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Whole Grains and Health


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p‐coumaric, sinapic, vanillic acid and caffeic acids are detected in small amounts (Rhodes et al. 2002; Parker et al. 2005; Barron et al. 2007; Li et al. 2008). The aleurone fraction generally also contains the intracellular compounds of the aleurone layer (60–70% of the aleurone layer dry mass) which are proteins (15% of the total protein and 30% of the total wheat grain lysine content), minerals (40–60% of the total wheat grain content) associated with phytic acid (around 80% of the total phytic acid content), B‐vitamins (notably 80% of the total niacin) and phytosterols (Evers and Bechtel 1988; Pomeranz 1988; Buri et al. 2004; Brouns et al. 2012).

      Dry processes, avoiding solvents, may better preserve aleurone structure integrity and composition, and thus, preserves the content of phytochemicals and functional properties. Dry processes are mainly based on the differences in mechanical properties between the aleurone layer and the other tissues composing bran. They include two main steps composed of one or several grinding steps followed by one or several separation steps according to the size and/or density of the generated particles. Milling is realized with roller, hammer or centrifugal impact devices (Stone and Minifie 1988; Bohm et al. 2003; Chen et al. 2013b). Separation methods based on electrostatic properties have been developed to sort out aleurone particles from bran (Hemery et al. 2007; Hemery et al. 2011b) based on the differences in charging properties between aleurone and pericarp tissues (Antoine et al. 2004a). These processes consist of two steps, tribo‐charging then electrostatic separation in an electric field. Electrostatic separation allows to obtain an aleurone enriched fraction with high aleurone purity. This method has been first described in 1988 (Stone and Minifie 1988). It has allowed the recovery of an almost pure (95%) fraction, however with a yield hardly reaching 10%. In the last ten years, Bühler company has patented (Bohm et al. 2003; Bohm and Kratzer 2008) a dry fractionation process using an electric field separation step in order to recover a fraction containing from 60–90% aleurone tissue in particles made of 5 to 40 intact cells (Amrein et al. 2003; Hemery et al. 2009). Chen et al. (2014b) used a non‐uniform electric field to isolate tribo‐charged particles allowing the enrichment of aleurone cell‐cluster with a final yield reaching 42%. The same authors (Chen et al. 2014a) have demonstrated the importance of process conditions such as air flow rate and nature of the tribo‐charging device (Teflon vs. Nylon or stainless steel pipe) in the efficiency of electrostatic separation.

      Due to high and diverse content of micronutrients and dietary fibres, the aleurone layer may contribute to health effects (Brouns et al. 2012) and could therefore be of interest for food enrichment. Price et al. (2012), for example have prepared ready‐to‐eat products and bread rolls enriched with aleurone (to deliver about 27 g/day of aleurone per person) to feed healthy, older, overweight adults. These authors have demonstrated a significant lowering of one of the inflammatory markers in plasma, the C‐reactive protein of which a high level is associated with the risk of cardiovascular disease. Wheat aleurone or sub‐aleurone enriched flours have also been produced in order to enrich pasta in protein and fibre (Bagdi et al. 2014). This addition has appeared more applicable in value‐added pasta production than those of wheat bran fractions or whole grain flour.

Schematic illustration of the processing of wheat fractions leads to changes in their structural parameters which can improve their health effects.

      Adapted from Rosa‐Sibakov et al. 2015. © 2015 Elsevier.

       3.6.2 From a grain tissue separation to isolation of macromolecules

      The improvement of dry fractionation methods in order to decrease the size of the obtained particles and the development of separation methods