is strongly influenced by the morphology and composition of the kernel (Mariotti et al. 2006). Two technological processes are generally used in order to decrease the bulk density of puffed cereals. Oven puffing involves the sudden application of heat at atmospheric pressure to a prewetted cereal. The product expands due to the vaporization of water inside the kernel. Gun puffing is based on the sudden transfer of a piece containing superheated water to a lower pressure, thus allowing the water to suddenly vaporize (Delcour and Hoseney 2010b). The degree of expansion with gun‐puffing is 15–20 times, much greater than with oven puffing (2–5 times). Oven‐puffed cereals are made almost exclusively using whole grain rice or maize, while rice and wheat are the only types of grain used in gun‐puffed whole grain production (Fast 2000). Puffed products must keep less than 3% moisture to maintain crispness, but due to their high porosity, they absorb moisture rapidly. The greater the expansion, the more difficult is to maintain the moisture. In this way, gun‐puffed cereals require special packaging.
The ultrastructure of the grains is severely affected by puffing and is reflected in some physical properties such as bulk density and water uptake. Puffed rye and rice presents a very porous matrix made up of numerous cavities of increasing size from the center of the kernel outwards, whereas wheat and barley show a more compact and non‐homogeneous structure (Mariotti et al. 2006). The changes in the structure after compression of puffed cereals have also been studied (Roopa et al. 2009).
1.8.3 Extruded breakfast cereals and snacks
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).
1.8.4 Crispbread
Crispbread is a dry cereal‐based baked and extruded product very popular in the Nordic countries. It is a light, flat and dry type of cracker with relatively long shelf life. Crispbread traditionally consists of wholemeal rye flour, salt, and water. However, different crispbread products containing wheat, other grains and spices can be found nowadays. The air cells can be introduced using leavening, mechanically or submitting the dough under pressure in an extruder. The traditional method involved rolling, sheeting and baking the dough. However, the introduction of the extrusion process has complemented and often replaced the traditional methods. Extrusion process conditions have a great influence in the porosity and texture of crispbread (Gondek et al. 2013). Furthermore, differences in insulin response between leavened and non‐leavened (whipped) whole grain rye crispbread have been recently reported (Johansson et al. 2015). Crispbread contains only about 5–8% water. However, this product is hygroscopic, depending on the process, due to its chemical composition, porosity and presence of starch in the amorphous state (Colonna et al. 1984). If the moisture of these crispy products increases due to water sorption during storage, loss of crispness may occur, leading to consumer rejection. Adsorbed water is supposed to behave as a lubricant at high water activities and reduce the friction between surfaces, which results in low strength. This can be explained by differences in the microstructure of the products (Jakubczyk et al. 2008). Therefore, mechanical properties and fracture behavior of crispbread are strongly affected by its structure.
One of the most commonly used technique to determine bread structure is based on light microscopy (Jakubczyk et al. 2008). However, the observation of crispbread by light microscopy is complicated due to the dry and brittle structure of the product. X‐ray micro‐computed tomography (micro‐CT) is a non‐invasive technique that enables the visualization of the internal microstructure of food products and has been applied for the characterization of extruded crispbread microstructure (Gondek et al. 2013). Extruded crispbread has a highly porous structure (porosity is higher than 90%), with numerous thin cell walls surrounding a multitude of irregular air inclusions in various shapes, sizes and orientations (Figures 1.7A and 1.7B). Scanning electron micrographs of extruded crispbread show that the structure of the surface layer is different than that of the interior due to the extrusion process (Marzec et al. 2007). The surface structure consists of numerous small air cells with rather thick cell walls, while the interior is highly porous with large air cells and thin cell walls (Figure 1.7C). Scanning electron microscopy has also been used to study the improvement of consistency and refinement of crispbread by addition of sodium stearoyl lactylate to the dough (Li et al. 2013) and the effect of different water activities on the mechanical properties of the product (Jakubczyk et al. 2008).
