Noncellulosic polysaccharides

Several procedures have been used to hydrolyze polysaccharides in cell walls and cell wall fractions. For example, the noncellulosic polysaccharides can be hydrolyzed using 1 M sulfuric acid for 2 to 3 hr at 100°C (Selvendran and Ryden, 1990). One of the simplest procedures is that of Albersheim et al. (1967) in which hydrolysis of the noncellulosic polysaccharides is achieved by incubating in 2 M trifluoroacetic acid (TFA) at 121 °C for 1 hr. The advantage of the TFA procedure is that it is quick and the acid can be removed by evaporation in a gentle stream of air or nitrogen. However, neither the 1 M sulfuric acid or TFA procedures hydrolyze cellulose. Hydrolysis of cellulose can be achieved by an initial dispersion in 72% (w/w) sulfuric acid (Saeman et al., 1963 Selvendran et al., 1979 Fry, 1988 Harris et al., 1988 Selvendran and Ryden, 1990) followed by hydrolysis in 1 M sulfuric acid. 

In this procedure, the cell walls are first treated with TFA to determine the neutral monosaccharide composition of the noncellulosic polysaccharides. The TFA-insoluble residue is then hydrolyzed using a two-stage sulfuric acid procedure to determine cellulose content. 

Qualification of different cellulose sources for the various end use applications is determined on the basis of purity, molecular size, and a-cellulose content, a-cellulose refers to the portion of cellulose insoluble in 18% aqueous sodium hydroxide. Whereas the content of noncellulosic polysaccharides has proven to be a hindrance to the clarity of cellulose esters (determined as haze in otherwise clear films), a-cellulose content is important for the spinnability of cellulose solutions into regenerated fibers, and for viscosity characteristics of cellulose ethers. Molecular weights play an important role in various cellulose ethers. 

Hemicelluloses repr s ni a class of noncellulosic polysaccharides that is associated with cellulose in plant cell walls [56]. The term hemicelluloses was first used by Schulze in 1887 in the belief that (hemi- or half)-celluloses were perhaps components that were on the way of becoming cellulose a term meant to distinguish this group of noncellulosic polysaccharides from that which makes up the cell wall stmcture. It has long been recognized that the term is unfortunate and misleading, and that polyoses or heteropolysaccharides are better descriptors [57]. However, hemicelluloses is an often-used designation for the noncellulosic heteropolysaccharide components in plants. The latter term shall be used in this text. 

Solid-state NMR has been used in the molecular characterization of the undegraded wheat straw and degraded samples. The NMR spectra confirmed the increase in carboxyl content but indicated that the overall lignin and methoxyl contents remained relatively constant, although some nonsyste-matic variations were observed. The spectra also showed a decrease in amorphous noncellulosic polysaccharides in relation to the crystalline cellulose upon degradation. 

Hydrolysis. A range of hydrolytic procedures has been employed for the release of neutral sugars from plant cell walls. A high proportion of the neutral noncellulosic polysaccharides can be hydrolyzed quantitatively using 1M H2SO4 for2.5 hat 100°C (Selvendranetal., 1979),or 2M trifluoroacetic acid for 2 h at 120°C (Albersheim et al., 1967). 

Structural polysaccharides, also referred to as nonstarch polysaccharides (NSP), in SB are diverse, and some have complex structures. Total NSP concentration is the sum of water-insoluble and water-soluble NSP fractions. Nonstarch polysaccharides also can be divided into cellulosic and noncellulosic polysaccharides. The noncel-lulosic polysaccharides consist of a variety of monosaccharides (arabinose, galactose, glucose, mannose, xylose, and uronic acids Table 9.3) that are arranged in complex combinations. As the difference in fiber fractions between SBM and SH (Table 9.2) indicates, the NSP composition is rather different in SB cotyledons compared to SH. Purified SB cotyledon cell walls contain approximately 73% NSP, and small amounts of noncarbohydrate matter consisting of protein, minerals, and phenolics (BriUouet Carre, 1983). 

Table 9.3. Approximate Monosaccharide Composition of Nonstarch, Noncellulosic Polysaccharides in Soybean Meal ...

The cotyledon co-product is processed, dried, and sold as a dietary fiber in competition with other sources such as a-cellulose, psyllium seed, guar gum, locust bean gum, pectin, and wheat, corn, and oat brans. Manufacturer s specifications for a domestic product include 75% (mfb) dietary fiber (65% noncellulosic polysaccharides and 10% cellulosic), 12% moisture, 0.2% fat, and 4.5% (as is) ash. 

Epidemiological evidence in causative relationships between fiber-depleted diets and certain characteristically Western diseases has stimulated research Interest in the physiological and physico-chemical properties of various dietary fibers in the past decade (1). Dietary fibers are composed of cellulose, lignin, and noncellulosic polysaccharides, all of which are the... 

Lignin is synthesised in plants from monomeric molecnles, whose functionality varies from two to four. Thus, both branched chain and the crosslinked structure may be formed. In plant tissue, the polymer chains of lignin are snrronnded by macromolecules of noncellulosic polysaccharides, with which they form an amorphous lignocarbohydrate matrix. The experimental methods that allow the stndy of the complex topology of macromolecules in a multicomponent solid composite are very limited. Therefore, most of the data are interpreted using theoretical methods developed from polymer chemistry. 

Noncellulose polysaccharides are referred to as hemicelluloses whereas the cellulose gives rise to the various structural microfibrils in the cell wall, the hemicelluloses surrounding the microfibrils compose the matrix cell wall component. One may expect to find several monosaccharide building blocks in the hemicelluloses including glucose, galactose, manose, xylose, and arabinose. Work has been performed to determine the cell wall carbohydrates and starch in alfalfa using NIR [57]. 

Differences in the structure of cell walls of various plants and their parts are related to the differentiation of the primary cells, cellulose crystalinity and the type and amount of non-cellulose polysaccharides. Deposition of additional layers of cellulose (in the form of clearly oriented parallel microfibrils), deposition of noncellulose polysaccharides and lignification of the polysaccharide network, which is due to polymerisation of phenolic compounds, results in the formation of thick secondary cell walls, which have various special functions. For example, they ensure the rigidity of tissues, transport of water and have protective and other functions. 

Hemicelluloses he-mi- sel-yo- l6s n [ISV] (1891) The principal noncellulosic polysaccharides in wood. Wood contains 28—35% hemicelluloses, the balance being cellulose and lignin. (Morrison RT, Boyd RN (1992) Organic chemistry, 6th edn. Prentice Hall, Englewood Cliffs, NJ)... 

---