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.
4.3.4 Starch
Granular starch is the most abundant material in whole grains with variation of size shape, and surface properties depending on its source and processing treatment (Krok et al. 2000). Starch granules have different levels of structural features and the granule is a semi‐crystalline material. Whole grain cereal and pseudocereal (quinoa and amaranth) starches are of the A‐type showing peaks at 2θ of 9.9, 11.2, 15, 17, 18.1 and 23.3°, which differentiates them from B‐type tuber starches showing peaks at 5.6, 15, 17, 22 and 24°. A‐ and B‐type starches reflect the difference in the geometry of their unit cells, the packing density of double helices and the amount of bound water within the crystal structure (Buléon et al. 1998; Qian and Kuhn 1999). When starch is gelatinized and dispersed, the linear amylose (essentially α‐(1→4) linked) and highly branched amylopectin [containing both α‐(1→4) and α‐(1→6) linkages] can be separated by their structural difference and size. The percentage of α‐(1→6) linkage is one aspect of the fine structure of amylopectin, for example, 4.6% for normal maize starch and 5.7% for waxy maize starch (Shin et al. 2008).
4.4 Carbohydrate quality of whole grain foods
The term carbohydrate quality refers to the health‐associated aspects of carbohydrates in foods and generally is more specifically related to the quality of glycemic carbohydrates in foods. It is not usually used in reference to the dietary fibre component. For whole grain foods, the glucose‐generating starch is the glycemic carbohydrate. While the entire whole grain package is generally considered important to the beneficial health outcome of whole grains (Fardet et al. 2008), the nutritional property of its starch is an essential though understudied component.
4.5 Slow digestion property of starch
Much about the notion of carbohydrate quality of whole grains is centered on the idea that whole grain foods have a low GI compared to refined grain foods. In this case, low GI is equated with a slow digestion property of starch that moderates and extends glycemic response. Low GI foods (GI<55) are considered to be beneficial to health by preventing or therapeutically addressing obesity and associated metabolic diseases (Ludwig 2002; Livesey 2005; Fabricatore et al. 2011). High GI foods (GI >75), which have been associated with consumption of refined grain products (Ludwig et al. 1999). Thus, whole grain foods with starch of the same chemical structure can generate different
