Arabinoglucuronoxylans (AGXs) (arabino-4-O — metylglucuronoxylans) are the major components of nonwoody materials (e. g. agricultural crops) and a minor component of softwoods (5—10% of dry mass). They consist of a linear b-(1,4)-D-xylopyranose backbone containing 4-O-methyl-D-glucuronic acid and a-L-arabi — nofuranosyl linked by a-(1,2) and a-(1,3) glycosidic bonds (Table 17.2) (Girio et al., 2010). The xylopyranose backbone might be slightly acetylated (Peng). The typical ratio arabinose:glucuronic acid:xylose is 1:2:8. Conversely to hardwoods xylan, AGXs might be less acetylated, but may contain low amounts of galacturonic acid and rhamnose. The average DP of AGXs ranges be­tween 50 and 185 (26). In addition, because of their fur — anosidic structure, the arabinose side chains are easily hydrolyzed by acids (Peng et al., 2012).

Arabinogalactan

The heartwood of larches contains exceptionally large amounts of water-soluble arabinogalactan (AG), which is only a minor constituent in other softwood species (Peng et al., 2012). Its concentration and quality are not affected by seasonal variability. AGs are highly branched polysaccharides with molecular weights ranging from 10,000 to 120,000 Da. All larch AGs iso­lated from the Larix sp. are of the b-(3,6)-D-galactan type and consist of galactose and arabinose in a 6 to 1 ratio. Larch AG has a galactan backbone that features b-(1 / 3) linkages and galactose b-(1 / 6) and arabi­nose p-(1 / 6 and 1 / 3) side chains (Peng et al., 2012) (Table 17.2). The highly branched structure is responsible for the low viscosity and high solubility in water of this polysaccharide (Peng et al., 2012). It has the ability to bind fat, retain liquid, and dispersing prop­erties and AG also possesses a high biological activity. Larch AG is currently used in a variety of food, beverage, nutraceutical, and medicine applications (Peng et al., 2012).

Arabinoxylan

AXs are the main hemicelluloses of the grasses (Gra — mineae). AXs have been generally present in a variety of tissues of the main cereals: wheat, rye, barley, oat, rice, corn, and sorghum, as well as other plants (Peng et al., 2012). AXs are generally present in the starchy endo­sperm (flour) and outer layers (bran) of cereal grain. They are similar to hardwood xylan, but the amount of L-arabinose is higher. In AX, the linear b-(1 / 4)-d — Xylp backbone is substituted by a-L-Araf units in the positions 2-O and/or 3-O (Table 17.2). In addition, the AXs are also substituted by a-D-glucopyranosyl uronic unit or its 4-O-methyl derivative in the position 2-O, as can be found in wheat straw, bagasse and bamboo.

O-acetyl substituents may also occur (Peng et al., 2012). According to the amount of glucuronic acid and arabinose, the types of AXs are classified as AGX and glucuronoarabinoxylan (GAX), respectively (Ebringerova et al., 2005). AGXs are the dominant hemi — celluloses in the cell walls of grasses and cereals, such as sisal, corncobs and straw. Compared to AGXs, the GAXs have an AX backbone, which contains about 10 times fewer uronic acid side chains than arabinose, and also contains xylan that is double substituted by uronic acid and arabinose units. Ferulic acid and p-coumaric acid can occur esterified to the C-5 of arabinosyl units of GAXs (Peng et al., 2012). The physical and/or covalent interaction with other cell wall constituents restricts xylan extractability (Girio et al., 2010).