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Aleurone develops from surface endosperm cells and is therefore located in the outer part of the cereal grain starchy endosperm. A unicellular layer made from block‐shaped cells in wheat (37–65 x 25–75 micrometre; Evers and Bechtel 1988), the aleurone layer is multi‐layered in other cereals such as barley (2–3 parallel cell layers), rice and oat (Stone 1985). Aleurone represents 7–9% (w/w) of the wheat kernel (Buri et al. 2004). In wheat, the cell‐wall of its constitutive cells are larger and thicker than in other cereals (Xiong et al. 2013).

The biochemical composition of the wheat aleurone fraction has been recently reviewed in Rosa‐Sibakov et al. (2015) and Brouns et al. (2012). Even if this fine composition depends on the wheat sample and the fractionation processes used, the aleurone fraction is always particularly rich in fibres (44–50% dm, Amrein et al. (2003)), mainly arabinoxylans (65%) and β‐glucans (30%) coming from the non‐lignified cell‐walls (Bacic and Stone 1981; Saulnier et al. 2007). The main part of these arabinoxylans (95%) are water unextractable (Saulnier et al. 2007; Rosa et al. 2013b) and esterified with ferulic acid that is the main phenolic acid compound found in the aleurone layer (constitutes 95%). Ferulic acid is mainly bound to arabinoxylans and only minor amounts of ferulic acid are under free or even conjugated forms (Rosa et al. 2013b). Beside ferulic acid, 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).

According to Chen et al. (2013a), the antioxidant capacity is higher in aleurone than in bran (412 vs. 205 micromol TE/gram; ORAC oxygen radical absorbance capacity test) and also presents higher oxidation inhibition (42% vs. 30%). Ferulic acid, tocol and other phenolic acids (vanillic, p‐coumaric acid, benzoic and caffeic acids) contribute to the antioxidant properties of wheat aleurone fraction in given order (Chen et al. 2013a). Beside ferulic acid located in the aleurone cell walls, two main carotenoids, lutein and zeaxanthin, are present in the lipid fraction of the aleurone layer in wheat, oat and corn grains (Hentschel et al. 2002; Adom et al. 2003; Panfili et al. 2004; Konopka et al. 2006; Ndolo and Beta 2013). Their total content has been positively correlated to the antioxidant properties of the aleurone (Ndolo and Beta 2013). Due to its high nutritional composition, several processes have been developed to produce enriched or pure aleurone fractions using wheat bran, ranging from lab to industrial scale (Hemery et al. 2007; Brouns et al. 2012). The first developed protocols to isolate aleurone from wheat and barley used a prior step of bran maceration in solvents followed by milling, sieving, air classification and centrifugation steps (Bacic and Stone 1981; Pomeranz 1988). Mechanical treatments of grains (successive debranning or pearling steps) have also been used to obtain bran fractions enriched in aleurone (Dexter and Wood 1996; Harris et al. 2005; Liyana‐Pathirana and Shahidi 2006). More recently, new large dry‐fractionation processes have been attempted in order to obtain aleurone enriched fraction with a percentage of enrichment over 60% (Hemery et al. 2007; 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.

Depending on the degree of integrity of the aleurone layer, modification of the bio‐accessibility and bioavailability of compounds of interest has been observed (Figure 3.3). Indeed, reduction of the aleurone particle size has increased the iron bioaccessibility (Latunde‐Dada et al. 2014) and ferulic acid bioaccessibility as shown with the in vitro gastrointestinal Caco‐2 model (Hemery et al. 2010). An ultrafine grinding of an aleurone fraction (particle size under 50 micrometre, Rosa et al. 2013b) increased its antioxidant capacity by a two‐fold factor (44 mmol TEAC/kg). This change was linked to the opening of 50% of the aleurone cells, which increased the accessibility of ferulic acid, the major contributor to the antioxidant properties in cereals (Anson et al. 2008). Degrading the aleurone cell to a higher extent with enzymes such as xylanase, coupled or not with ferulate esterase, drastically increases (by a four‐fold factor) the aleurone antioxidant capacity. This additional increase was due to the release of 85% of the ferulic acid in its bio‐accessible form and possibly also to additional antioxidant compounds released from the aleurone cells with the enzymatic treatment (Rosa et al. 2013b).

Figure 3.3 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.

The effect of aleurone disintegration, especially its ability to counteract metabolic disorders in diet‐induced obesity and its effect on weight reduction, adiposity, inflammation and oxidative stress has been studied in mice (Rosa et al. 2014). The aleurone grinding does not positively affect these health parameters and has even been demonstrated to slightly increase mice body weight gain. On the contrary, the enzymatic aleurone disintegration with both xylanase and ferulate esterase has a tendency to lower the mice weight gain, reduce the accumulation of fat in internal organs and also the fasting plasma insulin and leptin levels (Rosa et al. 2014). These positive effects are probably related to the high content of soluble AX and bioavailable FA of enzymatically treated aleurone (Rosa et al. 2014). AX depolymerization is indeed often related with a better degradation in colon and in high SCFA production even if the relationship between AX structure and SCFA production remains unclear (Rosa et al. 2014). FA enzymatically released from AX by esterase appears available in the upper intestinal tract (Zhao and Moghadasian 2008; Anson et al. 2009; Rosa‐Sibakov et al. 2015) and rapidly transformed in metabolites by colonic microflora. Some of these metabolites have possible anti‐inflammatory properties (Rosa et al. 2014). Recently, it has been demonstrated that enzymatically treated aleurone displaying a high amount of ferulic acid under free form allows to change the metabolite profile in urine of obese mice (Pekkinen et al. 2014). However, even if positive effects have been registered to counteract some of the metabolic disorders caused by obesity in mice, they remain limited probably because obesity was already settled before aleurone introduction in the mice diet. Native or enzymatically–treated aleurone should be more efficient when introduced into the diet as prevention, before obesity is established.