Whole Grains and Health. Группа авторов

Figure 1.3 Microstructure of rolled rye grain. A: bright field micrograph with iodine and light green staining for observation of starch (purple) and protein (green), respectively; B: epifluorescence micrograph with Calcofluor and Acid Fuchsin staining for observation of cell walls (blue) and protein (red), respectively.

1.7 Protein network‐based products

      The formation of gluten is essential for the structure and texture of bread, cakes and pasta.

       1.7.1 Bread

      With the addition of water to wheat flour and subsequent mixing, the storage proteins of the wheat endosperm (glutenin and gliadin) hydrate and interact to develop the gluten matrix, which provides viscoelastic properties to the dough and allows the incorporation of air (Autio and Salmenkallio‐Marttila 2003; Boitte et al. 2013). The gluten forms the continuous phase in which starch granules, lipid, added yeast cells, and cell wall fragments are dispersed (Figure 1.2C). The aggregation of gliadins and glutenins are greatly determined by noncovalent bonds and define the structural and physical properties of the dough. The quantity and quality of gluten proteins largely determine dough mixing requirements and the rheological properties, which influences the gas retention properties and the volume and crumb structure of the resulting bread (Chavan and Chavan 2011). Apart from gluten, the other components affecting the distribution of water in the dough, such as starch and cell walls, have a direct influence on the final texture of bread, too (Flander 2012).

      During yeast fermentation, carbon dioxide is generated and the dough expands. At the beginning of the fermentation process, the gas cells are embedded in the starch‐protein matrix. At a later stage of expansion, the matrix fails to enclose the gas cells and areas containing only liquid film are formed between adjacent gas cells. The end of dough expansion is marked by the rupture of the film, and not that of the starch–protein matrix (Gan et al. 1995). The major structural changes at the microscopic level during baking are starch gelatinization, denaturation of protein, melting of fat crystals and their incorporation into the surface of air cells and, sometimes, fragmentation of the cell walls (Autio and Salmenkallio‐Marttila 2001). Therefore, the final crumb structure is dependent not only on the shape, number and size of gas bubbles, but also on the structural properties of gluten‐starch matrix and probably on liquid film composed of surface‐active material (Autio and Salmenkallio‐Marttila 2003). The addition of shortenings stabilizes the gas cells. The fat crystals from the shortenings migrate towards the gas‐liquid interface and they melt during baking, allowing the bubbles to grow without rupture (Brooker 1996). Light microscopy is the most commonly used technique to determine bread structure (Jakubczyk et al. 2008). Scanning and transmission electron microscopy have also been used to study the effect of baking on starch granule structure (Bechtel 1985).

      Water evaporation occurs at the surface layers of the dough once it is placed in the oven. The lower water content in the surface compared to the core, together with the enhanced release of gasses due to the proximity to the interface with the oven air, generating smaller‐sized cells are structural characteristics of the crust that contribute to its mechanical properties (Vanin et al. 2009). The distribution of protein and partially gelatinized starch in the bread crust affects its fracturability (Primo‐Martín et al. 2006; Primo‐Martín et al. 2007). Migration of water, which acts as plasticizer, from the crumb to the crust during storage of bread can be influenced by the morphology of the product (porosity, gas cell size). The firmness of the bread increases during storage, which is called staling. It is mainly caused by the retrogradation of starch, specifically of the short amylopectin side chains (Gray and Bemiller 2003).

One of the main challenges in breadmaking is to increase the content of whole grain and dietary fibre to improve the nutritional value of the product. However, the presence of the outer bran layers of the grain alters not only the continuity and rheological properties of the starch–protein matrix, but also the water availability for lamellar film formation. This has an effect on gas cell stabilization and gas retention in wholemeal dough (Poutanen et al. 2014). Rye flour, which is typically used for baking in Northern and Eastern Europe, is suitable for making bread but it has marked differences in rheological properties and gas retention capacity compared to wheat bread. The elasticity of rye gluten is lower than that of wheat gluten and it holds less moisture. In this way, the increase of volume in rye dough during fermentation and baking is lower and resulting bread is more compact than wheat bread. Furthermore, rye starch gelatinizes at lower temperatures and is more susceptible of enzymatic degradation during baking than wheat starch (Weipert 1997). Oat can only be used in limited quantities for bread production because of its inferior baking quality, too. This is due to the lack of gluten proteins and the high content of β‐glucan and other dietary fibres (Flander 2012). Brown rice provides better nutritional properties than white rice since it contains considerably higher amounts of proteins and minerals (Lamberts et al. 2007), and is naturally gluten‐free and hypoallergenic. However, rice proteins lack the visco‐elastic properties necessary for the production of yeast‐leavened products. For this reason, the addition of hydrocolloids and gums (Sivaramakrishnan et al. 2004; Sciarini et al. 2010), and more recently, enzymatic processing of rice flour (Gujral and Rosell 2004; Renzetti and Arendt 2009) have been investigated in order to improve the breadmaking performance of rice and brown rice flour. Enzymatic processing promotes protein cross‐linking, and thus increases the elastic and viscous behavior of batters. As shown by Renzetti and Arendt (2009), treatment with protease affected the crumb microstructure, where smaller protein aggregates were more widely dispersed in the dominating starch phase, in comparison to the large aggregates of the control bread (Figure 1.4). The enrichment of gluten‐free breads with soluble fibres improves not only nutritional quality but also physicochemical characteristics, such as specific volume, crumb cohesiveness and cell density (Martínez et al. 2014).