Читать книгу Whole Grains and Health - Группа авторов - Страница 63

Arabinoxylans belong to the hemicellulose component of the cell wall of cereals, and are also termed pentosans as they are composed of the 5‐carbon monosaccharides of arabinose and xylose. In cereal grains, they are in highest concentration in the bran layer, though are also found in the endosperm cell walls. Structurally, the simplest arabinoxylan consists of a backbone of D‐xylanopyranosyl units linked through β‐(1→4) bonds with branches to single L‐arabinofuranose by α‐(1→2) and/or α‐(1→3) linkages (Izydorczyk and Biliaderis 1995; Ebringerová and Heinze 2000) (Figure 4.4). Thus, the structural property of arabinoxylan is often expressed by the ratio of arabinose and xylose (Ara/Xyl). Although most of side branches are single arabinose, there are side chains consisting of arabinose residues and other sugar units such as xylose and galactose and glucuronic acid. Depending on the botanical source of the arabinoxylan, the branch pattern may be of low or high complexity depending on the substituent position and density and composition of the side chains (Rumpagaporn et al. 2015). Additional to the sugar units, there are also phenolic compounds, mainly ferulic acid linked to arabinoxylans. In whole grain foods, arabinoxylans, in most cases, are cross‐linked with themselves and other polysaccharides in a cell wall form, which further increases its complexity lowering gut microbiota fermentability (Rose et al. 2010).

Figure 4.3 The molecular structure of the major non‐starch polysaccharides found in plant cell walls. Heteroxylan (arabinoxylan), (1,3/1,4)‐β‐glucan and cellulose are the major non‐starch polysaccharides in whole grain cereals; and pectin, xyloglucan, and cellulose are the major non‐starch polysaccharides in whole grain pseudocereals (amaranth, quinoa).

Source: Burton et al. 2010. © 2010 Springer Nature.

Figure 4.4 The general structure of arabinoxylan composed of side chains of L‐arabinose, glucuronic acid, galactose and xylose, and a backbone of xylose.

Sources: Based on Ebringerová and Heinze 2000; Mendis and Simsek 2014.