Galactomanana de sementes de Cassia fistula: extração, caracterização e modificação. Um potêncial substituínte da galactomanana da Locusta bean

Abstract

The demand for biodegradable materials extracted from renewable sources that have high performance and lower cost is increasingly growing. The galactomannan extracted from Cassia fistula seeds was characterized by several methodologies: Elemental Analysis; Protein content; Uronic acid content; Gel Permeation Chromatography (GPC); Intrinsic Viscosity; Rheological Studies;Turbidity; Thermogravimetry; Infrared and 1H, 13C NMR. The yield of the extraction was 26.5 % m/m. The values obtained for protein content, humidity and uronic acid content were: 2.5%, 11.9% and 3.6% respectively. The values of molar weight (Mw = 2.2 x106g/mol) and intrinsic viscosity (9.73 dL/g) found for GCF were close to those from locusta bean galactomannan. For galactomannan of locusta bean is the second galactomannan commercializes most in the world, for this reason a comparative between GCF was done andGLB. The rheological analysis showed that GCF solutions exhibited pseudoplastic behavior at concentrations higher than 1%. Thermogravimetry analysis for GCF showed three events: 1° related to water loss; 2° related to matter decomposition and the 3° related to loss of organic matter which is more resistant to degradation. The infrared spectrum showed characteristic bands for polysaccharide and the 1H and 13C NMR spectra showed galactomannan characteristic peaks. The ratio mannose/galactose of GCF determined by 1H NMR spectrum analysis was 3.2, value close to that one from locusta bean galactomannan. The GCF was modified by reacting with sodium periodate at several concentrations to obtain the modified polysaccharide with oxidation degrees of: 10%; 20%; 50%; 65% and 80% [(periodate molar rate/glycosidic unities) x100]. The yield values obtained for the reactions were around 70% w/w. The oxidated materials were characterized by Rheology; GPC and 1H ,13C NMR. It was observed a decrease of molar mass (Mw) of 2.3 x 106 g/mol (GCF) to 6.4 x 104 g/mol (GCFO10%). As the degree of oxidation increases there is a reduction of the molecular weight, confirming that there was a degradation of the polymeric chain by the oxidation reaction with sodium periodate. The rheological analysis for 1% (w/v) non-modified GCF solutions and of GCFOX’s showed a reduction of apparent viscosity with oxidation degree increase (at 596 s-1): of 60.0 mPa.s (GCF) to 14 and 1.7 mPa.s (GCFO10) and 80%, respectively. The 1H NMR spectra showed for samples until 20% a few signals on anomeric range, when compared with samples with oxidation degree higher than 50 %. The ratio Man/Gal increased from 3.2 for the original sample to 3.3 and 4.6 for those samples of 10 and 20%, respectively. That increase means that the oxidation takes place preferably at the galactose units.