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Whole Grains and Health


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rel="nofollow" href="#fb3_img_img_35374a29-0a15-55a9-a66d-15f4ab7d443b.jpg" alt="Photos depict the microstructure of rolled rye grain. A: bright field micrograph with iodine and light green staining for observation of starch and protein, respectively; B: epifluorescence micrograph with Calcofluor and Acid Fuchsin staining for observation of cell walls and protein, respectively. Porridge is a dish made by boiling rolled or crushed cereal, usually oats, in water, milk or both, usually served hot in a bowl or dish. From the structural point of view, porridge is a compact cereal product where insoluble fragments of grain are dispersed in viscous grain extract."/>

      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).

Photos depict the CLSM images of untreated (A) and 0.01-percent protease-treated (B) brown rice bread crumbs showing starch and proteins.

      Adapted from Renzetti and Arendt 2009.

       1.7.2 Sourdough bread

      Sourdough fermentation provides improved flavour and structure to rye and wheat bread. In the case of rye, the swelling of flour constituents is favoured and the enzyme activity is limited under acid conditions, which prevents early staling (Narvhus and Sørhaug 2012). The interest in the significant potential of sourdough fermentation to improve the nutritional properties of rye, oat and wheat products is increasing nowadays (Komlenić et al. 2012). The whole grain or fractions of cereal grain can be modified by sourdough fermentation to improve nutritional value or promote healthfulness of cereal foods (Chavan and Chavan 2011). Sourdough fermentation has been associated to positive effects in whole grain bread such as improved of palatability, mineral bioavailability and levels of bioactive compounds, and