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Extrusion cooking handles cereal flours at relatively low moisture contents (12–20%) and limited amounts of fibre and fat. It is a continuous process that uses both temperature and pressure to expand the product (Delcour and Hoseney 2010b). The dough is forced through an extruder to give it a specific shape and dried. This process causes starch gelatinization and mechanical damage in cell walls (Salmenkallio‐Marttila et al. 2004). The presence and gelatinization of starch is essential for optimal sensory properties of extruded products. Porosity is a key characteristic that determines quality properties such as crispness in this kind of products. Crispness is indeed the result of breaking behavior of complex structures at different length scales (Chanvrier et al. 2014). Extruded flours of maize or oat are usually puffed by extrusion at high temperature. In extruded whole grain rye, all starch granules are completely destroyed during processing, resulting in a continuous homogenous starch phase consisting of a mixture of amylose and amylopectin (Figure 1.2D). This also results in a very low content of resistant starch, according to Johansson et al. (2018). This recent study on rye has established a relationship between microstructure and product composition and in vitro glucose release. A later glucose peak was detected in extruded whole grain rye compared to wheat bread and fermented crisp rye bread. This was partially attributed to less degraded fibres, such as β‐glucans and arabinoxylans, in the extruded rye contributing to higher viscosity of the food digesta which would favor a slower diffusion of enzymes. Additionally, it was suggested that the extruded rye was more resistant to disintegration in the gastric compartment.

Although extrusion cooking can be used for the production of fibre‐rich products, bran particles act as inert fillers within the extrudate matrix, and thus affect the mechanical and physical properties (Robin et al. 2012). The addition of dietary fibre can affect the density and textural properties of the product since it can limit the extent of starch gelatinization and the proper formation of air cells (Stojceska 2013). Fibres such as pericarp cell wall remnants are not easily plasticized and, therefore, are not readily expandable under normal commercial extrusion conditions. Fibre particles disrupt the amorphous regions of plasticized starch and protein, decreasing the gas‐holding capacity and generating denser and less expanded products (Lue et al. 1991; Jin et al. 1995; Robin et al. 2012). Therefore, the cereal fibre content either isolated or in the form of whole grains, is a key factor in extruded products. Structural differences are obtained in extruded cereals depending on the type and level of fibre as well as on the base recipe (Chanvrier et al. 2013). As revealed by X‐ray tomography and 3D image analysis, higher porosity is obtained in whole wheat products compared to corn products (Chanvrier et al. 2014). Moreover, it could also be observed in the same study that porosity decreases with the amount of added fibres (Figure 1.6). The structural differences of the walls depending on the amount of added fibres induced also different breaking behaviors and noise creation. In this way, according to Chanvrier et al. (2014), the organization of the wall would have a greater impact on the breaking properties than the wall thickness. For instance, the presence of soy protein may induce different molecular interactions between fibres and the corn/soy matrix. As a result, the viscoelasticity of the extrudate changes and the interactions between all the components may be modified (Chanvrier et al. 2014).