Source |
Extraction |
Solvent/Enzyme |
AXs Yield * |
A/X Ratio |
Reference |
De-starched wheat bran |
Alkali |
0.44 M NaOH |
20.80 |
0.94 |
[83] |
Corn fibre |
Alkali |
0.25–50 M NaOH |
26.80 ** |
n.d. |
[84] |
De-starched plan materials |
Alkali |
NaOH (pH 11.5) |
14.30–59.9 *** |
n.d. |
[57] |
Chinese, black-grained wheat bran residue (after removal of water-extractable polysaccharides) |
Alkali |
Saturated Ba(OH)2, 1% NaBH4 |
~5.8 |
0.6 |
[85] |
Wheat bran |
Alkali |
Saturated Ba(OH)2, 0.26 M NaBH4 |
24 |
0.7 |
[86] |
Corn husk |
Alkali |
0.9% (w/v) Ca(OH)2 |
n.d. |
0.75 |
[87] |
De-starched wheat |
Alkali/Enzymatic + alkali |
0.16 mol/L NaOH, 0.5% H2O2//xylanase and cellulase (sodium acetate buffer) + 0.16 mol/L NaOH, 0.5% H2O2 |
19.83//5.27 and 14.95 |
1.14//0.25 and 1.52 |
[13] |
[ | 28 | ] |
Rye |
Endosperm |
3.56–4.25 |
|
[31] |
Main chain of 4-linked β-D-xylopyranosyl residues. A terminal α-L-arabinofuranosyl residue substitutes (on average) every second unit at position 3 and a small portion of the xylose units at position 2 and 3. |
[32][33][34] |
Bran |
12.6 |
2.1 |
[31] |
Oat |
Endosperm |
1.2 |
0.2 |
[35] |
(1–4)-linked β-D-xylopyranosyl residues making up the main chain, with terminal L-arabinofuranosyl residues substituting at O-3, but also at both O-2 and O-3. |
[35][36] |
Bran |
5.2 |
0.7 |
[35] |
2.1. Structure of Wheat Arabinoxylan
Wheat AXs are present in endosperm (3–5% of total endosperm), aleurone, and bran cell walls (approximately 60–70% of the entire cell wall)
[12][15]. In the specific case of wheat bran, AXs represent between 10.9 and 26% of all the bran fractions
[37][38][39][40].
In wheat AXs, side chains are linked by α-(1→2) and/or α-(1→3) bonds along the xylan backbone. The xyloses can be di-substituted, mono-substituted (the most common substitution), or not substituted at all
[16][17]. These side chains are mainly formed by single arabinose units (α-l-arabinofuranose), but side chains linked to xyloses of α-d-glucuronic acid (and its methyl ether, 4-O-methyl-glucuronic acid) also occur
[16]. The structure of wheat AXs presents a wide variability, as reported by several authors
[40][41][42]. These differences are influenced by the wheat variety and the wheat grains’ maturation stage. It has been reported that the arabinose/xylose ratio decreases upon maturation
[43][44], having a positive influence on wheat AX’s water solubility
[45] According to Barron et al.
[39], AXs of the endosperm present a higher water solubility than AXs from bran, as well as a lower arabinose/xylose ratio (A/X) (~0.6) than that of AXs derived from bran (~1). Other studies confirm these findings
[46][47][48]. However, Kaur et al.
[40] reported A/X ratios to be considerably lower than 1 for wheat brans of four different wheat varieties (between 0.33 and 0.62). These authors also reported different A/X ratios for bran fractions rich in AXs. They found A/X ratios between 0.09 and 1.37 (for water-extractable fractions), 0.33 and 1.82 (for alkali-extractable fractions), and 0.38 and 0.7 (for cellulosic arabinoxylans). This variability is a good indication of the complexity and variability of wheat AX’s structure. However, it seems clear that the A/X ratio is lower for AXs located in the endosperm than for those located in other parts of wheat grains. The A/X ratio plays an important role in modulating the hydration and swelling capacity of AX
[49]. Maes and Delcour
[50] observed that wheat AX extracted from wheat bran had an A/X ratio of 0.45, but the gradual precipitation of AX with ethanol changed the ratio significantly from 0.31 to 0.85, depending on the percentage of ethanol used, demonstrating the influence that the type of extraction method can have on the A/X ratio. In addition to the xyloses, arabinoses and α-d-glucuronic acid units that form part of the AX’s other short sugar side chains can also be present in wheat AX’s structure. These side chains are constituted by xylopyranosyl and galactopyranosyl residues associated with arabinofuranosyl residues
[16]. Additionally, arabinose units/chains can also carry acetic acid and hydroxycinnamic acids (ferulic and p-coumaric esters)
[16][45].
2.2. Structure of Barley Arabinoxylan
The basic structure of barley AXs is the same as that of wheat AXs (polysaccharides mainly composed of xylose and arabinose). However, there are some notable differences. For example, barley AXs present side chains of xylose units in the 2 and/or 3 C of the xyloses, forming the backbones of AXs
[19][20]. On average, barley AXs have a higher A/X ratio than wheat AXs
[51], since their arabinose side chains are more numerous. The molecular weight (Mw) of barley AXs is also distributed in a wide range for kDa
[19][22][52], having a higher Mw for water-soluble AXs
[22]. Barley AXs are distributed along all the grain, representing around ~10–14% and ~1.2–1.3% of the bran fraction and endosperm, respectively
[20][22], and around 25–40% of barley cell walls
[8]. Evidence supports a positive relationship between higher A/X ratios (implying more branching) and improved water solubility. Izydorczyk et al.
[20] reported both AX’s higher solubility and higher A/X ratios (from the water-soluble AXs) from bran fractions (~0.8–1) than from an endosperm fraction (~0.65–75)
[20]. However, when comparing the A/X ratio of water-soluble and -insoluble AXs, these authors observed that insoluble AXs from the endosperm had a higher A/X ratio than that of soluble AXs. In disagreement with these results, Lazaridou et al.
[8] reported a higher A/X ratio for water-soluble AXs than for non-water-soluble AXs originating from the endosperm. These differences between studies could be related to the barley variety investigated, the DPav of the AXs, the germination state, or the nature of the other polymers in the grain, among other causes. In such regards, Izydorczyk et al.
[20] found a relationship between starch structure and AX’s solubility, reporting a positive relationship between the water solubility of these carbohydrates and the amylose content of the starch of barley grains. In addition, these same authors reported AXs with higher ferulic acid content in high amylopectic grains.
2.3. Structure of Corn Arabinoxylan
Corn is also a good source of AXs, although it is much less studied than AXs from wheat or barley
[24][53][54]. Around 51% of corn bran has been identified as AXs, or 67% if residual starch is not considered
[24]. However, other authors have reported lower yields of AXs from corn bran (around 35–40%)
[55][56]. These AXs have a highly branched structure with a xylose backbone and arabinose residues as side chains on primary and secondary hydroxyl group structures, with an A/X ratio of around 0.6
[24]. Glucuronic acid (linked to the o-2 position of the xylose forming the backbone), galactose (linked to the arabinose branches), and some xylose residues also form part of corn AX’s structures
[24][25][26]. In addition to this, p-coumaric acid, ferulic acid, and acetic acid have also been found to be esterified to the monomers forming the corn AXs
[24].
3. Extraction and Production of AXs as a Food Ingredient
Extraction of AXs from cereals can be performed using various techniques from different parts of the grains. The most common source from which AXs are extracted is cereal brans, where the concentration of AXs is greatest (between 10 and 25% of the total bran)
[20][22][37][39][40]. Extraction of AXs can be performed by water treatments, mechanical treatments, chemical treatments, enzymatic treatments, or by combining these techniques
[12][13][42][57][58].
Figure 1 illustrates the different treatments that can be performed for AX’s extraction, including water and chemical treatments and other mechanical approaches.
Figure 1. Schematic illustration of a water treatment approach (
A) to extract AXs from cereal grains. (
B) demonstrates a different approach using acidic or basic chemical solutions to extract AXs. Other treatments (
C), including mechanical (milling and extrusion, steam-pressure, ultra-sound, microwave) and enzymatic treatments, are also included.
3.1. Water Extraction of Arabinoxylans
Water extraction of AXs is the easiest and least aggressive extraction method capable of preserving AX’s native structure. As previously discussed, the water solubility of AX is dependent on several factors, such as the type of grain, the degree of germination, and the nature of the polymers forming the grain
[8][9]. These factors will undoubtedly impact the yield of AXs when extracting with water.
The extraction procedure involves solubilising the AXs by placing the milled grains (or grain fractions) in water at temperatures that can range from 45 to 90 °C for a fixed time (usually longer when using lower temperatures)
[8][43][59][60]. This solution will then be precipitated using an organic solvent. To inactivate the grains’ endogenous enzymes, samples can also be pre-treated with an aqueous ethanol solution (80% v/v)
[8]. After extraction, insoluble polymers are removed by centrifugation. The supernatant rich in AXs can be directly lyophilized to retain a pellet rich in AXs
[8] and other water-soluble polymers. To overcome this, an alternative step following the first centrifugation can be performed. The AXs in the supernatant can be precipitated with 95% ethanol or another organic solvent at around 4 °C for a fixed time (typically 12 h), followed by centrifugation and drying steps
[4][60]. Before measurement, the lyophilized sample can be treated to remove denatured proteins by filtration with celite or an equivalent compound (e.g., Fuller’s earth), and by adsorption on Vega clay (or equivalent) for the residual non-denatured proteins
[50][59][61]. Depending on the raw material used for the extraction, removing other polymers such as starch and other carbohydrates may be required. Removal is typically achieved using specific enzymes that target these polymers. Free sugar is then removed using dialysis while the enzymes are heat-inactivated
[59]. These proteins and non-AX carbohydrate removal steps can also be achieved before the lyophilisation of the pellet rich in AXs
[59]. The main limiting factor of these extraction methods is that the crosslinks between potentially soluble AXs and other polymers of the cell wall matrix are not broken, limiting the extraction yield
[12]. Thus, it might be more appealing to couple water extraction of AXs with mechanical treatments to increase solubility. The following paragraph reviews the most critical mechanical treatments to improve AX’s extractability.
3.2. Mechanical Extraction of Arabinoxylans
Mechanical extraction helps to improve the extraction yield by making AXs more accessible. In addition, other mechanical treatments are available, such as milling and extrusion
[
Rye bran |
Alkali + enzymatic |
First extraction: 0.17 M Na2CO3 or 0.17 M Ca (OH)2 or water |
|
Second extraction: xylanase |
First extraction: 2.92–3.85 |
| Second extraction: 7.5–9.85 |
First extraction: 0.48–0.59 Second extraction: 0.23–0.28 |
[88] |
Wheat and barley straw |
Alkali and steam pretreatment + enzymatic |
1–2 wt% NaOH (steam pretreatment) + β-glucosidase and xylanase |
18–35 (Wheat) 17–47 (Barley) |
n.d. |
[89] |
Wheat bran |
Ultrasound + Enzymatic |
Xylanase (sodium acetate buffer) |
4.25–12.88 |
n.d. |
[66] |
Wheat bran |
Enzymatic |
Xylanase |
23.1 |
0.44 |
[90] |
Corn fibre |
Enzymatic |
Xylanase and cellulase (sodium acetate buffer) |
30–45 |
n.d. |
[90] |
* AX extracted yield by raw material dry basis (% of Dw). ** Maximum yield achieved at optimized NaOH concentration, time, and temperature (0.5 M, 2 h, 60 °C). *** Yields were dependent on the material; yield could be influenced by pretreatments of these plant materials carried out by manufacturers. n.d.: not determined.
3.4. Enzymatic Extraction of Arabinoxylans
Enzymatic extraction of AXs with the use of endoxylanases and cellulases can be as efficient as chemical methods, with the benefit that it is more environmentally friendly and AX degradation can be better controlled. Treatment conditions influence the yield, Mw, and A/X ratio of extracted AXs. The enzymatic effect on the AX extraction yield is influenced by the enzyme source and concentration, and it depends on whether they are used alone or in combination with another enzyme (
Table 2). The combined use of endoxylanases and cellulases provides higher extraction yields of AXs
[91][92]. A more common approach is to couple an enzymatic extraction of AXs with other extraction methods, typically chemical extractions with alkali solvents and chemical treatments
[13][66][69]. The enzymatic extraction can be performed after extracting the water-soluble AXs of the raw material to maximise the yield of AXs.