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The aleurone layer surrounds the starchy endosperm and part of the embryo, and makes up 5–8% of the kernel. It is usually constituted by a single cell layer except in the case of barley, which contains two to three layers. Aleurone cells do not contain starch but they have high protein content and they are rich in lipids. They release active enzymes that help the cereal grain grow into a plant. The main component of wheat and rye aleurone cell walls is arabinoxylan (Karin 2006). The aleurone cell walls in wheat are relatively thick (3–4 μm) and have been reported to possess high cellulose content and relatively linear arabinoxylans with low arabinose‐xylose ratio (Saulnier et al. 2007). Whole grain wheat contains less than 1% β‐glucan and the majority is located in the aleurone and subaleurone layers, while in whole grain rye (Figure 1.1B), β‐glucan accounts for 1.5% of dry matter and seems to be more evenly distributed throughout the grain (Dornez et al. 2011; Frølich et al. 2013). The content of β‐glucan is higher in other cereals such as oat and barley, which contain 5% and 4.6%, respectively (Frølich et al. 2013). β‐Glucan in oat is more concentrated in the sub‐aleurone layers, while in barley it is evenly distributed across the starchy endosperm (Cui and Wang 2009). The aleurone layer also contains an important amount of minerals like magnesium and phosphorus, vitamins, and phenolic compounds, such as ferulic acid, and p‐coumaric acid. For instance, the aleurone layer contains almost all of the niacin and almost one‐third of the lysine found in wheat (Delcour and Hoseney 2010a).

Cereal grains possess a well‐organized microstructure. The endosperm is rich in starch and storage proteins located in protein bodies, and the cells are compartmentalized by cell wall polysaccharides (Figure 1.2A). Indeed, especially refined cereal foods may be considered at the structural scale as a composite of starch and proteins blended with other ingredients such as fat, sugar and fibre. Processes such as milling, dough mixing, baking, rolling and extrusion cause large changes in the structure of proteins, starch and cell wall components and therefore affect the structure and quality of the end product (Autio and Salmenkallio‐Marttila 2001; Della Valle et al. 2014). The organization of the grain components at different structural levels contributes to the different characteristics among cereal products, as shown in Figure 1.2, and Figure 1.2B shows the microstructure of a rye porridge, where fragments are easily detected. The effect of baking and extrusion on the microstructure is shown in Figures 1.2C and 1.2D, where dough mixing and baking provides a continuous protein network while extrusion provides a continuous starch phase. In both cases, the generation of an aerated structure and the presence of pores are essential for the desired properties of this kind of products. The mechanical properties, which are highly associated with texture and to a great extent consumer acceptance, depend on the morphology of the product. For instance, the structure of porridge is composed of grain fragments in a continuous phase of released amylose from the starch granules and storage proteins (Figure 1.2B). Wheat bread and cakes obtain their solid foam structure due to the continuous gluten network being able to retain air in open or closed cellular network (Figure 1.2C), while many crispbread products and most ready‐to‐eat cereals, which include extruded products, obtain their crunchiness from the multiple air cell layers retained in a continuous starch phase (Figure 1.2D).

Figure 1.2 Schematic representation of a typical endosperm cell (A) and microstructure of different cereal products such as rye porridge (B), wheat bread (C) and extruded rye breakfast cereal (D). Starch and protein can be observed in blue/purple and green, respectively.

Structure is also essential for nutritional properties since it influences the bolus structure before swallowing as well as further gastrointestinal digestion. Most studies have been focused on defining or creating food structures, but knowledge on food structure breakdown during consumption is also needed. In this way, gelatinized starch is more rapidly digested than in its native crystalline form and compact products such as pasta or porridge are more slowly digested than porous ones. At the molecular level, the amylose component of starch is more slowly digested than amylopectin, and the use of high‐amylose raw materials helps lowering the glycaemic response (Alminger and Eklund‐Jonsson 2008; Poutanen et al. 2014). This could be partly due to the lower swelling capacity of high amylose starch granules compared to waxy starch granules (Lii et al. 1996). The presence of sucrose or emulsifiers has also been shown to delay the swelling of starch granules (Richardson et al. 2003). The length of the amylose chains and interactions between starch and other components in the product such as proteins, dietary fibre or lipids can also influence the digestibility of starch (Sajilata et al. 2006).

Structural characteristics of cells and tissues influence grain quality and performance in food processes. These microstructural changes can be studied using a variety of microscopy techniques such as light microscopy, confocal laser scanning microscopy (CLSM) and electron microscopy. Light microscopy provides lower magnification than, for instance electron microscopy but it allows specific staining of different chemical components. This is particularly useful in cereal products, which are complicated multicomponent multiphase materials. For instance, amylose and amylopectin can be identified under the light microscopy by staining with iodine (Langton and Hermansson 1989; Autio and Salmenkallio‐Marttila 2001). Iodine staining also allows subsequent image analyses and obtaining quantitative results (Srikaeo et al. 2006). Some staining systems can also be used in CSLM, which requires a lower degree of sample preparation compared to other microscopy techniques (Autio and Salmenkallio‐Marttila 2001). Moreover, the use of fluorescently labelled antibodies has high potential due to the combination of immunological specificity with the sensitivity of fluorescence (Vázquez‐Gutiérrez and Langton 2015). Scanning electron microscopy (SEM) permits three‐dimensional observation of a wide spectrum of food structures. Since this technique does not require sectioning, it is appropriate for visualization of porous structures and sample preparation is easier compared to light microscopy (Sanchez‐Pardo et al. 2008). SEM has been used to observe cooking‐induced changes in starch granules and protein matrix (Srikaeo et al. 2006; Sanchez et al. 2008).