestibility of treated biomass but also the glucan digestibility. Xylanase reduces the hindrance caused by resilient hemicellulose layer on the cellulose microfibril, thus, improving cellulase accessibility to the cellulosic part. 

Partial etherification of the beech wood MGX with p-carboxybenzyl bromide in aqueous alkali yielded fully water-soluble xylan ethers with DS up to 0.25 without significant depolymerization the Mw determined by sedimentation velocity was 27 000 g/mol [400,401]. By combination of endo- 6-xylanase digestion and various ID- and 2D-NMR techniques, the distribution of the substituents was suggested to be blockwise rather than uniform. The derivatives exhibited remarkable emulsifying and protein foam-stabilizing activi-... 

Little information is available on the enzymatic hydrolysis of xylan. The kinetics of breakdown of xylan are reported.128 Hemicellulose from linden wood is said to be extensively hydrolyzed124 by the digestive juices of Helix pomatia. Xylanase128 of Aspergillus niger, while active on xylan between pH 2-8, has its optimum at pH 4.0. 

Xylanases act on the P-l,4-linked xylan, the most abundant of the hemicelluloses that constitutes over 30% of the dry weight of terrestrial plants.116 They resemble cellulases and cooperate with cellulases and xylosidases117 in digestion of plant cell walls.110116118-121 Galactanase digests the P-l,4-linked component of pectins.122... 

Yang, B., and Wyman, C. E., 2000, Effect of xylan and lignin removal by batch and flowthrough pretreatment on the enzymatic digestibility of com stover cellulose, Biotechnol. Bioeng. 86 88-95. 

The ultimate goal of the AFEX treatment is to increase the yields of fermentation products such as ethanol by increasing the digestibility of the biomass. Therefore, selected runs that showed higher glucan and xylan conversion were chosen for further SSF analysis. The best treatment temperature is to be selected based on the fermentation results, not just on enzymatic hydrolysis the fermentation results are presented later in this article. 

Fig. 7 -35. Adsorption of xylan on cotton fibers present in the digester during kraft pulping (Yllner and Enstrom, 1956).

The encrusting of cellulose fibrils by lignin-carbohydrate complexes within the cell wall is still a possible mechanism for lowered cellulose availability. The nutritive availability of the different fractions of hemicellulose and cellulose is quite different as shown by the data of Gaillard (26) in Table IV. Xylan is generally less digestible than other fractions (26, 30), and Lyford et ah (36) have shown that the micellular pentosans have a low digestibility. 

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