Cereals hemicelluloses

Hemicellulose Fiber polysaccharides that include xylans, mannans, galactans, and glucomannans found in the cell walls of cereals. Hemicelluloses contain pentoses and uronic acid in addition to hexoses. 

Xylan-type polysaccharides are the main hemicellulose components of secondary cell walls constituting about 20-30% of the biomass of dicotyl plants (hardwoods and herbaceous plants). In some tissues of monocotyl plants (grasses and cereals) xylans occur up to 50% [6j. Xylans are thus available in huge and replenishable amoimts as by-products from forestry, the agriculture, wood, and pulp and paper industries. Nowadays, xylans of some seaweed represent a novel biopolymer resource [4j. The diversity and complexity of xylans suggest that many useful by-products can be potentially produced and, therefore, these polysaccharides are considered as possible biopolymer raw materials for various exploitations. As a renewable resource, xylans are... 

AGX are also the dominant hemicelluloses in the cell walls of hgnified supporting tissues of grasses and cereals. They were isolated from sisal, corncobs and the straw from various wheat species [4]. A more recent study on corncob xylans [30] showed the presence of a hnear, water-insoluble polymer... 

PENTOSAN. A complex carbohydrate (hemicellulose) present with the cellulose in many woody plant tissues, particularly cereal straws and brans, characterized by hydrolysis to give five-carbon-atom sugars (pentoses). Thus the pentosan xylan yields the sugar xylose (HOH,C CHOH CHOH CHOH CHO) that is dehydrated with sulfuric acid to yield furfural (C5H4O2). 

Pentoses contained in hemicellulose are used to produce furfural, a useful industrial chemical, used as a solvent for resins and waxes and in petrochemical refining. It is also used as a feedstock for a range of aromatic substances (it has an almond odour) including preservatives, disinfectants and herbicides. Furfural can be converted to furfuryl alcohol and used to make resins for composite applications with fibreglass and other fibres. These are of interest in the aircraft component and automotive brake sectors. Furfural is commercially derived from acid hydrolysis of waste agricultural by-products, such as sugarcane bagasse, com cobs and cereal brans. Around 450 000 tonnes is produced by this method per year. 

Carbohydrates would be the predominant raw materials for future biorefineries. The major polysaccharides found in nature are cellulose, hemicellulose and starch (see Chapter 1). These molecules would be mainly utilised after they are broken down to their respective monomers via enzymatic hydrolysis, thermochemical degradation or a combination of these two. Cellulose and hemicellulose, together with lignin, constitute the main structural components of biomass. Starch is the major constituent of cereal crops. This section would focus on the potential utilisation of carbohydrates and lignocellulosic biomass for chemical production. 

The noncellulose (1-n-glucans in hemicellulose are generally 1,3- and 1,4-bonded arabinans, xylans, arabinoxylans, arabinogalactans, etc. Their aqueous dispersions are quite viscous to the detriment of filters used to clarify cereal-based fermented beverages. Oat (3-glucans are Theologically... 

Hemicellulose and oligosaccharides are characterized according to their water and alkali solubility branching and density are the major distinguishing features. Where the lignin content is low, e.g., in the endosperm of cereals, hot water extracts a considerable percentage. 

To determine if any component of the dietary fibre in the cereals investigated here was correlated with zinc bioavailability, hemicellulose, cellulose and lignin were determined by the method of Mongeau and Brassard (16). The results are summarized in Table 5. 

There have been many articles on the hemicelluloses,1-17 and this one, on the hemicelluloses of the Gramineae (grasses and cereals), is written in the knowledge that, when it is completed, Norman s state-... 

The term hemicellulose is also often extended to include certain carbohydrates in cereal endosperms, namely, non-starch polysac-... 

Since 1889, improved grass and cereal cultivars have been bred.41 It is difficult, and may be commercially impossible, to obtain viable seeds of cultivars required for studies extending over several years, as the cultivar may meanwhile have been replaced by another. For this reason, it may not be possible to re-examine the hemicelluloses of a cereal or grass cultivar studied earlier. 

Investigations on hemicelluloses by pure chemists, agricultural and cereal chemists, and others are often devalued with respect to one another by inattention to factors that are now known, or may later be found, to obscure significant differences in the hemicelluloses present in the grass. Conversely, some differences, mainly those of a quantitative nature, that have been remarked on may arise from unsuitable... 

As used in this Chapter, the term non-endospermic hemicellulose refers to hemicelluloses from aerial tissues and organs (bran, cob, hull, husk, leaves, stems, and other parts), but excludes those from root tissues. The hemicelluloses associated with cereal starches and flours are collectively termed endospermic hemicelluloses. 

The caryopses of grasses and cereals are anatomically complex. The starch endosperm of the caryopsis is surrounded by the aleurone layer and pericarp, both of which can be partly or completely removed, along with the embryo, by the abrasive process known as pearling, which removes the bran. The flour obtained by milling a pearled grain consists of protein, starches, and a low proportion of endospermic hemicellulose and non-endospermic hemicellulose from the bran. 

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