Structural and physicochemical changes in guar gum by alcohol–acid treatment

•Alcohol-acid treatment could degrade guar gum in large quantities.•Alcohol-acid degradation of guar gum followed first-order kinetics.•Increase in the number of alcohol carbon atoms and acid content enhanced degradation.•Alcohol-acid degradation had no significant effect on guar structure.•Degraded guar had decreased viscosity, reduced thermo-stability, and browned color. The aim of this study was to evaluate the effects of alcohol-acid degradation on the structural and physicochemical characteristics of guar gum. Guar gum (50wt.%) was treated with methanol, ethanol, 2-propanol, or 1-butanol containing 1 or 2mL 36% hydrochloric acid at 75°C for 30–240min. The linear relationship between 1/Mw and time suggested that alcohol-acid degradation followed first-order kinetics with rate constants varying directly with the number of alcohol carbon atoms and acid concentration. Alcohol-acid treatment exerted no effect on the chemical and crystal structure of guar but significantly decreased shear viscosity, slightly increased solubility, mildly reduced thermostability, and dramatically darkened colour. This study extended the knowledge of the effect of alcohol on the acid hydrolysis of guar and will contribute to the development of a large-scale procedure for guar gum degradation.