Radiation Induced Degradation of Galactomannan Polysaccharides

Galactomannans are neutral polysaccharides that occur in substantial amounts in the endosperm of the seeds of some leguminous plants. Structurally they consist of a β(1-4)-D-mannose backbone to which galactose units are attached α(1-6). Among various galactomannans known, guar gum (GG), tara gum (TG) and locust bean gum (LBG) are the most widely used in applications in, for example, the food, pharmaceutical, and chemical industries as thickening agents or stabilizers due mainly to the high viscosity they impart at low concentrations. In many industrial applications, the use of low molecular weight polysaccharides is essential. For example, guar solutions, which are used as hydraulic fracturing fluids in oil and gas recovery, need to be degraded to facilitate the outflow of oil. In addition, to understand the solution properties of guar as well as other water-soluble biopolymers, it is often necessary to degrade the native polymer to prepare samples with various molecular weights (MW. Degradation of polysaccharides has been widely studied. Though acid and enzymatic hydrolysis are most common, other methods such as thermal, γ-irradiation, extrusion, ultrasonication and free radical degradation are also reported. In this study, radiation induced degradation of galactomannan polysaccharides has been investigated. GG, TG and LBG samples were irradiated with gamma rays in air at ambient temperature in the solid state at low dose rate. The change in their molecular weights was determined by SEC analysis and the change in their viscosity values as a function of temperature and irradiation dose was determined. Chain scission yields, G(S), and degradation rates were calculated. As a result of irradiation the molecular weight and viscosity of all galactomannans sharply decreased up to 50 kGy, no significant change was observed beyond this dose value. We observed that mannose-to-galactose ratio is an important factor controlling the G(S) and degradation rate of galactomannans. The G(S) values were found to follow an order of guar gum > tara gum > locust bean gum. When the chemical structures of these gums are examined it is seen that GG has one galactomannan unit attached to the backbone per two monomeric units of the backbone. This is one per three monomeric units for TG and one per four monomeric units for LBG. It can be concluded that the G(S) value increases with an increase in the galactose to mannose ratio and/or molecular weight of the unirradiated sample