Enzymatic Hydrolysis of Hemicelluloses

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. 

Popa V.I., Enzymatic hydrolysis of hemicelluloses and cellulose, in Polysaccharides. Structural Diversity and Functional... 

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. 

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. 

The foregoing observations confirm the conclusions derived from former experiments with beechwood holocellulose (10) (1) A partial degradation of the hemicelluloses is imperative before the cellulose fibrils can be attacked. (2) The hemicelluloses seem to be deposited between the cellulose fibrils or even to be encrusting them. (3) The enzymatic hydrolysis of the cellulose is governed by the porosity of the tissue (enzyme diffusion), the impediment of the hemicelluloses, and the properties of the cellulose (e.g., crystallinity). 

The xylose produced via enzymatic hydrolysis of the hemicellulose fraction could be converted via microbial fermentation into xylitol (which has been listed by the US Department of Energy among the top 12 value-added platform chemicals) for the production of a range of chemicals, including xylaric acid, propylene glycol, ethylene glycol and a mixture of hydroxyl-furans and polyesters (Werpy and Petersen, 2004). 

Schwald, W., Brownell, H. H., and Saddler, J. N. 1988. Enzymatic hydrolysis of steam treated aspen wood influence of partial hemicellulose and lignin removal prior to pretreatment. J. Wood Chem. Technol, 5(4), 543-560. 

XOS are manufactured by enzymatic hydrolysis of xylan from corn cobs, oat spelt xylan or wheat arabinoxylan [257]. Xylan, a principle t)q)e of hemicellulose exists as a linear polymer of -D-xylopyranosyl units linked by 1 4 glycosidic bonds. Depending on the origin, different substituents such as arabinfuranosyl, 4-0-methylglucuronosyl and acetyl groups are present. Typical raw materials for XOS production include hardwoods, com cobs, straws. 

Xylose is the main product of enzymatic hydrolysis of angiosperm hemicellulose. In the past, it was regarded that xylose was unfermentable, until Wang et al. [2] found that some microbes were able to utilize xylose to produce ethanol in 1980. Now, more than 100 kinds of microbes that are capable of fermenting xylose have been found, including bacteria, fungi, and yeasts [3]. 

Fig. 3 Time course of cellulose and hemicellulose conversion and cellobiose production during enzymatic hydrolysis of wet-exploded wheat straw at 14% DM and an enzyme loading of 10 FPU/g cellulose. Degree of cellulose and hemicellulose conversion was calculated as amount of glucose and xylose, respectively, released during the hydrolysis relative to the maximum theoretical. Results are average of duplicates...

The enzymatic hydrolysis of intractable cellulosic and hemicellulosic substrates of the plant cell-wall is efficiently performed by multienzyme machines designated under the collective name of cellulosomes. Cellulosomes are large extracellular enzyme... 

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