“Dietary fibre means carbohydrate polymers with ten or more monomeric units,* which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories:
1 Edible carbohydrate polymers naturally occurring in the food as consumed,
2 Carbohydrate polymers, which have been obtained from food raw material by physical, enzymatic or chemical means and which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities,
3 Synthetic carbohydrate polymers which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities.
*Decision on whether to include carbohydrates from three to nine monomeric units should be left to national authorities.
By this definition, dietary fibres that have been isolated or synthesized and used as ingredients must also be shown to have a physiological benefit, but importantly whole grain fibres do not have this requirement. This fibre requirement of physiological benefit is also incorporated into the new FDA dietary fibre definition of 2016 (US FDA 2016). Cellulose, hemicellulose, lignin, gums, modified celluloses, mucilages, oligosaccharides, pectins and associated minor substances such as waxes, cutin, and suberin all fall within the scope of dietary fibre. By the late 1990s, the methodologies for dietary fibre analysis had been refined by AOAC and AACC (DeVries et al. 1999). However, novel digestion‐resistant materials developed by the food industry were not always within their scope, and a new method, (AOAC Official Method 2009.01, 2009), was developed to capture analytically all fibre materials meeting the definitions. Based on Footnote 2 found in the AOAC method, resistant oligosaccharides, resistant starch and resistant maltodextrin all satisfy the definition of dietary fibre (Jones 2014).
Dietary fibre is, in a general way, often separated into water‐soluble and water‐insoluble types. Some water‐soluble dietary fibres, such a β‐glucans and pectins, have a high viscosity property that can slow the transit of food in the gastrointestinal tract, reduce the postprandial glucose spike, and aid in the excretion of bile acids and cholesterol. They are also readily fermented in the colon and produce short chain fatty acids (acetate, propionate and butyrate) that have certain health benefits and modulate the microbiota. Some water‐insoluble dietary fibres have water retention capacity to promote bowel movement and generally have a lesser degree of fermentation. In whole grain foods, β‐glucans and some arabinoxylans are the main water‐soluble dietary fibres in cereals (Carpita and Gibeaut 1993) and xyloglucans and pectins in pseudocereals like amaranth and quinoa (Lamothe et al. 2015), while the major water‐insoluble dietary fibres are cellulose, the insoluble cross‐linked arabinoxylans, and lignin (Table 4.2). Depending on the botanical source of the dietary fibre as well as individual genotypes and growth conditions, there may be variability in degree of heterogeneity in structure within the same type of fibre polysaccharide type. Such a wide range of fibre structures in whole grains could differentially influence the gut microbiome and resulting metabolic and physiological responses (Hamaker and Tuncil 2014). Additionally, as illustrated from the molecular architecture of the plant cell wall (Figure 4.1), dietary fibre polysaccharides in whole grain usually exist in a cell wall form consisting of a fibrillar phase of cellulose microfibrils in a matrix composed of non‐cellulosic polysaccharides with a variety of different structures along with structural proteins, glycoproteins and phenolic components. At a more macro‐level, whole grain cell wall structures may contain starch (Figure 4.2), and make it more difficult to access, and thus slowing its digestion or making it resistant. The principal whole grain non‐starch polysaccharides and their molecular structures (shown in Figure 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,
