Temperature fractionation, physicochemical and rheological analysis of psyllium seed husk heteroxylan

Psyllium husk is a source of natural dietary fibre with marked water absorbability and gelling properties, which makes it an attractive functional ingredient for applications in the food industry, such as gluten free bread and breakfast cereals. The main functional component of psyllium husk is a complex branched heteroxylan. In this study, a straightforward sequential fractionation of hydrated psyllium seed husk powder based on temperature-dependent behaviours was applied. The F20 (20 °C fraction) showed the highest yield followed by F60 while 13.5% of the husk is unextractable (residue). The obtained fractions showed unique rheological properties as analysed using small amplitude oscillatory shear rheometry and the time-temperature superposition (TTS) technique. The results indicate that: 1) only F20 was influenced by heat treatment, 2) high temperature fractions showed stronger gel properties, 3) a three-step softening/melting was observed, 4) F60 has longest relaxation time as shown in TTS master curves. The four fractions were also characterised using the monosaccharide analysis, FTIR and 13C NMR. The arabinose/xylose ratio was found to increase with the increase in fractionation temperature. FTIR and 13C NMR spectra supported that low temperature fraction is less branched. Two hypotheses were therefore proposed: The first one based on models by Haque, Richardson, Morris, and Dea (1993) and Yu et al. (2019) focusing on chemical and structural properties of the molecules. The second hypothesis highlights differences in hierarchical molecular conformations of polysaccharides which is proposed by Diener et al. (2019). Sidechain substitution and composition and length of sidechains are critical and significantly influence the properties of each fraction. [Display omitted] •Psyllium husk heteroxylan is fractionated by a straightforward method.•Fractions show distinct rheological properties.•Arabinose/xylose ratio can be estimated by 2nd-derivative FTIR and 13C NMR spectra.•Composition and spatial arrangement of sidechains are influential.•Two hypotheses were proposed.