Cellulose holocellulose

Relatively pure xylan isolated from the holocellulose of aspen (Populus) wood is said to contain 85% of xylose residues.78 One of the characteristic properties of xylan is its ease of hydrolysis. Because it hydrolyzes much more readily than cellulose, mild acid treatment may be employed to bring about preferential hydrolysis of xylan from plant material. Xylose is ordinarily prepared in the laboratory by direct sulfuric acid hydrolysis of the native xylan in ground corn cobs.74 Hydrolysis in hydrochloric acid proceeds rapidly, but decomposition to furfural also occurs to some extent.76 A commercial method for the production of D-xylose from cottonseed hulls76 and straw77 and from corn cobs17 78 has been described. 

Partially degraded cellulose is called hydrocellulose or oxycellulose, depending on the agent used for degradation. The term holocellulose is used to describe the residue after lignin has been removed from wood pulp. 

One of the favored organisms for study of cellulolysis by Trichoderma is T. reesei. Consequently, many mutant strains which hyperproduce cellulase have been obtained by treatment with ultraviolet light, gamma irradiation, the linear accelerator, diethyl sulphate and N-methyl-N -nitro-N-nitroso-guanidine (7). Whereas much of the study of T. reesei has been with cellulose as substrate, it is relevant to consider the other fractions of natural lignocelluloses hemicellulose and holocellulose (the combined cellulose and hemicellulose fraction). 

Table I. Growth of Trichoderma reesei QM9414 (Tr) and T. harzianum IMI275950 (Th) on wheat straw lignocellulose (lignocell.) or derived cellulosic materials from straw (cell., cellulose hemicell-A, hemicellulose-A holocell., holocellulose)...

Holocellulose Cellulosic material obtainable from wood after removal of lignin. The term therefore means total carbohydrates (cellulose and hemicellulose) present in the wood (Refs 1 2) Ritter (Ref 3) succeded in isolating holocellulose from wood pulp by repeated chlori-nations, followed by extraction with alcohol containing 3% of monoethanolamine. The resulting product was white but changed color on standing... 

Under the condition chosen cellulose decomposes much faster than lignin. Adding nitrogen accelerates the decomposition of holocellulose as well as... 

The whole procedure normally takes about 1 hr. The acid is then evaporated, and the dry matter can be analyzed. This method can be applied to cellulose from wood, as a-cellulose or pulp, or to other celluloses (e.g., cotton) as well as to cellulosic materials with higher amounts of other polysaccharides (e.g., holocellulose). The chromatograms of the hydrolysates of a-cellulose from beechwood and of holocellulose from sprucewood (Figure 6) are examples of the application of this method. Compared with sulfuric acid hydrolysis, the total sugar yield from the spruce holocellulose is higher after the hydrolysis with concentrated TFA (Table II). Regarding the individual sugars, it can be seen that the... 

Figure 6. Chromatograms of the hydrolysates of a-cellulose from beech-wood and holocellulose from sprucewood...

Sprucewood holocellulose was treated with an endo-p-1,4-mannanase isolated from Aspergillus niger and an endo-/3-1,4-xylanase, two avicelases, and a cellobiohydrolase C isolated from Trichoderma viride. The mannanase hydrolyzed about a quarter of the mannan in 2-3 days without xylan or cellulose degradation. The xylanase hydrolyzed about half the xylan with 10% mannan solubilization. The three cellulases hydrolyzed up to 45% of the cellulose and 20% of the xylan, accompanied by 40-70% solubilization of the mannan. Combined xylanase-mannanase treatment hydrolyzed about half the xylan and mannan. Addition of mannanase to to cellulose-treated samples increased the degradation of the cellulose and mannan. Micromorphological studies of the variously treated specimens revealed a loss of substances in P/Slf T, and adjacent zones of S2 of the tracheid wall. 

Holocellulose (Enzyme Treatment) Mannan Xylan Cellulose Total... 

Avicelases or Cellobiohydrolase C. Treatment of sprucewood holocellulose with the three different cellulose-splitting enzymes gave very similar results. The cellulose was hydrolyzed to about 25-45% in 48 hr (Table I). Cellobiose was the predominant reaction product, but the amount of glucose increased considerably with incubation time (Table II). Acid hydrolysis of the reaction solutions showed that higher-... 

The cellulose was either not degraded by the xylanase treatment or degraded only to a small extent. This is in line with former investigations on holocelluloses of sprucewood (5) and beechwood (7,10,22). 

The three cellulases decomposed about 25-45% of the cellulose accompanied by solubilization of about 40-70% of the mannan and, by partial hydrolysis, of about 20% of the xylan present in the untreated sprucewood holocellulose. Based on the degradation products (cf. Table III, Columns 13-15, and Table II), the catalytic actions of the three cellulases—all isolated from Trichoderma viride—are similar or identical. The lower absolute degradation values obtained with cellobiohydrolase C might merely be a result of enzyme concentration. 

The degradation rates of cellulose obtained with the individual cellulases were, in each case, higher than the corresponding values reached under the same conditions with beechwood holocellulose (10). The same tendency had already been found in preliminary tests with crude cellulase preparations (13 cf.23). 

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. 

About a quarter of the mannan in the sprucewood holocellulose seems to be accessible to the mannanase. More mannan can be hydrolyzed only when the second hemicellulose becomes at least partly dissolved. The remaining mannan—less than half the initial amount— appears to be removable only together with cellulose degradation. In beechwood holocellulose, most of the main hemicellulose—i.e., the xylan —can be hydrolyzed rather selectively by the sole action of xylanase. This phenomenon could be due to the different molecular size of the two hemicellulases, mannanase about 24 A, xylanase about 18 A (JO), resulting in better diffusing conditions for the xylanase. However, the porosity of the... 

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 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. 

Hemicellulose Group of carbohydrates found in the cell wall in more or less intimate contact with cellulose. The hemicelluloses are more soluble than cellulose and much more readily hydrolyzed into sugars. Holocellulose Total carbohydrate fraction of wood remaining after the removal of lignin and solvent extractable substances. 

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