Arabinoxylans from cereal by-products: insights into

Arabinoxylans from cereal by-products: insights into structural features, recovery, and applications

8

Rita Bastos, Elisabete Coelho, Manuel A. Coimbra Universidade de Aveiro, Aveiro, Portugal

8.1

Introduction

Arabinoxylans (AX) were identified for the first time by Hoffman and Gortner in 1927 and described as a viscous gum present in wheat flour. AX were often referred to as pentose-containing carbohydrate polymers called “pentosans,” since they are composed of the pentoses xylose (Xyl) and arabinose (Ara). They consist of a linear backbone chain of (b1 / 4)-linked D-xylopyranosyl residues to which a-L-arabinofuranosyl residues are linked. Because cereals are rich in starch, these polysaccharides are usually also referred to as “nonstarch polysaccharides.” They occur in various tissues of cereal grains and are the predominant matrix polysaccharides in cereal grain cell walls. AX, together with cellulose microfibrils, form a matrix contributing to the maintenance of cell wall integrity. The structural similarity between the (b1 / 4)-linked glucose of cellulose and the (b1 / 4)-linked Xyl of AX confers on the latter properties resembling cellulose. In addition, because they can be solubilized from plant cell walls by alkali solutions, they are called “hemicelluloses.” In lignified cell walls, AX can be covalently linked to lignin and therefore provide higher strength and rigidity to the cell walls (Izydorczyk, 2009). AX have high technological importance. As dietary fiber, they have an impact on the nutritional quality of cereal-based foods for human and animal feed. Also AX affect the physicochemical properties and processing behavior of cereal grains in milling, brewing, and baking due to their high viscosity and water retention properties (Izydorczyk and Biliaderis, 1995, 2000). These properties are also important concerning AX application as food-thickening and -stabilizing agents (Mansberger et al., 2014; Yadav and Hicks, 2015). Furthermore, several biological activities have been reported for AX, such as support of phenolics with antioxidant activity (Katapodis et al., 2003; Paz-Samaniego et al., 2014), prebiotic activity of arabinoxylooligosaccharides (AXOS) (Vardakou et al., 2008; Reis et al., 2014), cholesterol-lowering agents (Mendis and Simsek, 2014), blood sugar modifiers (Lu et al., 2000), and immunity enhancers (Mendis et al., 2016).

Sustainable Recovery and Reutilization of Cereal Processing By-Products https://doi.org/10.1016/B978-0-08-102162-0.00008-3 Copyright © 2018 Elsevier Ltd. All rights reserved.

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Sustainable Recovery and Reutilization of Cereal Processing By-Products

Moreover, the availability of AX sources is huge. First, AX are part of dietary fiber intake in a wide variety of cereals, one of the major components of the human diet worldwide. Second, as most cereals need to be processed for food production, there is an enormous generation of cereal by-products that are unfit for human consumption. A major share of cereal by-products is their use for animal feed. However, considering the availability and technological importance of AX, efforts have been made to increase their value by extracting and purifying AX from cereal-processing by-products (Izydorczyk, 2009). The extractability of AX depends on the cereal source and tissue localization, as well as on AX structural features (branching degree, molecular weight, and type of side chain substitutions) and interactions with the matrix. Several methodologies have been applied to extract AX from cereal by-products, mainly hydrothermal treatments, chemical extractions, and enzymatic hydrolysis. Moreover, microwave- and ultrasound-assisted technologies have been used to improve the recovery yields and sustainability of the processes, with particular importance for chemical extractions. According to the methodology and the type of cereal by-product tissues, the macromolecular characteristics of extracted AX as well as extraction yields can exhibit huge differences. Moreover, the structural details of recovered AX or oligosaccharides strongly influence their functionality, technological value, and further potential valorization. Toward an effective valorization of cereal by-product-derived AX, AXOS, Xyl, and Ara, it is important to understand the native structural heterogeneity of AX as well as the influence of various methodologies on the recovery and structure of final products. This chapter explores the occurrence and structural heterogeneity of AX from cereals and their by-products, discussing the main extraction approaches and their influence on the recovered structures. It provides an overview of the main applications for cereal by-product AX and AXOS in the food, health, and materials fields.

8.2

Occurrence and distribution of arabinoxylans in cereals and their by-products

AX have been reported in several grains, including all the commercially relevant ones: wheat, barley, corn, rice, oat, and rye. In cereal grains, AX are present in the cell walls of starchy endosperm, aleurone, in the bran tissues, and in the husk or hull. However, depending on the cereal species, the amount and molecular structure of AX in a particular tissue may vary (Izydorczyk, 2009). Generally, cereal whole grains contain AX from 1.2% (w/w) in rice to 8.5% in rye, whereas xylans of more lignified tissues of cereals, such as leaves, straws, and husks, can represent 25%e35% of dry biomass (Fig. 8.1) (Aspinall, 1970). Quantitatively, most AX are present in bran layers that represent about 25% of grain dry weight. Cereal by-products arise from the field, as well as from the processing of cereals. Primarily, wastes and by-products from cereals are generated during harvest, postharvest processing, and postproduction stages. During harvesting of cereal grains in the field, the main by-products generated are the straws (dry stalks) that are highly

Arabinoxylans from cereal by-products

% of total AX (dry basis)

Whole grain

229 Bran

Flour

Straw

Husk

45 40 35 30 25 20 15 10 5 0 Wheat

Barley

Corn

Rice

Oats

Rye

Figure 8.1 Average content of total arabinoxylans (AX) in whole grains and tissues. Based on values reported by Izydorczyk, M., Biliaderis, C. 2006. Arabinoxylans. Functional Food Carbohydrates, CRC Press and Collins, H.M., Burton, R.A., Topping, D.L., Liao, M.-L., Bacic, A., Fincher, G.B. 2010. Review: variability in fine structures of noncellulosic cell wall polysaccharides from cereal grains: potential importance in human health and nutrition. Cereal Chemistry Journal 87, 272e282.

recognized as valuable raw material for biogas/biofuel production. In addition, during the harvesting of maize, the corncob may be discarded as part of the corn stover (mixture of stalks, leaves, husk, and cob) in the field. Processing of cereals include dry milling (to produce flour), wet milling (for starch and glucose production), and brewing (brewer’s spent grain [BSG]), all relevant sources of AX (Fallows and Verner Wheelock, 1982). During milling, bran and germ are separated from the endosperm of cereal grain (mostly wheat) that is further ground to a uniform particle size called flour. Generally, 28% of the grain is removed during the production of white flour with the concomitant production of huge amounts of bran (Elmekawy et al., 2013). Bran is the main milling by-product and includes the coarse outer shell of the grain with small amounts of flour. Bran represents the most important source of cereal fibers, which includes the highest amount of cereal grain AX (Fulcher and Duke, 2002). During the milling of rice, oats, and corn there is a prior removal of the hard protective shell that surrounds the grain before fine milling, which is discarded as grain husks by-product (Elmekawy et al., 2013; Krishna and Chandrasekaran, 2013). In the brewing industry, the major byproduct produced from barley grains (w85%) is BSG. In the early stages of the brewing process, the barley grains are germinated and dried (kilned) to produce the malt, which is then subjected to a mashing process that extracts the fermentable compounds. The insoluble undegraded part is discarded as BSG and is composed mainly of barley grain husks and minor fractions of pericarp and fragments of endosperm (Mussatto et al., 2006). In general, the nutritional value of cereal grain milling and malting by-products varies from the original cereal grain. An average of AX contents of the most common cereal grains and by-products (dry basis) is represented in Fig. 8.1.

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8.3


Cereal arabinoxylans: general structural features

AX are composed of a linear chain backbone of (b1 / 4)-linked D-xylopyranosyl (D-Xylp) residues (Fig. 8.2(a and b)). Cereal xylans are mainly substituted with monomeric L-arabinofuranosyl (L-Araf) residues at the C-2, C-3, and/or both C-2,3 D-Xylp positions, resulting in a molecular structure with unsubstituted, mono-, and disubstituted D-Xylp residues (Izydorczyk and Biliaderis, 1995; Ebringerova and Heinze, 2000; Izydorczyk, 2009). The distribution of L-Araf units along the xylan chain is not regular, and AX from different tissue sources can differ highly in their structural complexity. Monosubstituted D-Xylp units with (a1 / 2)-linked L-Araf groups are usually found in alkali-extractable barley flour AX (22% of substitution), but they occur in small amounts in all cereal AX (Viëtor et al., 1992). Water-extractable wheat bran AX have the highest amounts of doubly C-2,3 substituted D-Xylp units (40% of substituted units) (Shiiba et al., 1993). Although L-Araf residues are present in AX as monomeric substituents, small amounts of side chains with two or more L-Araf residues linked by (a1 / 2), (a1 / 3), and (a1 / 5) linkages also occur (Izydorczyk, 2009). Besides the L-Araf residues, the D-Xylp units can also be substituted with acetyl groups, D-glucopyranosyluronic acid (D-GlcA) and its 4-O-methyl ether (4-OMe-a-D-GlcA). AX with acid residues are referred to as glucuronoarabinoxylans, and can also be substituted with small amounts of xylopyranose and galactopyranose residues (Izydorczyk, 2009). A unique feature of AX is the presence of the hydroxycinnamic ferulic and p-coumaric acids esterified at the C-5 position of L-Araf that is linked to C-3 of D-Xylp residues (Aspinall, 1959; Saulnier et al., 1995a; Ishii, 1997). Ferulate can dimerize into dehydrodiferulate esters via phenolic oxidative coupling, and form the covalent cross-linking between AX. Also ferulic acid residues can establish the linkage between AX and cellulose, lignin and cell wall proteins, contributing to the insolubility of some AX structures (Izydorczyk and Biliaderis, 1995; Izydorczyk, 2009). Water-insoluble AX are reported with 8e39 times more diferulic cross-linking bridges than the water-soluble structures (Bunzel et al., 2001). The structure of AX shows differences according to the cereal grain tissue. However, it is not possible to attribute a defined structure to the different AX. Nevertheless, different motifs (Fig. 8.2(c)) can allow the distinction between AX from nonlignified tissues (starchy endosperm), lower-lignified tissues (bran), and more lignified tissues of cereals (which occur in straws, husks, and corncob). AX from lignified tissues are usually lower branched than AX from cereal bran (Izydorczyk and Biliaderis, 2006). The most lignified tissues of cereals have the highest diversity of AX side chains, as, for example, single terminal D-Xylp, a variety of disaccharide side chains, including D-Galp(b1 / 4)-D-b-Xylp, D-Galp(b1 / 5)-L-a-Araf, D-Xylp(b1 / 2)L-a-Araf, and a trisaccharide of L-Galp(b1 / 4)-D-Xylp(b1 / 2)-L-a-Araf (Whistler and Corbett, 1955; Aspinall et al., 1963; Saulnier et al., 1995b). BSG AX are also composed of 4-O-methyluronic acids, acetylated structures, and side chains with terminally linked galactose residues (Coelho et al., 2016), showing that the AX from different cereals have similar structural details and that AX heterogeneity is mainly related to their location among tissues.


(a)

231

O OH OH OH

HO

OH

OH

OH

D-Xylp

L-Araf

COOH O

OH

H3CO

OH

COOH O

O H3CO OH

OH

4-O-Me-α-D-GlcAp

β4

α3

OH

HO

OH

D-GlcA

β4

D-Galp

OH

OH

(b)

CH2OH OH O OH OH

OH

O OH

β4

β4

α3

α2

β4

β4

β4

α3

Ferulic acid

OMe

β4 α3

β4

β4

β4

α3

FeA

β4

α2

α5 FeA

FeA

AX

(c)

OAc β4

β4

β4

β4

β4

β4

β4

β4

β4

α3

α3

α5

β5

FeA

β4

β4

β4 β3

FeA β4

β4

Lignin

α3 β4

β4

β4 β2

β3 β4

β4

β4

β4 α2

β4 OMe β4

β4

β4 α3

β4

β4

β4 α2

β4

β4

β4 β3

β2

β4

β4

Figure 8.2 Arabinoxylan (AX) structural features. (a) Structural units that constitute cereal AX. (b) General representation of an AX from less lignified tissue cell walls. (c) Structural building blocks that may occur in AX from lignified tissues cell walls.

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The structural heterogeneity of AX is also observed on the degree of polymerization (DP) of the xylan backbone, as well as on the ratio of Ara to Xyl residues. The DP indicates the length of the AX chain, which is usually estimated based on the principle that Xylp do not occur as AX branching residues. Based on this assumption, the DP is calculated by the relative amount of total Xylp divided by the amount of terminally linked Xylp (Coelho et al., 2016). However, because Xylp can in fact occur as substituent side chains of AX (Aspinall, 1959), estimation of the DP by this methodology should give lower values than the real ones, especially in samples containing a higher content of branched Xyl residues. For AX from nonlignified tissues, this is a relevant parameter for comparison of AX from a large number of samples. The ratio of the content of Ara/Xyl of AX is an indication of the proportion of Araf groups to Xylp units and is therefore an average measure of the degree of branching of AX. Nevertheless, the Ara/Xyl ratio does not illustrate how Araf units are distributed over the backbone chain, neither does show if Xylp units are mono- or disubstituted with Araf. The Ara/Xyl ratio generally ranges from 0.5 to 0.8 in cereal flour, from 0.4 to 1.2 in bran, and is lower than 0.3 in lignified husks and straws (Izydorczyk and Biliaderis, 1995). AX in cereals can be water soluble or insoluble. Unsubstituted xylans are nearly water insoluble due to high hydrogen bonding occurring intra- and intermolecularly. With the increase of Ara residues as side chains, the polysaccharides become more water soluble due to the restriction of intermolecular hydrogen bonding. AX from lignified tissues can establish covalent linkages with lignin through ester and ether linkages of AX hydroxycinnamic acids (Saulnier et al., 2007), which contribute to their insolubility in water. Moreover, AX can also be directly linked to lignin through ester linkages between uronic acids and lignin hydroxyl groups or through ether linkages involving the primary hydroxyl groups (C-5) of Ara (Iiyama et al., 1994). AX of cereals and cereal by-products are divided into water extractable (WEAX) and water unextractable (WUAX) types. As, for all cereals, the amount of WEAX is much lower than the amount of WUAX (Izydorczyk, 2009), the improvement of their extractability is a priority to ensure a higher recovery and an efficient utilization of cereal by-products biomass.

8.4

Extraction and structural modifications of arabinoxylans from cereal by-products

The majority of AX are included in a water-insoluble matrix where only a small fraction can be solubilized with water at atmospheric pressure, giving origin to WEAX (Izydorczyk and Biliaderis, 1995; Fry, 2004). The presence of variable amounts of proteins, phenolic compounds, and other solubilizable polysaccharides prevents the extraction of AX and their further purification (Ebringerova and Heinze, 2000). Consequently, harsher and multistep methodologies (even at pilot scale) have been proposed for the extraction of water-insoluble AX, such as hydrothermal (compressed hot


233

water), chemical (alkali or diluted acids), and enzymatic treatments (Izydorczyk, 2009; Zhang et al., 2014). According to the extraction method used, the AX fractions obtained can present different compositions and, consequently, their properties may differ (Izydorczyk, 2009). As an example, AX isolated from the same barley husks by four different methodologies led to variable film-forming properties (H€oije et al., 2005), as will be presented in Section 8.5. From a sustainability point of view, the methodologies to be used should also promote higher recovery from cereal by-products (higher yields of AX extraction) using safer solvents and a minimum of wastes generated (Anastas and Warner, 1998).

8.4.1

Water and hydrothermal extractions

Water extractions are typically carried out at temperatures in the 25e100 C range, allowing the recovery of high molecular weight AX with the highest preservation of their native structure. However, the resulting extraction yields are relatively low and other water-soluble compounds are coextracted, lowering the purity of the AX obtained. To improve the extraction yield and purity of WEAX, some initial treatments such as destarching of cereal biomass are usually performed. In addition, to remove hydrophobic compounds that could prevent the extraction of the AX, a defatting treatment is also included (Izydorczyk and Biliaderis, 1995; Vinkx and Delcour, 1996; Ebringerova and Heinze, 2000). The pretreatment of wheat bran with hot ethanol (70% [v/v]; 80 C) at a pilot scale is able to remove fat and low molecular weight carbohydrates and inactivate AX-degrading enzymes, allowing WEAX composed of 81% of feruloylated AX with water 1:12 (w/v) at a mild temperature of 40 C to be obtained, although the water extraction yield was only 2.8% (Hollmann and Lindhauer, 2005). The amount of ferulic acid linked to wheat WEAX is only 0.2%e0.4% (w/w), which represents two to four ferulic acid residues per 1000 D-Xylp. Higher amounts of ferulic acid residues are reported for insoluble AX (0.6%e0.9%) (Saulnier et al., 2012). To increase the efficiency of water-based extractions, hydrothermal technologies have been proposed as green processes with high yields of hemicellulose extraction (58%e70%) (Peterson et al., 2008). In general, as this process only uses hot water as solvent (no hazardous chemicals are used), it has a low environmental impact. The hydrothermal process that uses liquid water under a temperature higher than 100 C and pressures between 5 and 40 MPa is called autohydrolysis. During autohydrolysis, the hot compressed water penetrates the biomass, hydrates cellulose, and depolymerizes the hemicelluloses, namely, AX, through hydronium-catalyzed reactions. The pressurized hot water undergoes autoionization (Eq. 8.1) generating acidic hydronium ions (H3Oþ) that are responsible for the first stages of hemicellulose hydrolysis (Carvalheiro et al., 2016). H2O (aq)

H+ (aq) + OH- (aq)

H+ (aq) + H2O (aq)

+ H3O+ (aq)

Equation 8.1: Water autoionization with formation of hydronium ion.

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The hydronium ions promote the selective hydrolysis of the hemicellulose ester linkages of acetyl groups that will further contribute to the formation of more hydronium ions in solution, therefore increasing the hydrolysis reaction kinetics (Heitz et al., 1986). Moreover, the glycosidic linkages of AX are further hydrolyzed by in situ acetic acid into low molecular weight oligosaccharides and monosaccharides (Josefsson et al., 2002; Carvalheiro et al., 2016). Autohydrolysis treatment can promote the solubilization of almost all hemicellulose content, mostly AX. Depending on the operational conditions, the products of AX autohydrolysis are mainly a mixture of AXOS, Ara, Xyl, acetic acid, and degradation products such as furfural and levulinic acid (4-ketopentanoic acid). Autohydrolysis is the hydrothermal approach that enables the highest recovery of oligosaccharides; up to 75% AXOS yield (g/100 g xylan). Moderate autohydrolysis conditions should be applied to balance the formation of AXOS over degradation products (Conde et al., 2009; Carvalheiro et al., 2016). It is generally accepted that short reaction times at higher temperatures led to a higher yield of pentose release (Li et al., 2015). Also lower particle size led to an increase in hydrolysis extent. Particle size is particularly important in lignocellulosic biomass, since higher particle sizes have poor impregnation and solubilization, therefore lowering the extraction yields due to an incomplete hydrolysis of the interior (Carvalheiro et al., 2016). Also grain particle fractions influence the structure of AX recovered, as observed in BSG, where different grain particle sizes led to the extraction of AX with distinct polymerization and branching degrees (Reis et al., 2015b). Reactor operation and configuration modes also influence the extent of AX autohydrolysis (Liguori et al., 2016). Batch procedures usually require low water and energy consumption and are mainly assisted by microwave-assisted extraction (MAE) technology (Galia et al., 2015). MAE uses microwave energy to rapidly heat the biomassesolvent mixture with shorter extraction times than other high heat treatments. Typically, MAE procedures take a few minutes and use small solvent volumes, about 10 times smaller volumes than those used by conventional extraction techniques (Eskilsson and Bjorklund, 2000; Coelho et al., 2014). Microwave irradiation has been proven as an effective and reliable means of producing soluble feruloylated AXOS from cereal by-products. The treatment of maize bran with MAE at 180 C for 10 min or 200 C for 2 min led to the release of about 50% original AX content as AXOS, with a wide variety of molecular weights. Lower temperatures and shorter MAE treatment times are consistent with low AXOS yields, while higher temperatures and longer reaction times result in an increase in monosaccharides, free ferulic acid, and side products levels. The highly feruloylated AXOS that can be recovered (6.62e8.00 g of esterified ferulate/100 g of AXOS) are reported to have health benefits, including prebiotic effects and antioxidant activity. Furthermore, the side products, namely, furfural, levulinic acid, and formic acid, can have application in the pharmaceutical or chemical industries (Rose and Inglett, 2010). Different cereal by-products could lead to the recovery of structurally different AXOS under hydrothermal treatment. The mild treatment of wheat bran, BSG, and corncobs (150e160 C, 60 min) led to the recovery of 22%, 33%, and 35% AXOS, respectively, and molecular weight lower than 104 Da, with different Ara/Xyl ratios and substitution pattern. AXOS mostly branched with Ara were found in the wheat


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bran and BSG hydrolysates, whereas from corncob, the more lignified by-product, AXOS substituted with 4-O-Me-a-D-GlcA were recovered (Kabel et al., 2002a). Another approach of hydrothermal processes is steam explosion (SE) treatment. SE has been described as a thermomechanochemical process where the breakdown of biomass is assisted by heat as steam (thermo-), by shear forces from the expansion of moisture (mechano-), and by autocatalyzed hydrolysis of hemicellulose glycosidic linkages (-chemical) (Gírio et al., 2012). In SE, the biomass is quickly heated by introducing high-pressure saturated steam in a reactor (preferably at temperatures between 160 and 240 C and pressures of 0.69e4.83 MPa), during a short period (seconds or minutes). Under high pressure, the steam condensates and permeates the biomass initiating the autohydrolysis reactions, catalyzed by the acetic acid released from hemicelluloses. Furthermore, when the pressure is immediately released, the condensate within the material evaporates again, causing mechanical disruption of the cell wall matrix, with the consequent breaking down of inter- and intramolecular linkages leading to the extraction of hemicelluloses (Avellar and Glasser, 1998; Sun and Cheng, 2002; Cara et al., 2006). The fractionation of wheat straw using a two-stage process based on SE pretreatment (200 C/15 bar for 10e33 min or 220 C/22 bar for 3e8 min) results in xylooligosaccharides (XOS) and monosaccharides accounting for 20.5%e28.5% of the dry starting material. The recovery yields continuously increased as SE temperature and time increase (Sun et al., 2005). To increase the extraction efficiency and higher sugar yields recovered from AX, hydrothermal processes can be assisted by heterogeneous acid catalysts. The addition of heterogeneous catalysts, supported on mesoporous silica, can enhance the hydrolysis rate of cereal AX. Relatively high temperatures, short extraction times, and solid catalysts favor the extraction of high amounts of low molecular weight AX with high acidity. The hydrothermal treatment of wheat bran (180 C, 10 min) using RuCl3-based catalysts led to 78% recovery yield of AXOS with around 9 kDa, resulting in reduced operation time and energy consumption, and formation of sugar degradation products. Moreover, the easy removal of the solid catalysts from the liquid extract allows their recovery and reutilization (Sanchez-Bastardo et al., 2017).

8.4.2

Chemical extractions

AX extraction yields can be greatly improved by the extraction of WUAX with aqueous alkali or acid solvents. These extractions can promote the recovery of WAUX by disruption of hydrogen and/or covalent bonds within the cell wall matrix. So far, sequential alkaline extraction using increasing strength is one of the most important approaches to recover water-insoluble hemicelluloses (Izydorczyk, 2009). Alkaline extractions are usually performed with sodium hydroxide, potassium hydroxide, and barium hydroxide solutions (Zhang et al., 2014). Under alkaline conditions, the hydroxyl ions promote the swelling of cellulose and the disruption of intermolecular hydrogen bonds. They also hydrolyze the ester linkages between ferulic acid and Ara residues, the acetyl groups, and the carboxyl groups of uronic acids and benzyl groups of lignin (Jeffries, 1994). The disruption of these linkages leads to the solubilization of hemicellulose from the cell-insoluble matrix, with the concomitant

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loss of acetic and ferulic acid groups. The sequential fractionation of BSG and wheat bran by alkali with increasing strength (50 mM Na2CO3, 0.5 M KOH, 1.0 M KOH, and 4.0 M KOH) allows the recovery of AX with lower Ara/Xyl ratios as the alkali strength increases, due to the extraction of less substituted, more compact, and insoluble AX structures with higher alkali strength. This fractionation results, however, in loss of ferulic acid (Mandalari et al., 2005). Beyond the differences in branching, sequential extraction with increasing alkali strength also promotes recovery of AX with different molecular weights (Zheng et al., 2011). An integrated process with increasing alkali (KOH or NaOH) concentrations of 0.1, 0.5, and 4 M led to the recovery of 62%e86% AX from BSG without pretreatment. The process allows the selective and sequential precipitation of the proteins by acidification of the medium with a solution of citric acid and the precipitation of AX by HCl and ethanol. This environmentally clean process (Fig. 8.3) allows the recycling of citric acid and ethanol, which can be used in the subsequent steps, saving 93% in costs, as well as the recovery of the sodium chloride or potassium chloride (Vieira et al., 2014).

Brewer's spent grain 0.1 M KOH or NaOH

Residue 1

Extract 1

HCI ethanol

Citric acid

AX 1

P1 0.5 M KOH or NaOH

Residue 2

HCI ethanol

Citric acid

Extract 2

AX 2

P2

4.0 M KOH or NaOH

• Citric acid • Ethanol • KCI/NaCI

Residue 3

Extract 3

HCI ethanol

Citric acid

AX 3

P3 Water

Final residue

Extract 4

HCI; ethanol

AX 4

Figure 8.3 Scheme of the integrated extraction of proteins and arabinoxylans (AX) from brewer’s spent grain (BSG) using alkaline reagents. Adapted from Vieira, E., Rocha, M.A.M., Coelho, E., Pinho, O., Saraiva, J.A., Ferreira, I.M.P.L.V.O., Coimbra, M.A. 2014. Valuation of brewer’s spent grain using a fully recyclable integrated process for extraction of proteins and arabinoxylans. Industrial Crops and Products 52, 136e143.


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Despite the high yields reported in laboratory-scale experiments, cost, process safety, and adverse environmental effects may limit the industrial production of AX with alkali solvents. This can be illustrated by laboratory-scale extraction performed on destarched and delignified wheat bran with 0.5 NaOH, where an AX yield of w35% on a dry weight basis for 80% of NaOH/bran was observed, independently of the extraction temperature. However, because the use of 80% of NaOH/bran is not suitable for further application at an industrial scale, at a pilot scale, to avoid excessive energy costs and environmental difficulties, 40% of NaOH and 40 C were used. This led to the recovery of 13% AX, approaching little more than one-third of the yield obtained at a smaller scale (Bataillon et al., 1998). Dilute alkaline solutions of hydrogen peroxide are effective in promoting delignification and improving the recovery of hemicelluloses from cereals (Doner and Hicks, 1997). In alkaline medium, hydrogen peroxide forms the active hydroperoxide anion (HOO") that causes the oxidation of lignin and cleavage of interunit bonds, promoting the dissolution of lignin and hemicelluloses (Pan et al., 1998). Also the use of an H2O2/ NaOH delignification step before alkaline extraction has been successfully used to substitute the hazardous and industrially more expensive NaClO2 delignification (Peng et al., 2012). In corn bran, the addition of H2O2 enhanced the yield of extraction by 8.8% (100 C, 60 min) to a total recovery of 37% of branched AXOS (44 kDa, Ara/ Xyl ratio of 0.65) (Doner and Johnston, 2001). H2O2 treatments are also reported for the extraction of higher molecular weight alkali-resistant AX, promoting in corn the cleavage of alkali-resistant linkages and the extraction of higher molecular weight AX (491 kDa) than those of alkali-extracted AX (348 kDa) (Yadav et al., 2007). Barium hydroxide solutions are AX selective promoters. However, the mechanism is not clear. Although Fincher and Stone (1986) hypothesized that Ba2þ ions specifically interact with pentoses and facilitate their extraction, it is possible that the formation of insoluble complexes between Ba2þ and b-glucans be the responsible mechanism that avoids their coisolation (Gruppen et al., 1991). The use of Ba(OH)2 as a primary extractant for WUAX led to the extraction of 80% of wheat flour WUAX (Gruppen et al., 1991) and 50% of wheat bran WUAX (Bergmans et al., 1996). As glycosidic linkages are resistant to alkaline treatments, the degradation of xylan under alkaline conditions is a slow reaction proceeding only from the reducing end group, in a process called “alkaline peeling” (Aspinall et al., 1961). This is observed, for example, by the increase from 0.8 to 1.0 of the Ara/Xyl ratio after the alkali extraction of destarched wheat bran with 0.4 M NaOH at 80 C during 15 h, which is consistent with a decrease in total Xyl in the extracted AX (Aguedo et al., 2013). Nevertheless, the AX glycosidic linkages can be chemically hydrolyzed with acid solutions. Acid extractions can promote the hydrolysis of AX side chains and labile a-Ara glycosidic linkages. At higher temperatures, hydrolysis of b-linkages of xylan backbone can occur. Like hydrothermal treatment, this approach led to high recovery yields of AXOS and monosaccharides. Acid extractions are usually performed with diluted acetic or formic acid. For example, wheat straws submitted to various acid treatments with acetic acid, formic acid, methanol, and ethanol (combined with 0.1% of HCl at 85 C during 4 h) allowed yields of 30% AXOS poorly substituted (Ara/Xyl < 0.1), evidencing the acid degradation of xylan backbone, as well as the side chains (Xu et al., 2006).

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Like hydrothermal treatments, chemical extractions can be assisted by microwave technology, which improves the efficiency of extraction and the sustainability of the process, since MAE significantly lowers the quantity of reagents used. The recovery of BSG AX and AXOS using MAE witnessed an increase in the AX þ AXOS yield with an increase in temperature in the range from 140 to 210 C during 2 min. The best conditions for promoting a compromise between yield and loss of structural details, minimizing the thermal degradation of the fractions extracted, were a sequential procedure. This starts with water extraction at 140 C to remove the residual starch mixed with b-glucans, followed by a second water extraction at 180 C, and a third extraction with 0.1 M KOH at 180 C. This sequential procedure promotes the extraction of 62% of BSG AX þ AXOS, presenting degrees of polymerization ranging between 7 and 24 Xyl residues, and a degree of phenolic acid esterification between 5% and 21% (Coelho et al., 2014). Ultrasound-assisted extraction (UAE) has also been proven to increase AX alkali extraction yields. The application of high-intensity ultrasounds causes pressure fluctuations in water that propagates through the material. The pressure waves and resulting cavitation phenomena promote the collapse of cavitation bubbles and highly localized temperature that breaks cell walls, promoting the release of the contents of the cell into the extraction medium. Ultrasound treatment promotes the splitting of the a-ether linkages of AX with lignin, improving therefore the extraction yield. Moreover, UAE substantially reduces the extraction time, solvent consumption, and extraction temperature, resulting in higher yields and purity of polysaccharides with no significant structural changes and no negative effects in their functional properties (Hollmann et al., 2009; Reis et al., 2014, 2015a). Short ultrasound treatments of up to 30 min intensify the extraction process of corncob AX (29% yield) without compromising their biological activity. Moreover, Ebringerova et al. (1998) reported that the mitogenic and comitogenic activities of sonicated extracts were comparable with those of the alkali classically extracted AX, and higher than those of the commercially controlled immunomodulator Zymosan. The pretreatment of biomass using ultrasound and water was reported as an important refining step in the removal of starch and proteins from cereal by-products. The washing ultrasound water treatment of BSG allows further ultrasound alkali extraction of higher purity AX extracts, almost absent of starch (Reis et al., 2015a).

8.4.3

Enzymatic extraction

Enzymatic strategies for AX extraction appear as a viable alternative to chemical methods. It has been proved that enzymatic methods for the conversion of WUAX into solubilized AX were as efficient as the chemical methods, and are more acceptable from an environmental point of view (van Craeyveld et al., 2010). The most common enzymes used in AX extraction are the endo-(b1,4)-xylanases (EC 3.2.1.8) (Escarnot et al., 2012). Endoxylanases can attack the xylan backbone, cleave internal (b1,4)-linkages in a random manner, and penetrate the cell wall network, producing a mixture of nonsubstituted and substituted XOS. The action of xylanases led to the partial solubilization and extraction of WUAX and depolymerization of WEAX that are further degraded into their single components: Ara and Xyl. These monosaccharides


239

could be further hydrogenated to obtain valued xylitol and arabinitol, two of the top 12 value-added products derived from biomass (Courtin and Delcour, 2001). The most effective xylanases reported are the ones from the glycoside hydrolase (GH) family, namely, GH10 and GH11. Xylanases of the GH10 family require two consecutive unsubstituted Xylp residues to attack the xylan backbone and are capable of cleaving the glycosidic linkage to MeGlcA- or Ara-substituted Xylp residues. In the case of the xylanases of the GH11 family, they require three consecutive unsubstituted Xylp residues and do not hydrolyze substituted residues (Biely et al., 2016). GH10 xylanases are most active on soluble AX, whereas GH11 xylanases have higher selectivity for insoluble substrates and showed greater ability to penetrate the cell wall network (Zhang et al., 2014). On destarched wheat bran, AX hydrolyzed by GH11 xylanases exhibit lower A/X ratios (0.23e0.28) than those produced by GH10 xylanases (0.38e0.43), indicating the distinct patterns of action. The simultaneous use of both xylanases does not result in a synergistic action on wheat bran AX, but instead leads to the production of mixtures with a profile similar to the single action of GH10 xylanases. Upon treatment with either xylanases, yields of 50.7% (raw material AX basis) of AX can be recovered, with unaltered diferulic acid levels in residual bran, although the content in ferulic and p-coumaric acids may decrease (Beaugrand et al., 2004). In addition to endo-(b1,4)-xylanases, other enzymes are capable of hydrolyzing AX (Fig. 8.4), which is particularly important when it is necessary to modify the structure for further applications. The exo-(b1,4)-xylanases (EC 3.2.1.37) are enzymes capable of hydrolyzing the Xyl chain from the nonreducing end, but their effectiveness decreases by increasing the XOS chain length. Also the hydrolysis of AX side chains can alter intrachain and interchain AX interactions, leading to an increase in their solubility/extractability (Faulds et al., 2003; Zhang et al., 2014). The a-L-arabinofuranosidases (EC 3.2.1.55) release terminally linked a-L-Araf side residues. The a-L-arabinofuranosidases are classified in 43, 51, 54, and 62 GH families and can be divided into two groups. The m2,3 group is disseminated in all mentioned a-L-arabinofuranosidase GH families and is active on b-D-Xylp residues monosubstituted by a-L-Araf at either position, 2 or 3. The d3 type is a minor group of the a-L-arabinofuranosidase GH43 family, specific for doubly arabinosylated b-D-Xylp residues. The d3 a-L-arabinofuranosidases selectively release only the (a1 / 3)-L-Araf, leaving the (a1 / 2)L-Araf residues on the main chain (van Laere et al., 1999; Biely et al., 2016). Only the d3 type is commercially available on the market, which is a drawback to the enzymatic extraction or purification methodologies. For total debranching of Ara side chains, the combined action of d3 and m2,3 a-L-arabinofuranosidases is required. The side chains of AX can also be hydrolyzed by (1) xylan (a1,2)-glucuronosidases (EC 3.2.1.131) that hydrolyze the linkage between uronic acids and the b-D-Xylp unit; (2) acetyl xylan esterases (EC 3.1.1.72) that cleave the acetyl groups from positions C-2 and C-3 of b-D-Xylp units; and (3) feruloyl esterases (EC 3.1.1.73) that are able to hydrolyze the ester linkages between ferulic or p-coumaric acids and a-L-Araf units. Feruloyl esterases are particularly important in AX isolation from the cell wall matrix, since the ferulic acid bridges AX and lignin. Moreover, since a-L-arabinofuranosidases are not able to hydrolyze substituted Ara residues, feruloyl esterases are important to remove first the hydroxycinnamic acids from Ara side chains of ferulated AX.

240


Endo-(β1,4)-xylanase GH10

β4

β4

β4

β4

β4

Endo-(β1,4)-xylanase GH11

β4

β4

β4

β4

β4

β4

α3

α2

α3

β4

β4

Exo-β-(1,4)-xylanase

OMe

α-L-Arabinofuranidase m2,3

Acetyl xylan esterases

α-L-Arabinofuranidase d3

OAc β4

β4

β4

β4

β4

β4

α2

α3

α3

Ferolyl esterase β4

β4

β4

β4

β4

β4

β4

β4

α2

Xylan α-1,2-glucuronosidase β4

β4

β4 α2

α3 α5 FeA β4 FeA

β4

β4 α2 OMe

Figure 8.4 Enzymes used for hydrolysis of the arabinoxylan (AX) backbone and side chains.

To promote the extraction of AX from the cell wall-insoluble matrix and their purification, enzymes such as a-amylase to remove the starch and cellulases to remove cellulose can also be used together with the foregoing reported enzymes to degrade AX (Doner et al., 1998). Enzymatic treatments usually exhibit lower AX extraction yields when compared to chemical extractions with alkalis or acids. For example, the extraction yield of 12.4% of AX obtained from destarched wheat bran with endoxylanases is lower than the 18.5% achieved with alkaline hydrogen peroxide. Alkali-extracted AX were shown to have an average molecular weight of 350 kDa, 10 times higher than the 32.5 kDa of enzymatically extracted AX. In addition, enzyme-extracted AX were shown to preserve 43.5 mg/100 g of ferulic acid, while these structural features completely disappeared in alkaline-extracted AX (Zhou et al., 2010). Obstacles such as the crystalline structure of lignocellulose and the presence of enzyme inhibitors often limit enzyme hydrolysis, resulting in low yield compared to alkali extractions. However, the alkaline approach does not create a friendly environment and promotes the release of AX ferulic acid with reported antioxidant activity. In addition, enzyme-extracted AX show higher ferulic acid content and greater in vivo immune-enhancing activities than alkaline-extracted AX (Zhou et al., 2010) Also enzymatic approaches have advantages over the chemical method when used in industrial-scale food production.


8.5

241

Potential application fields for cereal by-products AX and AXOS

In the last two decades the food industry has exploited ways of improving the overall nutritional balance of carbohydrate-rich foods by increasing their dietary fiber content. Dietary fibers are mainly nonstarch polysaccharides, undigested by the endogenous enzymes in the small intestine of humans (Phillips and Cui, 2011). Cerealprocessing by-products have been exploited as an inexpensive source of insoluble and soluble dietary fiber, mostly AX. These polysaccharides have been reported with multiple beneficial effects on the human digestive system, which is advantageous to improve the functional value of foodstuffs. Moreover, AX can also improve the viscosity, texture, sensory characteristics, and shelf-life of food products (Mudgil and Barak, 2013). Also AX-derived oligosaccharides have aroused scientific and commercial interest mostly because of their prebiotic character. The most recognized functional property of XOS and AXOS is their ability to stimulate the growth of beneficial microflora in the gut (Rastall, 2010). According to Gibson and Roberfroid (1995), “A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well-being and health.” The ingestion of prebiotics typically stimulates the growth of Bifidobacterium and/or Lactobacillus species, recognized as health-promoting live microorganisms. The fermentation of prebiotics by colon bacteria led to the production of short chain fatty acids (SCFAs) such as acetate, propionate, butyrate, and lactate, that contribute to a lower pH in the intestine and therefore inhibition of potentially harmful bacteria (Broekaert et al., 2011). Since AXOS can differ in their DP and degree of substitution according to the preparation methodology, the diversity of these compounds is higher than in other commercial prebiotic carbohydrates, such as inulin and fructooligosaccharides. Thus the relationship between oligosaccharide structure and beneficial health effects as a prebiotic has been investigated. The prebiotic potential of wheat bran-derived AXOS in relation to their structure has been evaluated in vivo using rats. AXOS and XOS with DP #3 promote the increase of colonic acetate and butyrate concentrations as well as Bifidobacteria. In contrast, AXOS with the highest average DP of 61 do not increase butyrate concentration or stimulate fecal Bifidobacteria development. These high DP effectively seem to suppress SCFA concentrations, shifting the balance away from protein fermentation. Two AXOS preparations with DPs of 12e15, and different Ara/Xyl ratios of 0.69 and 0.27, were shown to have similar prebiotic effect, suggesting that the influence of AXOS DP is more pronounced than Ara substitution. Overall, the best gut health effects seem to be obtained with AXOS DP of 5 and Ara/ Xyl ratio of 0.27 (van Craeyveld et al., 2008). The influence of substituents on AXOS fermentation by human fecal inocula was studied in vitro with differently substituted oligosaccharide recovery from the autohydrolysis of BSG (Kabel et al., 2002b). The nonsubstituted XOS and AXOS ferment faster than the more complex XOS structures with acetyl or 4-O-methylglucuronic acid groups since they are not easily degradable. The fermentation of these compounds

242


resulted in the production of SCFAs and lactate, where the highest production occurs for the nonsubstituted XOS and AXOS. Substituted XOS obtained by autohydrolysis of corn straw have also reported bifidogenic activity on in vitro human fecal inocula, with a better performance for the low DP when compared with the medium-high DP. The slow assimilation of substituted XOS suggests that their fermentation in humans may proceed in the distal part of the colon (Moniz et al., 2016). Overall, the XOS obtained from autohydrolysis of cereal by-products appear to be promising distally fermentable substrates. Combinations of enzymatically hydrolyzed wheat bran fractions of WUAX, WEAX, and AXOS included in a standardized diet for laboratory rats showed that, in general, WUAX are only partially fermented in the cecocolon leading to an increase in butyrate levels. On the other hand, extensive fermentation occurs for WEAX and AXOS, which reduces the intestinal pH, suppresses the proteolytic breakdown, and induces a selective bifidogenic response. Interestingly, the partial replacement of WUAX with AXOS causes a striking synergistic increase in cecal butyrate levels. In a human diet context, the addition of AXOS to a bran-containing food product containing WUAX would lower bran levels in the product while maintaining or even raising its health-related physiological benefits, thereby improving the properties of the food products (Damen et al., 2011). AX have been exploited as promising polymers for the development of renewable and environmentally friendly biobased materials aimed at decreasing the environmental impact of petroleum-based materials used in food packaging. The main AX polymer-based studies have focused on the formation of films and coatings, foams and gels, and also chemical modifications of xylans to enhance film/foam/gel properués et al., 2014). AX exhibit relevant functional properties that are advantaties (Eg€ geous to form films for food applications, namely, their capacity to form a continuous and cohesive matrix with high water absorption capacity (up to 100 g of water per gram of dry polymer), their stability to temperature and pH changes, and their neutral taste and odor (Izydorczyk and Biliaderis, 1995). Moreover, the mechanical and barrier properties of AX-based films are reported as the same order of magnitude as those of gluten, whey protein isolate, hydroxypropyl methylcellulose, methylcellulose, or starch (Ni~no-Medina et al., 2010). WEAX-feruloylated AX possess a unique capacity to form covalent gels through ferulic acid cross-linking upon oxidation by free radical-generating agents, such as the laccase and peroxidase/H2O2 system (Izydorczyk and Biliaderis, 1992). Laccase (p-diphenol oxygen oxidoreductase, EC 1.10.3.2), a blue multicopper enzyme, can oxidize ferulic acid from WEAX into different diferulic structures (5-5ʹ-, 8-O-4ʹ-, 8-5ʹ-, and 8-8ʹ di-FeA), with the 8-5ʹ and 8-O-4ʹ forms being the predominant ones (Fig. 8.5) (Carvajal-Millan et al., 2005). This covalent cross-linking has commonly been considered as responsible for gel network development, even if weak interactions also contribute to the final properties (Figueroa-Espinoza and Rouau, 1999). Polysaccharide films can be used to protect perishable food products from deterioration, and the incorporation of antimicrobial compounds or biocontrol microorganisms into the films has been reported to maintain the stability of food storage and improve the control of postharvest diseases. The capacity of wheat WEAX to form


HO

O

243 O HO

O OH HO

MeO

O

H O OMe

HO HH

OMe

HO

HO

O

O OMe

OMe OH

OH

O

OH OH

O

O

OMe OH OH OMe

OMe OH

8–0–4ƍ dimer

8–5ƍ dimer

8–8ƍ dimer

5–5ƍ dimer

Figure 8.5 Ferulate dehydrodimers formed by enzymatic oxidation of ferulic acid.

covalently cross-linked films in the presence of Debaryomyces hansenii yeast, a microorganism used to competitively control blue mold decay of lemon, has been exploited. The covalently cross-linked films containing D. hansenii can be prepared by casting, resulting in a slight reduction of WEAX gel elasticity, probably due to decreased physical interactions between WEAX chains. These films are promising in functional yeastentrapping bioactive packaging (Gonzalez-Estrada et al., 2015). The chemical structure of AX, namely, Ara/Xyl ratio, has been proven to influence the properties of films. AX samples from rye with Ara/Xyl ratios from 0.2 to 0.5, enzymatically prepared using an a-L-arabinofuranosidase, are able to form cohesive films upon drying without addition of external plasticizers. In contrast with the completely amorphous film prepared with untreated AX, the films with AX enzymatically modified are reported to be semicrystalline. The degree of crystallinity increases with the decrease in Ara content. All films are reported to be strong and relatively stiff, with stress behavior similar to synthetic semicrystalline polymers. Furthermore, the decrease of Ara content seems to lead to a decrease in oxygen permeability of the films, which is very attractive for packaging applications (H€oije et al., 2008). Also fractions from barley husks, prepared using different enzymatic and chemical approaches, resulted in variable AX and lignin content. Films prepared with the fraction with the lowest AX and highest lignin content (50% and 12%, respectively) were dark, while films prepared with the richer AX fraction (83%) and fewer amounts of lignin (2%) were light, homogeneous, and transparent, probably conferred by AX-unsubstituted regions. The presence of larger unsubstituted AX regions can give rise to strong hydrogen bonds, causing chain interactions, turning the material partly crystalline (H€oije et al., 2005). Moreover, it seems that AX form films without the addition of plasticizers. The prebiotic properties of AX are an interesting way to extend the value of functional edible packaging, similar to those based on chitosan films. Chitosan films with 0.2% of AX or AX/AXOS prepared by solvent casting did not change the permeability and elongation of chitosan-based films. However, the incorporation of AX with a molecular mass of 462 kDa results in a stronger and more flexible material, probably due to higher intermolecular interactions. Phenolics of AX/AXOS promote the opacity of the films, contributing also to darkening of the products. Overall, AX can be successfully incorporated into chitosan-based edible films, providing potential added functional properties (Costa et al., 2015).

244


D-Xylose

L-Arabinose NAD(P)H

1 NAD(P)+ NAD(P)H

L-Arabitol

1 NAD(P)+

NAD+

2 NADH

3 L-Xylulose

Xylitol NAD(P)+

NAD(P)H

Figure 8.6 Pathways for utilization of D-xylose and L-arabinose by fungi. 1, Aldose reductase (EC 1.1.1.21); 2, L-arabinitol 4-dehydrogenase (EC 1.1.1.12); 3, L-xylulose reductase (EC 1.1.1.10).

Another relevant valorization approach for cereal by-product AX and AXOS is their complete hydrolysis into monosaccharides to further produce fine chemicals. D-Xyl and L-Ara resulting from AX hydrothermal processes, acid extractions, and enzymatic hydrolyses can be reduced to xylitol (Fig. 8.6), a noncariogenic sweetener that is metabolized insulin independently and therefore is suitable for diabetics. Due to its anticariogenicity, tooth rehardening, and remineralization properties, D-xylitol has been widely applied in the odontological industry. Currently, xylitol is manufactured at the industrial level by chemical hydrogenation of Xyl in the presence of a nickel catalyst at elevated temperatures and pressures (Prakasham et al., 2009). To make the process more economical and eco-friendly, the bioconversion D-Xyl into xylitol by fermentation of cereal by-products hydrolysates has been exploited (Rao et al., 2006).

8.6

Summary

Depending on cereal by-products’ main tissues, AX can present different structural complexities and properties. Therefore several methodologies have been developed to achieve the highest recovery of AX from diverse by-products. Water extractions are an environmentally friendly approach that preserves the structural features of extractable AX. However, due to the low recovery yields of water extractions, new hydrothermal approaches using high pressures and high temperatures have been proposed. Chemical treatments with alkali and acid solutions are nevertheless the most efficient for AX extraction. The main disadvantage of these approaches is the loss of AX structural features, namely, the breaking down of linked ferulic acid, with reported antioxidant activity. Also chemical extractions may lead to the recovery of low branched structures with low Ara substitution ratios, which may be relevant or not, depending on the application. Cost, process safety, and adverse environmental


245

effects may limit the industrial production of AX with chemical solvents. Chemicalassisted extractions with microwave and ultrasound technology have been proposed to decrease the consumption of solvent and reaction times, improving their sustainability. Enzymatic treatments are less efficient than alkaline extractions. However, this environmentally friendly methodology enables the control of AX degradation according to the type of enzyme applied. In addition, this is a high-valued method for the specific modification of AX to increase their technological relevance. Overall, different polysaccharides, oligosaccharides, and monosaccharides can be recovered according to the different methodologies used. The AX recovered can be further applied in food products to improve the dietary fiber content. Moreover, as AXOS have prebiotic properties as bifidogenic enhancement, they are relevant for the formulation of functional foods. Despite their antioxidant activity, feruloylated AX possess a unique capacity to form covalent gels in the presence of free radical-generating agents. Films from AX have been shown to provide good oxygen barrier properties and their application as packaging materials has been suggested. Also the sugar monomers that result from AX extractions can be valorized to produce fine chemicals, such as xylitol. In conclusion, the huge amounts of by-products generated every year from cereal crops and the processing industry represent an excellent and inexpensive source of carbohydrates for different and valuable applications. The challenge is to match the structural features of AX with the required properties of the final products.

List of abbreviations Ara Ara/Xyl AX AXOS BSG DP MAE SE UAE WEAX WUAX XOS Xyl

Arabinose Arabinose to xylose ratio Arabinoxylans Arabinoxylooligosaccharides Brewer’s spent grain Degree of polymerization Microwave-assisted extraction Steam explosion Ultrasound-assisted extraction Water extractable arabinoxylans Water unextractable arabinoxylans Xylooligosaccharides Xylose

Acknowledgment The authors acknowledge to FCT/MEC for the financial support to the QOPNA research Unit (FCT UID/QUI/00062/2013), through national founds and where applicable cofinanced by the FEDER, within the PT2020 Partnership Agreement and the individual grants of Rita Bastos (PD/BD/114579/2016) and Elisabete Coelho (SFRH/BPD/70589/2010).

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