Wheat, barley, oats and rye belong, as all true cereals do per definition, to the grass family (Poaceae). This family is a very diverse family, covering plants that humans have used to grow a lawn to plants that can grow several meters tall (bamboo) [13]. Cereal grains have a complex structure that is characterized by different cell layers. Although the individual structural parts of the different cereal kernels may differ significantly in terms of composition and size, the general cereal structure remains largely the same [8]. Three main parts can be distinguished: the embryo or germ, the endosperm, and the outer kernel layers that cover the embryo and endosperm or the so-called bran [14]. The starchy endosperm accounts for 80–85% of the grain. It is mainly composed of starch and protein. Bran and germ represent 12–18% and 2–3% of the dry grain weight, respectively [15]. The embryo is vital for the germination process, as it comprises the embryonic axis and scutellum. The embryo has the highest content of lipids and lipid-soluble vitamins of all fractions in the cereal kernel [16]. The endosperm has the highest economic importance. In the endosperm insoluble nutrients, mainly starch and proteins, are deposited as an energy source for the developing plant upon germination [15]. The aleurone layer is the outermost layer of the endosperm. It usually consists of 1 to 3 layers of cells, depending on the cereal. In some cases, pigmentation in the aleurone layer can give cereal kernels a distinct colour [16]. Although botanically, the aleurone layer is considered to be part of the endosperm, a major part of this layer has been shown to be removed during roller milling and, hence, is often not part of the refined white cereal flour [17]. The bran fraction consists of several different layers that can be distinguished from one another, i.e., outer pericarp, inner pericarp, testa, and nucellar epidermis (also called hyaline layer) (Figure 1) [15]. Inner and outer pericarp are rich in highly crosslinked polysaccharides, such as cellulose, lignin and heteroxylan [18,19]. The testa of barley, oats and rice is present as one cell layer, while wheat and rye testas generally have two distinct layers [14]. The nucellar epidermis is the maternal tissue that covers the endosperm, but is not prominent in all cereals [16]. In sorghum, this layer is very prominently present, but it is usually absent or only present as a thin layer in most other cereals [20].

Dietary fibres are defined as “carbohydrates with a degree of polymerization of 3 or more that naturally occur in foods of plant origin and that are not digested and absorbed by the small intestine” [22]. Dietary fibre can be classified according to its water solubility in insoluble dietary fibre (IDF) and soluble dietary fibre (SDF) [23]. IDF includes cellulose, water-insoluble hemicellulose and lignin, and are mainly present in plants as structural cell wall components [24]. SDF consists of a variety of non-cellulosic polysaccharides and oligosaccharides. Examples are pectins, β-glucans and water-soluble gums [25]. SDF and IDF differ largely in their functionality as food ingredients and their physiological effects upon consumption [23]. With regard to the latter, SDF, by increasing the viscosity of stomach and intestinal contents, is believed to reduce the overall intestinal enzymatic activity, and to decrease post-prandial plasma glucose levels [26,27]. In addition, SDF are highly fermentable and increase the production of short chain fatty acids (SCFAs), which are important contributors in the management of CVDs [28]. IDF, conversely, mainly serves as a bulking agent and laxative, hence, increasing faecal mass and decreasing intestinal transit time [29]. A potential mechanism of IDF related to management of NCDs might be related to increased satiety and reduction in body weight [28]. Both SDF and IDF help prevent constipation, decrease re-adsorption of bile salts, and lower the risk of colon cancer [25].

Dietary fibre can be obtained from different dietary sources, which include grains, fruits and vegetables. The amount and composition of dietary fibre can vary with the source [23]. Cereals are an important source of dietary fibre, contributing to about 50% of the total dietary fibre intake in Western countries [30]. Vegetables deliver about 30 to 49% of the daily dietary fibre intake, while fruits contributed about 16%. Equal weights of fruits and vegetables contain less total dietary fibre relative to cereal grains, due to the higher moisture content. The proportion of IDF of the total dietary fibre varies depending on the type of fruit or vegetable that is studied [23]. Cellulose is the main component in the IDF fraction in plants, while pectin is a major fraction in the SDF fraction of fruits and vegetables [31].

Pseudocereals are largely underutilized crops that have recently been gaining attention due to the nutritional properties that are associated with them. Pseudocereals can be processed into, and used as, a flour, in a very similar way to the way in which wheat is processed and used. The three pseudocereals that have been most widely studied thus far are amaranth (Amaranthus spp; Amaranthceae), quinoa (Chenopodium quinoa subsp. quinoa; Chenopodiaceae), and buckwheat (Fagopyrum esculentum; Polygonaceae) [55]. These are dicotyledonous plants, as opposed to cereals like wheat, rice and barley, which are monocotyledonous [77].

Pseudocereal seeds are, similarly to cereals, also composed of several ‘layers’ (Figure 4). Perisperm, germ and endosperm are the three main areas containing food reserves in pseudocereals [76]. The kernel and ‘layer’ composition varies for the different pseudocereals.

Amaranth is a small seed with diameters ranging from around 0.9 to 1.7 mm [76]. The major portion of the seed is the embryo, which is twisted in a circle. The embryo is large and encloses the perisperm and consists of the radicle and cotyledons, which is the main protein storage organ of the seed [78]. The seed coat is completely smooth and thin, and its colour can be white, cream, gold-yellow and even brown [55]. Quinoa kernels have the same structure as amaranth kernels. It produces small, spherical-shaped seeds, with diameters that vary between 1.0 and 2.6 mm. One gram of quinoa contains about 250 to 500 seeds [79]. Similar to what is the case for amaranth, the main storage tissues of quinoa seeds are the perisperm, embryo and cotyledons [80]. Buckwheat seed is pyramid-shaped with sizes ranging from 4 to 9 mm [79]. The seed is covered with a dull brown or grey pericarp that is tightly attached to the seed [81]. The embryo is embedded in the centre of the endosperm and has two cotyledons [82].

Amaranth, quinoa and buckwheat are pseudocereals with a long history of utilization as food ingredients and have very interesting nutritional characteristics. Pseudocereals have been gaining popularity in the last decade as ingredient for gluten-free products. Their use substantially increases the dietary fibre content of these products, which are typically deficient in dietary fibre [49]. Although the dietary fibre content varies between amaranth species, the total dietary fibre content of amaranth varies between 9 and 21% (dry weight basis) [83,84], while quinoa contains around 7 to 21% TDF [50,52,53]. Based on a monosaccharide analysis of dietary fibre extracted from amaranth and quinoa samples, the dietary fibre in these pseudocereals is mainly composed of galacturonic acid, arabinose, xylose, glucose and galactose. According to this monosaccharide composition and analysis of the linkages, the dominant fraction of both the soluble and insoluble dietary fibre in these pseudocereals is classified as pectic polysaccharide [85]. Xyloglucans are the second most prominent dietary fibre present in amaranth and quinoa whole grains. Cell walls of amaranth also contain significant amounts of phenolic acids. High levels of ferulic acid were found, while coumaric acid and caffeic acid are also present, but in lower amounts [78]. Both quinoa and amaranth have a high proportion (~22% of the total dietary fibre content) of SDF compared to wheat (about 15%), indicating promising potential with regard to colon health [86].

The total dietary fibre content of buckwheat groats (7–11.9%) is lower than the dietary fibre content found in most cereals such as wheat, barley and oats (Table 1). The majority (~70%) of dietary fibre from buckwheat groats is water insoluble [82]. The water soluble fibre of buckwheat seeds is mostly classified as pectin, arabinogalactan and xyloglucan [81]. Pectin was found in the cell walls of the outer and inner epidermis, and the endosperm of buckwheat seeds [81].