WATER-SOLUBLE YELLOW MUSTARD (SINAPIS-ALBA L) POLYSACCHARIDES - PARTIAL CHARACTERIZATION, MOLECULAR-SIZE DISTRIBUTION AND RHEOLOGICAL PROPERTIES

The water-soluble (WS) fraction of yellow mustard mucilage, exhibiting pronounced shear thinning behaviour in aqueous solutions, was separated into a CTAB-precipitated fraction (WSCP) and a CTAB-soluble fraction (WSCS) by precipitation with 5% CTAB (hexadecyltrimethylammonium bromide) under optimum conditions of complexation of CTAB with acidic polysaccharides. The chemical structure and molecular size distributions of WSCP and WSCS were determined by methylation, C-13 NMR spectra and gel filtration chromatography in order to examine their effect on the rheological properties of these materials in solution. The major fraction WSCP (52.0%) was a mixture of a pectic material consisting primarily of galacturonic acid, galactose and rhamnose and a 1,4-linked beta-D-glucan. The minor fraction WSCS (34.0%) also was composed of two polysaccharide fractions differing in their molecular size and consisted mostly of neutral sugars and non-reducing end residues of glucuronic acid. Both WSCP and WSCS contribute to the rheological properties of WS solutions although they have different structures and molecular weight distributions. WSCP exhibited similar rheological behaviour to WS as assessed by both dynamic and steady shear flow measurements. In contrast. the WSCS fraction only showed a steady shear rheological pattern similar to WS.