Whole Grains and Health. Группа авторов
4.3) are briefly discussed in the following, along with their potential association with starch in the context of the whole grain food matrix as related to carbohydrate quality in whole grain foods.
Figure 4.1 Depiction of plant cell wall structure consisting of cellulose microfibrils (blue), non‐cellulosic polysaccharides, termed hemicelluloses (green), and pectins (yellow/brown).
Source: Johnson et al. 2018.
Figure 4.2 The endosperm cell wall in oats and barley as it contains starch (dark spheres) and other constituents.
Source: Tosh 2013.
4.3.1 Arabinoxylan
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.
4.3.2 β‐glucans
The β‐glucans are composed of D‐glucopyranose units with β‐1→4 and 1→3 linkages types. The number of adjacent glucose units with the same linkage determines the complexity of β‐glucans with its various molecular weights, solubilities, viscosities and three‐dimensional configurations. For the water‐soluble β‐glucans such as the (1,3/1,4) β‐glucan that is rich in oats and barley, the (1,4)‐β‐linkages are present mainly in groups of two or three adjacent linkages (Figure 4.5). The ratio of randomly arranged cellotriosyl (DP3) and cellotetraosyl residues (DP4) is used to represent the structural property of (1,3/1,4) β‐glucan, and β‐glucans with a ratio of 1.5‐2.5 tends to form a gel‐like matrix, while lower and higher ratios tend to form aggregates (Burton et al. 2010). The ratio of DP3/DP4 from oats is in the range of 1.5–2.3, while a ratio of 1.8–3.5 was found for barley‐sourced β‐glucan, while the ratio is even higher in wheat (3.0–4.5) (Lazaridou and Biliaderis 2007). There are also long cellulosic oligomers (DP>4), such as in barley. The structural diversity of β‐glucans determines differences in physiochemical properties and related health benefits.
Figure 4.5 The general structure of cereal mixed linkage (1→3) (1→4)‐β‐D‐glucan
(Source: Lazaridou and Biliaderis 2007)
4.3.3 Other Cereal Dietary Fibres
While arabinoxylan and β‐glucan are the major non‐starch polysaccharides in whole cereal grains, pectin and xyloglucan are mainly present in whole grain pseudocereals (amaranth, quinoa) (Lamothe et al. 2015). A low amount of xyloglucan was found in rice (Choct 1997). Also, glucomannans comprised of β‐(1→4)‐linked glucose and mannose is in cereal grains and galactomannan in low amounts is found in amaranth and quinoa (Gartaula et al. 2018; Lamothe et al. 2015). Arabinogalactan, a polymer of β‐(1→4) galactan (comprised of (1→4)‐β‐D‐galactose) substituted with arabinose side chains was found in wheat flour. Although RS does not belong to the non‐starch polysaccharides, it is regarded as dietary fibre (Phillips and Cui 2011). Depending on the origin of RS, different types are recognized. RS1is physically inaccessible starch; RS2 refers to raw starch granule types that are resistant to digestion such as potato and banana; RS3 is retrograded starch, namely, of amylose after cooking to gelatinization; and RS4 is a chemically‐modified resistant starch. Although they are all termed resistant starch, processing can affect their resistance to digestion as RS1, RS2 and RS3 can disappear during some types of thermal and shear‐induced processing, while RS4 is generally not sensitive to such treatments.
Cellulose is a linear polymer of β‐(1→4)‐linked D‐glucopyranosyl residues with a dimer of anhydrocellobiose as the repeating unit (Brown et al. 1996). The strongly associated cellulose molecules into crystalline structures are one component of water‐insoluble dietary fibre in whole grains. The cellulose content of cereal grain cell walls varies among cereal species and location in the grain. In endosperm cell walls of wheat and barley, cellulose accounts for only a few percent of the total cell wall material, while in the outer, lignified layers it may comprise up to 20% of the cell wall weight (Fincher and Stone 1986). Lignin is a polymer of phenyl‐propanoides deposited in secondary thickened cells walls (Vanholme et al. 2010). As it is an intrinsic component of cell wall, it is considered as another component of dietary fibre.