The effect of gamma irradiation on guar gum, locust bean gum, gum tragacanth and gum karaya

Abstract

Changes in rheological properties, as measured by viscosity, of two galactomannans (guar gum and locust bean gum) and two acidic polysaccharides (gum tragacanth and gum karaya) were studied at a range of irradiation doses <10 kGy. Powdered samples were irradiated, and the viscosity of a 1% dispersion prepared at room temperature or by heating to 80°C for 1 h, was determined over a wide shear rate range. All samples showed pseudoplastic behaviour which approached Newtonian with increasing irradiation dose. Viscosities were calculated at a shear rate of 54 sec −1 to enable comparison across the samples. Both galactomannans showed a decrease in viscosity with increasing γ irradiation independent of temperature and a hypothesis is proposed that at low γ irradiation doses (<2 kGy) there is a reduction in polymer aggregation in solution, whereas at higher doses polymer hydrolysis occurs. Electron spin resonance spectroscopy data supports this hypothesis, with the detection of different free radicals at low and high irradiation doses. The viscosity of the acidic polysaccharides, gum karaya and gum tragacanth, following γ irradiation at low doses (<1 kGy) was unchanged or slightly higher when compared to the unirradiated control samples. Above 1 kGy dispersion viscosity decreased with increasing dose. For these polysaccharides chain hydrolysis seems to occur during irradiation at all doses resulting in an increase in the amount of soluble polymer and hence increased viscosity at low doses, whilst at high doses viscosity decreases due to extensive polymer hydrolysis. Similar electron spin resonance (ESR) spectra were obtained at low and high doses with a stronger signal at the higher dose.