Whole Grains and Health. Группа авторов
This network consists of highly swollen starch granules with some leaked amylose (Johansson et al. 2015 and Johansson et al 2018).
1.8.1 Whole grain flakes
Flaked cereals are made directly from whole grain kernels or parts of kernels. Corn flakes are made with maize endosperm. The maize grits are pressure cooked with a solution containing sugar, malt (nonenzymatic), and salt. The cooked grits are partially dried to remove stickiness and then tempered for 24 hours to allow the moisture to equilibrate. The grits are subsequently submitted to flaking and toasting, where the moisture decreases to less than 3%, and the product is browned and blistered. After cooling, the flakes are sprayed with a solution of vitamins and minerals. To make wheat or rye flakes, the whole kernel is used and each kernel makes one flake. A heating step is applied before the flaking to plasticize the kernel. Instead of cooked grains, flakes may also be made from extruded pellets in a similar way.
Collapse of structure and formation of rubbery and crystalline states affects strongly physical properties of cereal products (Boitte et al. 2013). The mechanical and sensory properties of grain flakes are affected by water content. Crunchiness is lost upon water adsorption and it is attributed to the plasticizing effect of water. However, water adsorption has either plasticizing or antiplasticizing effect depending on the water activity (Gondek and Lewicki 2006).
The type of grain affects the internal structure of the product. For instance, corn flakes present a porous structure with thick, continuous and homogenous air bubbles, whereas wheat bran flakes have a heterogeneous structure with many discontinuities, cracks and ruptures. These structural differences influence the behavior of the products against compression. Fragile breaking of the matrix occurs in corn flakes, while dislocations of one piece of wheat bran against the others manifest in the wheat bran flakes (Gondek and Lewicki 2006).
1.8.2 Puffed‐grain cereals
Puffed‐grain cereals are commonly used as ready‐to‐eat breakfast foods or as ingredients in snack formulations. They have characteristic lightness and crispness, qualities related to their cellular structure (Peleg 1997) and degree of expansion (Owusu‐Ansah et al. 1984). Moreover, the effect of the puffing treatment 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