Birch, acidic hemicelluloses

Xyloisosaccharinic acid [2,4-dihydroxy-2-(hydroxymethyl)butanoic acid] is one of the major, alkaline-degradation products of wood xylan, in particular, that of birch. The disaccharide, 2-O-D-xylopyranosyl-L-arabinose, which was isolated as a hydrolysis product of corn-cob hemicellulose, is readily degraded at 100° in 15 mM Ca(OH)2 to acidic products, primarily saccharinic acids. Xylan oligosaccharides from corn-cob hemicellulose produced 2,4-dihydroxy-2-(hydroxymethyl)buta-noic acid when exposed to 0.02 M Ca(OH)2 at 25°. However, it was noted that the xylan, itself, was stable at 100° in Af NaOH. The major acidic component of the hemicellulose fraction of slash pine Pinus el-liotti) after acid hydrolysis was identified as 4-O-methyl-D-glucuronic... 

Tritiated Water. Exchange with tritiated water and subsequent scintillation analysis of the tritium content is a very useful method for accessibility measurements when the samples are not transparent to IR and visible light. Thin shives of sapwood from black spruce (a common softwood), and white birch (a common hardwood) where studied before and after delignification by treatment with peracetic acid (17). Also wood cambium from the same softwood was studied. The cambium is the recently formed fiber layer, located close to the bark and not yet lignified. Spruce and birch wood contain about the same amounts of cellulose, 42 and 44%, respectively. Spruce wood has more lignin (28 vs. 18% ), while birch has more hemicellulose (35 vs. 2S% ), in particular more pentosans than spruce (24 vs. 14%). 

Hardwood hemicelluloses are represented by xylans and a small proportion of glucomannans. Hardwood xylans are linear polymers, constituted of [l,4]-linked xylanopyranosyl units that constitute the main skeleton. Every tenth D-xylanopyranosyl unit is substituted by a 4-0-methyl-D-glucuronic acid residue, linked to the birch xylan chain by [1,2] linkages, that has been found to retard the alkaline peeling reaction. Analysis of partially hydrolyzed xylan indicates that 4-0-methyl-D-glucuronic acid units linked to the C2 position are randomly distributed along the birch xylan backbone [9]. 

Acetic acid, formic acid and ievulinic acid are the most common carboxylic acids found in the hydrolyzates. Acetic acid is not only a by-product of hydrolysis (45) but is also a well-known by-product in fermentation (46). Acetic acid is mainly formed from acetylated sugars in the hemicellulose, which are cleaved off already at mild hydrolysis conditions. Therefore, die acetic acid yield in the hydrolysis does not significantly depend on the severity of the hydrolysis process (2). Hydrolysis of hardwoods (alder, aspen and birch) at 198-234°C, 0.5 g/1 H2SO4, and 33% wood dry materials for 7 min resulted in approximately 10 g/l acetic acid, whereas the hydrolysis of softwoods (pine and spruce) produced 3 g/1 acetic acid at similar conditions (2). 

C12H22O11 342.299 Structural unit in the glucomannans of plant hemicelluloses. Isol. from the partial acid hydrolysates of the glucomannans from Amorphophallus spp., white spruce (Pinus glauca), western hemlock (Tsuga heterophylla), red maple (Acer rubrum), eastern white pine (Pinus strobus), larch (Larix decidua), jack pine (Pinus bank-siana) and from Narcissus tazetta from hemicelluloses of lobolly pine (Pinus taeda) and in trace amounts from the acetolysates of the a-celluloses from white birch (Betula papyrifera) and slash pine... 

C12H20O11 340.283 Isol. from partial acid hydrolysates of maritime pine Pinus pinaster), birch (Betula verrucosa) and Scandinavian spruce (Picea abies) hemicelluloses. 

Isolated cellulose I and xylan extracted finm bleached birch pulp was mixed in order to study the process opposite to the removal of hemicelluloses fiom bleached kraft pulp by acid hydrolysis (16). The spectrum of hydrated tylan is shown in Figure 6. 

Shimizu, K., and O. Samuelson Uronic Acid in Birch Hemicellulose. Svensk Papper-stidning 76, 150 (1973). 

---