Structure and properties of arabinoxylans from rye

Rye arabinose xylans influence bread volume, form, crumb properties and shelf life. The mechanisms of their contribution to the quality of product are poorly understood, partly because the structural heterogeneity of this group was not taken into account in many investigations. Although aspects of the oxidative gelation of wheat arabinoxylans have been clarified, this phenomenon has received little attention in the case of rye arabinoxylans. In this work, the oxidative gelation of water-extractable arabinoxylans and the structural variability of not only the water- but also the alkali-extractable arabinoxylans from the rye kernel were investigated. Rye water-extractable arabinoxylans gel upon addition of hydrogen peroxide and peroxidase. The phenolic function of ferulic acid esterified to arabinoxylan is a conditio sine qua non for the gelling reaction. Interfering substances react with ferulic acid (e.g. ascorbic acid). Rye water-extractable arabinoxylans consist of a linear xylan chain carrying branches of terminal arabinoses. Using ammonium sulfate precipitation, a very highly branched polymer containing 64 per cent di- and 14 per cent 2-monosubstituted xyloses, was isolated. Differences were observed in the proportion of disubstituted xyloses that occurred isolated, paired or in longer sequences in the xylan chain. This provides evidence that a range of polymer structures exist, in contrast to previous suggestions of two classes. For the whole group of rye water-extractable arabinoxylans, the contents of the four structural features (un-, 3-mono-, 2-mono-, and disubstituted xylose residues) are linearly related to the arabinose-to-xylose ratio (A/X) of the arabinoxylan preparations. Most of the water-unextractable arabinoxylans can be extracted under alkaline conditions. Three classes of arabinoxylans, differing in extractibility, solubility and structure, are distinguished: arabinoxylans with A/X of about 0.2 (in the aleurone), arabinoxylans with intermediate A/X and arabinoxylans with an A/X of about 1.1. Substituted arabinose residues, terminal xylose and galactose units are present in considerable proportions in the latter fraction (located in the outer parts of the kernel). The linear relations between the proportions of structural features and the A/X, were strongly different for extractable rye bran arabinoxylans as for water-extractable rye arabinoxylans.