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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).

Figure 1.6 Cross‐sectional X‐ray tomography images (10 mm × 10 mm) of extruded cereals with added fibres (wheat bran. A: wheat recipe with 40% whole wheat; B: corn recipe; C: corn recipe in 3D rendering (carrot cut in the extruded ball).

Adapted from (Chanvrier et al. 2014).

Figure 1.7 Microstructure of rye crisp bread. A1 and A2: Reconstructed X‐ray micro‐CT cross‐sections and 3‐D structure of extruded rye crispbread (Gondek et al. 2013). A3: Scanning electron micrograph of extruded rye crispbread (Marzec et al. 2007). B1–C3: bright field (B1 and C1) and confocal (B2, B3, C2, C3) micrographs of non‐fermented and fermented (yeast leavened) rye crisp bread before and after in vitro digestion. Amylose in blue, amylopectin in purple, protein in yellow, β‐glucan in green fluorescence. Images from B1 to C3 kindly provided by Daniel Johansson

(Source: Department of Molecular Sciences, SLU, Uppsala).

As suggested by Johansson et al. (2018), factors related to the microstructure of cereal products can influence the digestive process and the release and absorption of nutrients, with possible later implications for the postprandial responses in vivo. Some microstructural differences between whipped (non‐fermented) and leavened (fermented) rye crispbread can be observed under confocal microscopy (Figure 1.7). Due to the inability of rye proteins to properly develop gluten, both types of crispbread present a continuous starch network consisting of highly swollen starch granules with some leaked amylose that encapsulates the protein (Figures 1.7 B1 and 1.7 C1). Visualization of the β‐glucans by using immunolabeling and confocal microscopy show that β‐glucans (green fluorescence) appear to be more degraded or solubilized in fermented rye crispbread (Figure 1.7 C2) compared to non‐fermented rye crispbread (Figure 1.7 B2). This has also been confirmed by analysis of the calcofluor average molecular weight of β‐glucans in these products (Johansson et al. 2018). Yeast fermentation has been shown to decrease the molecular weight of β‐glucans (Andersson et al. 2004; Tiwari and Cummins 2009; Rakha et al. 2010). In the same way, digested samples of non‐fermented rye crispbread show less degraded β‐glucans than the leavened one (Figures 1.7 B3 and 1.7 C3). Lower degradation of fibres could contribute to higher viscosity of the digesta and slower diffusion rates of enzymes, which has been related to later glucose peaks registered in in vitro experiments and lower insulin responses in human trials for non‐fermented rye crispbread compared to leavened rye crispbread (Johansson et al. 2015, 2018).