174
indicates the degree of branching in arabinoxylans. The ratio of Arabinose to xylose
may vary from 0.3 to 1.1 depending on the origin of arabinoxylans, being lower in
endosperm than in bran (Rattan et al. 1994). A higher ratio has been associated with
a lower content of 3-monosubstituted and unsubstituted xylose residues and a higher
content of 2-monosubstituted and disubstituted xylose residues (Vinkx et al. 1991;
Gruppen et al. 1992). Also, some of the arabinose moieties are ester- linked on C
(O)-5 positions to hydroxycinnamic acids such as ferulic acid and p- coumaric acid
(Ayala-Soto et al. 2016). In addition to monomeric ferulic acid, dimer and trimer
structures of this compound has been reported in arabinoxylans extracted from
wheat and corn bran (Lapierre et al. 2001). It has been reported that in water- soluble
arabinoxylan, ferulic acid substituents are involved in the oxidative gelation by formation of dehydrodiferulic acid units (Geissmann and Neukom 1973). Rattan et al.
(1994) reported that arabinoxylans extracted from several Canadian wheat varieties
contained ferulic acid in the range of 0.63–1.37 mg of ferulic acid per gram of purified polymer. Ayala-Soto et al. (2015) reported the ferulic acid content of glucoarabinoxylans from three sorghum varieties in the range of 0.09–0.16 (μg FA/mg). The
concentration of ferulic acid and p-coumaric acid of arabinoxylans extracted from
different maize fiber sources has been reported in the range of 0.52–1.197, and
0.1–0.22 μg/mg extract, respectively (Ayala-Soto et al. 2014a, b). Dervilly-Pinel
et al. (2001) found that the amount of ferulic acid in the wheat, barley, rye and triticale endosperm ranged from 18 to 60 residues per 10,000 xylose units with wheat
and barley arabinoxylans containing more ferulic acid than those from rye and triticale. In general, arabinoxylans are heterogeneous molecule consisting of a range of
structures with different degrees and substitution patterns. The molecular weight of
arabinoxylans extracted from cereals varies with the origin and the method used for
their estimation. Fincher and Stone (1986) reported molecular weight of water
extractable wheat arabinoxylans in the range of 800,000–5,000,000 Da as estimated
by gel filtration chromatography. Gruppen et al. (1992) reported a weight-average
molecular weight of 850,000 for alkali extractable wheat arabinoxylans, as determined by laser light scattering analysis. Girhammar and Nair (1992) reported
molecular weight of rye arabinoxylans in the range of 519,000–770,000 Da using
high performance gel permeation chromatography. The weight average molecular
weight of 124,000 Da was obtained for water-extractable arabinoxylan from rye as
determined by multiple angle laser light scattering (Nilsson et al. 2000). Girhammar
et al. (1986) reported that the molecular weight of water-extractable wheat arabinoxylans range from 65,000 to 66,000 Da as obtained by sedimentation (Fig. 1).
Classification, Isolation and Purification of Pentosans
Arabinoxylan are divided into water-extractable and water-unextractable fractions.
The differences in the water extractability of the arabinoxylans is related to the pattern of substitution and the extent of physical entanglement, covalent ester bonding
between the –OH group of arabinoxylans and –COOH group of uranic acids and the
A. Shah et al.
indicates the degree of branching in arabinoxylans. The ratio of Arabinose to xylose
may vary from 0.3 to 1.1 depending on the origin of arabinoxylans, being lower in
endosperm than in bran (Rattan et al. 1994). A higher ratio has been associated with
a lower content of 3-monosubstituted and unsubstituted xylose residues and a higher
content of 2-monosubstituted and disubstituted xylose residues (Vinkx et al. 1991;
Gruppen et al. 1992). Also, some of the arabinose moieties are ester- linked on C
(O)-5 positions to hydroxycinnamic acids such as ferulic acid and p- coumaric acid
(Ayala-Soto et al. 2016). In addition to monomeric ferulic acid, dimer and trimer
structures of this compound has been reported in arabinoxylans extracted from
wheat and corn bran (Lapierre et al. 2001). It has been reported that in water- soluble
arabinoxylan, ferulic acid substituents are involved in the oxidative gelation by formation of dehydrodiferulic acid units (Geissmann and Neukom 1973). Rattan et al.
(1994) reported that arabinoxylans extracted from several Canadian wheat varieties
contained ferulic acid in the range of 0.63–1.37 mg of ferulic acid per gram of purified polymer. Ayala-Soto et al. (2015) reported the ferulic acid content of glucoarabinoxylans from three sorghum varieties in the range of 0.09–0.16 (μg FA/mg). The
concentration of ferulic acid and p-coumaric acid of arabinoxylans extracted from
different maize fiber sources has been reported in the range of 0.52–1.197, and
0.1–0.22 μg/mg extract, respectively (Ayala-Soto et al. 2014a, b). Dervilly-Pinel
et al. (2001) found that the amount of ferulic acid in the wheat, barley, rye and triticale endosperm ranged from 18 to 60 residues per 10,000 xylose units with wheat
and barley arabinoxylans containing more ferulic acid than those from rye and triticale. In general, arabinoxylans are heterogeneous molecule consisting of a range of
structures with different degrees and substitution patterns. The molecular weight of
arabinoxylans extracted from cereals varies with the origin and the method used for
their estimation. Fincher and Stone (1986) reported molecular weight of water
extractable wheat arabinoxylans in the range of 800,000–5,000,000 Da as estimated
by gel filtration chromatography. Gruppen et al. (1992) reported a weight-average
molecular weight of 850,000 for alkali extractable wheat arabinoxylans, as determined by laser light scattering analysis. Girhammar and Nair (1992) reported
molecular weight of rye arabinoxylans in the range of 519,000–770,000 Da using
high performance gel permeation chromatography. The weight average molecular
weight of 124,000 Da was obtained for water-extractable arabinoxylan from rye as
determined by multiple angle laser light scattering (Nilsson et al. 2000). Girhammar
et al. (1986) reported that the molecular weight of water-extractable wheat arabinoxylans range from 65,000 to 66,000 Da as obtained by sedimentation (Fig. 1).
Classification, Isolation and Purification of Pentosans
Arabinoxylan are divided into water-extractable and water-unextractable fractions.
The differences in the water extractability of the arabinoxylans is related to the pattern of substitution and the extent of physical entanglement, covalent ester bonding
between the –OH group of arabinoxylans and –COOH group of uranic acids and the
A. Shah et al.