Xanthan-like ‘weak gel’ rheology from dispersions of ispaghula seed husk

Dispersions of isabgol, the milled seed husk from Plantago ovata Forsk (alternatively known as ispaghula), show ‘weak-gel’ properties broadly similar to those of xanthan and related polysaccharides with rigid, ordered structures in solution. The origin of this behaviour is attributed to tenuous association of fibrillar assemblies visualised by light microscopy. The network structure is retained to 80°C, but decreases steeply at higher temperatures. The melting process is accompanied by a sharp change in optical rotation of the extracted polysaccharide component of isabgol. An earlier change in optical rotation at lower temperature is tentatively attributed to conformational rearrangement of xylan chains within an ordered, intermolecular structure. Aqueous solutions of the extracted polysaccharide form gels which gradually contract on prolonged storage, consistent with progressive re-formation of the fibrillar structure seen for intact isabgol. Loss of gel-like character in isabgol dispersions occurs over the same temperature range as thermogelation of hydroxypropylmethylcellulose, suggesting opportunities for combined use of the two materials as a substitute (or supplement) for gluten in baked products.