Rheological Properties of Tamarind Xyloglucan Treated by Ultrasound and Enzymolysis

The effects of 1.5 and 2.5 h of ultrasonic treatment and 16, 32 and 48 h of enzymatic hydrolysis with galactosidase on the rheological properties of Xyloglucan (XG) from tamarind were studied. The viscosity and module of untreated and treated XG were studied under static and dynamic shear using a rotational rheometer. The molecular weight distribution and average particle size were analyzed using gel chromatography with multi-angle laser light scattering and dynamic laser light scattering instruments. In addition, the microstructure of XG samples were observed via scanning electron microscope. Results of the temperature sweep showed that the viscosity of XG treated by ultrasound and enzymatic hydrolysis significantly decreased at 5 ℃, but the difference of samples treated for different durations was small. In the range of 5 to 60 ℃, the elastic and viscous modulus of XG were close to 7 Pa. The viscosity of ultrasonic treated XG decreased with the increasing of temperature, and the rheological properties of ultrasonic samples was viscous-dominant. The elastic properties of XG treated with 48 h enzymatic hydrolysis were higher than those treated with 16 and 32 h enzymatic hydrolysis. The results of molecular weight analysis showed that the average molecular weight of ultrasonic-treated 1.5 and 2.5 h XG decreased from 885 kDa to 780 and 520 kDa respectively. The enzymatic hydrolysis of XG was composed of a low molecular weight (101~478 kDa) ratio and a high molecular weight (4591~12173 kDa) ratio, of which the low molecular weight component accounted for more than 80%. The results of dynamic light scattering showed that the average particle size of XG increased after galactosidase hydrolysis, while ultrasonic treatment had little effect on the particle size of XG. This study would provide experience on the physical and biological modification on xyloglucan to obtain carbohydrate colloids with various rheological properties, which is beneficial for its further application in food industry.