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Two important transformations in the main components of durum wheat pasta take place during cooking: gluten polymerization and starch gelatinization. The competition of both components for water determines the final texture properties of the product (Fuad and Prabhasankar 2010). Microscopy techniques such as scanning electron microscopy and light microscopy of stained sections have helped to increase knowledge regarding the induced structural changes of pasta during cooking (Heneen and Brismar 2003). There is a moisture distribution gradient from the surface to the center due to the penetration of water and the progress of starch gelatinization. This moisture gradient is essential for the texture properties of pasta. In this way, the ideal texture of pasta, known as “al dente” is characterized by a soft outer region and a very thin hard core (Sekiyama et al. 2012). Pasta is usually made from durum wheat semolina and is a good source of low glycaemic index carbohydrate (Brennan 2008). Bran and germ particles in semolina produce a less homogeneous mixture and the particles can physically interfere with gluten development (Manthey and Schorno 2002). For this reason, bran and germ, commonly referred to as pollard, are largely removed during milling of durum wheat. However, nutritionally‐enriched pasta is also available commercially, prepared using wholemeal, semolina/flour or ground whole‐wheat. Although negative effects in the cooking and sensory properties of whole‐wheat or bran enriched pasta have been frequently reported, spaghetti dried at high temperature can be prepared with pollard, with 10% substitution of semolina, causing minimal impact on sensory and technological properties (Aravind et al. 2012). High molecular weight inulin can also be incorporated with minimal effect on the technological and sensory properties below 20% incorporation (Aravind et al. 2012b).

Nuclear magnetic resonance imaging (MRI) is a non‐invasive and nondestructive method to visualize the water distribution and to study its influence in the properties of pasta (Bonomi et al. 2012). It has proved particularly useful in combination with microscopy techniques such as light microscopy or epifluorescence. This way, the water distribution has been related to differently cooked zones in pasta (Sekiyama et al. 2012) and the influence of raw materials in the microstructure of cooked pasta (Steglich et al. 2014). In pasta, the gluten network is continuous throughout the whole tissue encapsulating starch granules and fibre particles. However, according to the study carried out on spaghetti by Steglich et al. (2014), microstructure is not homogeneous throughout the product after cooking since starch granules are affected by heat differently depending on their distance to the surface. Starch granules in the core region did not gelatinize, while granule distortion and amylose leakage increased towards the surface (Figure 1.5A). The MRI and microscopy results of this study proved that the local water content and microstructure differed due to locally varying raw materials. The whole grain pasta showed lower (darker) T₂ ^* values, which was attributed to faster exchange of water protons with exchangeable fibre protons or to less swelled starch granules in the areas close to the fibre particles or both (Figure 1.5 B–D). Rice, being low in protein, has relatively poor technological properties for interacting and developing a cohesive network for gluten‐free pasta products. Severe parboiling, extrusion cooking and/or addition of pre‐gelatinized flour are required in order to obtain the desired texture and avoid excessive leaching of solids during cooking (Marti et al. 2013).

Figure 1.5 Representative cross‐sections of 10‐min cooked spaghetti made of 100% fine durum wheat semolina (DS) and durum whole grain flour (DS+WG). A: Bright field light micrographs at high magnification from three regions; B–D: Bright field light micrographs (B), polarized light micrographs (C), and T₂^* maps (D) of cross‐sections at lower magnification. Sections in A and B were stained with Light Green and Lugol’s iodine solution for observation of protein (green) and starch (blue/violet), respectively, while fibre particles were not stained

(Source: (B) and (D): Modified from Steglich et al. 2014).