Читать книгу Whole Grains and Health - Группа авторов - Страница 53

Bran fractions are considered by millers as by‐products and are therefore not standardized. They correspond to classical fractions obtained by debranning or milling from grains concomitantly with flours and semolina or hominies. They are mainly constituted of outer layers of grains ranging from the aleurone layer to the outer pericarp, without the eventual hulls, with a minor proportion of remaining starchy endosperm. Bran fractions differ greatly in tissue composition and thus biochemical composition and functional properties, depending on grains (oat vs. wheat), process (debranning vs. milling) and extraction rate used. Oat bran, but no other bran, is defined by AACC (http://www.aaccnet.org) based on its chemical composition and extraction rate, but not from the anatomical point of view. According to this definition “the oat bran fraction is no more than 50% of the original starting grain, and displays a total β‐glucan content of at least 5.5% (dry weight basis) and a total dietary fibre content of at least 16.0% (dry weight basis) such as at least one‐third of the total dietary fibre corresponds to soluble fibre.

A typical wheat bran contains about 60–70% carbohydrates, 13–18% proteins, 12% water, 3–4% lipids (Apprich et al. 2014). However, composition is variable according to the extraction rate (varying between 18–40%), the grain cultivar, but also environmental conditions (Lempereur et al. 1997; Shewry et al. 2010). Present carbohydrates are mainly arabinoxylans (around 25–35%), (Delcour et al. 1998; Wang et al. 2006), cellulose, and β‐glucans (Hemery et al. 2007). Brans are thus relatively rich in dietary fibre (50–60%, (Hell et al. 2014), but also contain variable proportions of starch from the remaining starchy endosperm, depending on the extraction rate. The amount of protein in bran can reach higher contents than those found in starchy endosperm or in whole grain (Sullivan et al. 2010), but the protein nature is different as shown by its amino acid composition. The starchy endosperm contains mainly the storage proteins richer in cysteine and amido‐acids whereas the aleurone layer contains more soluble functional proteins richer in lysine (Rhodes and Stone 2002; Buri et al. 2004). Moreover, bran fractions are also rich in phytic acid and minerals (Tabekhia and Luh 1979; Anson et al. 2012), vitamins B and E (niacin, folates, tocopherols and tocotrienols) (Watson 1987; Pomeranz 1988; Wang et al. 1993; Edelmann et al. 2012) as well as phenolic compounds such as phenolic acids, lignans, and alkylresorcinols (Tabekhia and Luh 1979; Anson et al. 2012; Landberg et al. 2014; Wang et al. 2014), betaine and choline (Bruce et al. 2010), and sterols (Nystrom et al. 2007b; Nurmi et al. 2012). Pigmented grains, rice, for example, are rich in phenolic acids and other bioactive compounds such as anthocyanins and proanthocyanins (Min et al. 2012; Shao et al. 2014) mainly recovered in bran (Paiva et al. 2014).

Bran fractions are commonly recovered as a by‐product of the milling industry. Extraction rate first influences the amount of starchy endosperm adherent to the whole outer layers and therefore the amount of starch in bran. Fractions recovered after processes such as pearling or peeling as well as those recovered after milling of peeled/pearled grains are also called bran fractions even if they do not have the same histological composition (Hemery et al. 2009; Chandra Lohani et al. 2012). Debranning technologies have first been developed from rice technology and have then further adapted to other cereal grains such as wheat (Fellers et al. 1976; Dexter and Wood 1996), barley (Izydorczyk et al. 2003), rye (Lampi et al. 2004), oat (Doehlert and Moore 1997; Peterson et al. 2001), millet (Chandra Lohani et al. 2012), sorghum (Viraktamath et al. 1971) and triticale (Lorenz and Al‐Husaini 1980) to remove hulls but also bran. Debranning is based on a sequential removal of the most peripheral layers through successive steps in modified rice polishing equipment allowing the control of the target debranning yield. However, taking into account the grain form (ovoid vs. elongate) and possible presence of a crease, the efficiency of the debranning process to successively recover the different outer layers surrounding the starchy endosperm varies. The histological composition is different between bran fractions obtained with distinct processing steps such as debranning, milling or milling after debranning (Hemery et al. 2009), as illustrated in Figure 3.2 for the last two processes. In consequence, the bran biochemical composition is also different. Low degree of debranning (< 2%) is used in the wheat milling industry to decontaminate grains by removing about 90% of heavy metal such as Pb or Hg, but also microbial load (Eugster 2005; Laca et al. 2006). The resulting brans are discarded taking into account their content in undesirable compounds.

A bran stabilization step is often carried out in order to avoid any rancidity problem caused mainly by lipase, in particular, in rice. Different technologies are used such as thermal treatment (dry heat treatment, microwave, ohming heating, toasting), gamma‐irradiation or infrared radiation, or treatment coupling thermal and mechanical processes such as extrusion. Grinding post‐treatments, such as micronization, can also be carried out. Regrinding bran modifies its functional properties when incorporating in food products (Noort et al. 2010; Kim et al. 2013), but can also modify its nutritional properties. Indeed, an ultrafine grinding increases the bioaccessibility of phenolic acids (mainly sinapic and ferulic acids) from wheat bran (Hemery et al. 2010). The specific surface area of wheat bran powder also appears to be positively correlated to its antioxidant capacity (Rosa et al. 2013a). Mechanical and biological bran pre‐treatments are the most efficient to increase the phenolic acid accessibility (Wang et al. 2014). Treatment with enzymes hydrolyzing the cell walls also appears to increase sterols accessibility (Nystrom et al. 2007a).

Figure 3.2 Tissue composition of different bran issues obtained from different processes (conventional milling technologies with or without previous peeling or pearling pre‐treatment), and for similar process but with different grain cultivars (Tiger, Crousty and Caphorn).

Until recently, bran has mostly been used in the animal feed industry. However, considering its richness in compounds of high nutritional value mainly located in the aleurone layer, the food industry has begun to use it as a food ingredient, especially in cereal products. Bran fraction is commonly added in a fixed proportion to white flours to obtain whole‐meal flour from wheat, barley, rye, oat and maize (van der Kamp et al. 2014). The addition of bran generally impacts technological properties (e.g., bread volume, color), but also the shelf‐life and sensory attributes (texture, bitter taste) of the food product. Various available processes can overcome such problems but also impact nutritional properties (Katina et al. 2007). Bran consumption has also increased for non‐food uses such as bioethanol production (Friedman 2013; Apprich et al. 2014; Pruckler et al. 2014). It has also been used as a raw material to produce ingredients by wet extraction, for example, proteins (Youngs 1974), oil (Friedman 2013) or dietary fibres such as β‐glucans from oat or barley (Knuckles et al. 1992).